Towards Understanding Understeer/Oversteer.
Discussion
I've set up this thread really to discuss oversteer and understeer largely in the context of FWD cars. Reason is, my understanding of how a vehicle behaves when steering has changed quite a bit since the days of Gran Turismo for the Playstation.
I used to think it was a rule of physics that FWD cars understeer and RWD cars oversteer. When I started hearing about RWD cars understeering, doubt had entered my mind.. When I started oversteering in my FWD car, I had to rethink my purpose in life.
I have read loads on the topic and there are still a few things I don't really understand:
1. Why do MR cars oversteer more than FR cars to the point that it is easier to lose control of an MR car? Surely less weight on the front wheels means less traction. Less traction on the wheels that are steering must mean more understeer. Also, a heavier back end must make the rear wheels have more traction and more traction on the rear wheels must mean minimal oversteer. Once it does oversteer though, I understand that the extra weight means more sideways momentum and hence why oversteer in an MR is more prolonged. But an MR should be more difficult to initiate oversteer shouldn't it?
2. Why does increased rear tyre tread/decreased front tyre tread and increased front tyre pressure/decreased rear tyre pressure result in increased oversteer/decreased understeer? I would think the opposite was true. For oversteer to happen the back wheels need to let go and the front wheels need to stay put in order to tell the vehicle to steer. If rear wheels have more grip though that means more traction and if front wheels have less grip that means less traction, which is surely a recipe for understeer. With regards to tyre pressure, it's well known that you let some air out to give the car more grip since there will be more surface area touching the road. This is a widely practised strategy in poor weather conditions. Again, if the rear tyres have less pressure that means they have more grip which means less oversteer, and vice versa for the front tyres.
3. I've heard of the concept of tyres that lose traction progressively rather than suddenly. My 106 gti felt like it would lose grip progressively rather than suddenly. Is this directly correlational to grip level in general or can you actually get a set up that is any combination of easy/hard to lose grip and progressively/suddenly lose grip? The grip level on my Swift Sport certainly doesn't feel that great, yet when I try to oversteer in it, it feels alot harder to achieve but a lot more sudden and therefore a lot harder to control and maintain a steady oversteer round a bend. I learned this in an empty car park. The 106 gti however was so easy that I could oversteer round roundabouts endlessly. It definitely flattered me in the sense that it made me out to be better than the beginner level drifter that I am. I'm wondering if it's because of the size of the wheels. The 106 gti is 15 inch IIRC? The swift sport is 17 inch. Notice how I am focusing on tyres since I have had to change one of mine lately and so I've been reading up about them. I'm probably ignoring other factors that determine understeer/oversteer like suspension set up perhaps?
Anyway, that's all for now but there are loads more questions I'm just struggling to remember them all at the moment, plus I'm typing all this out on my phone which is getting tiring. Thanks for any input in advance!
I used to think it was a rule of physics that FWD cars understeer and RWD cars oversteer. When I started hearing about RWD cars understeering, doubt had entered my mind.. When I started oversteering in my FWD car, I had to rethink my purpose in life.
I have read loads on the topic and there are still a few things I don't really understand:
1. Why do MR cars oversteer more than FR cars to the point that it is easier to lose control of an MR car? Surely less weight on the front wheels means less traction. Less traction on the wheels that are steering must mean more understeer. Also, a heavier back end must make the rear wheels have more traction and more traction on the rear wheels must mean minimal oversteer. Once it does oversteer though, I understand that the extra weight means more sideways momentum and hence why oversteer in an MR is more prolonged. But an MR should be more difficult to initiate oversteer shouldn't it?
2. Why does increased rear tyre tread/decreased front tyre tread and increased front tyre pressure/decreased rear tyre pressure result in increased oversteer/decreased understeer? I would think the opposite was true. For oversteer to happen the back wheels need to let go and the front wheels need to stay put in order to tell the vehicle to steer. If rear wheels have more grip though that means more traction and if front wheels have less grip that means less traction, which is surely a recipe for understeer. With regards to tyre pressure, it's well known that you let some air out to give the car more grip since there will be more surface area touching the road. This is a widely practised strategy in poor weather conditions. Again, if the rear tyres have less pressure that means they have more grip which means less oversteer, and vice versa for the front tyres.
3. I've heard of the concept of tyres that lose traction progressively rather than suddenly. My 106 gti felt like it would lose grip progressively rather than suddenly. Is this directly correlational to grip level in general or can you actually get a set up that is any combination of easy/hard to lose grip and progressively/suddenly lose grip? The grip level on my Swift Sport certainly doesn't feel that great, yet when I try to oversteer in it, it feels alot harder to achieve but a lot more sudden and therefore a lot harder to control and maintain a steady oversteer round a bend. I learned this in an empty car park. The 106 gti however was so easy that I could oversteer round roundabouts endlessly. It definitely flattered me in the sense that it made me out to be better than the beginner level drifter that I am. I'm wondering if it's because of the size of the wheels. The 106 gti is 15 inch IIRC? The swift sport is 17 inch. Notice how I am focusing on tyres since I have had to change one of mine lately and so I've been reading up about them. I'm probably ignoring other factors that determine understeer/oversteer like suspension set up perhaps?
Anyway, that's all for now but there are loads more questions I'm just struggling to remember them all at the moment, plus I'm typing all this out on my phone which is getting tiring. Thanks for any input in advance!
1. You're right, an MR vehicle is more difficult to initiate oversteer if you try to do it the same way as an FR car. An MR car is more sensitive to weight transfer and will under steer more than most people think. When it does go, though, the lower moment of inertia means it will try to swap ends at a quicker rate. Snap lift off oversteer is the biggest issue with a lot of MR cars.
2. Unless it is wet, tread depth doesn't really factor into the grip level of a tyre (unless you're talking extreme amounts of wear). So in the dry, the tyre with the most tread doesn't necessarily have the stickiest rubber.
Tyre pressures are a much debated subject by many who don't understand vehicle dynamics. Increasing tyre pressures increases the cornering stiffness on that corner; an increased cornering stiffness generates a smaller slip angle for a given lateral acceleration. It's been a while since I studied vehicle dynamics at uni but this essentially means a higher tyre pressure on an axle will cause that axle to grip more in a bend. Adding weight to an axle also has an effect on this. We got to play around with these ideas in a jaguar x-type and indeed, lowering the rear tyre pressures caused oversteer, with accelerometer read outs and lots of graphs to prove it.
It should go without saying that this is excluding extreme low pressures used in off-roading. And this is not to say that lower pressures don't produce more grip, just that it affects vehicle dynamics in a non-intuitive way.
I can't remember your 3rd question as I'm on my phone so apologies!
Hope I've answered some questions and given you something to read around.
ETA: 3. Higher profile tyres will be more progressive than low profile ones due to greater sidewall flex giving more warning before loss of grip.
2. Unless it is wet, tread depth doesn't really factor into the grip level of a tyre (unless you're talking extreme amounts of wear). So in the dry, the tyre with the most tread doesn't necessarily have the stickiest rubber.
Tyre pressures are a much debated subject by many who don't understand vehicle dynamics. Increasing tyre pressures increases the cornering stiffness on that corner; an increased cornering stiffness generates a smaller slip angle for a given lateral acceleration. It's been a while since I studied vehicle dynamics at uni but this essentially means a higher tyre pressure on an axle will cause that axle to grip more in a bend. Adding weight to an axle also has an effect on this. We got to play around with these ideas in a jaguar x-type and indeed, lowering the rear tyre pressures caused oversteer, with accelerometer read outs and lots of graphs to prove it.
It should go without saying that this is excluding extreme low pressures used in off-roading. And this is not to say that lower pressures don't produce more grip, just that it affects vehicle dynamics in a non-intuitive way.
I can't remember your 3rd question as I'm on my phone so apologies!
Hope I've answered some questions and given you something to read around.
ETA: 3. Higher profile tyres will be more progressive than low profile ones due to greater sidewall flex giving more warning before loss of grip.
Edited by yorkshireegg on Sunday 17th February 16:58
BadBanshee said:
1. Why do MR cars oversteer more than FR cars to the point that it is easier to lose control of an MR car? Surely less weight on the front wheels means less traction. Less traction on the wheels that are steering must mean more understeer. Also, a heavier back end must make the rear wheels have more traction and more traction on the rear wheels must mean minimal oversteer. Once it does oversteer though, I understand that the extra weight means more sideways momentum and hence why oversteer in an MR is more prolonged. But an MR should be more difficult to initiate oversteer shouldn't it?
The problem with a MR or RR car is that the transition to oversteer is sudden. Think about driving around a roundabout, gradually pressing the accelerator to the floor. A FR car is usually set up to understeer on a neutral throttle, helped by the weight at the front. As you apply more throttle the balance shifts towards oversteer gradually, making it easy to adjust the angle of the car by using the accelerator. With an MR or RR car the weight over the rear wheels means that as you accelerate more, the understeer gets worse and worse until you loose rear traction, and then it snaps into oversteer. The weight also acts as a pendulum once the slide starts, making it harder to hold in balance.
BadBanshee said:
2. Why does increased rear tyre tread/decreased front tyre tread and increased front tyre pressure/decreased rear tyre pressure result in increased oversteer/decreased understeer? I would think the opposite was true. For oversteer to happen the back wheels need to let go and the front wheels need to stay put in order to tell the vehicle to steer. If rear wheels have more grip though that means more traction and if front wheels have less grip that means less traction, which is surely a recipe for understeer. With regards to tyre pressure, it's well known that you let some air out to give the car more grip since there will be more surface area touching the road. This is a widely practised strategy in poor weather conditions. Again, if the rear tyres have less pressure that means they have more grip which means less oversteer, and vice versa for the front tyres.
Not sure whether you are right with regards tread (assuming we're talking about worn vs. not worn examples of the same tyre). The situation with pressure is more dynamic than that. At low speeds, low tyre pressure will get you more contact patch and help you with moving away. At speed it becomes a liability because the tread will flop about a bit. BadBanshee said:
3. I've heard of the concept of tyres that lose traction progressively rather than suddenly. My 106 gti felt like it would lose grip progressively rather than suddenly. Is this directly correlational to grip level in general or can you actually get a set up that is any combination of easy/hard to lose grip and progressively/suddenly lose grip? The grip level on my Swift Sport certainly doesn't feel that great, yet when I try to oversteer in it, it feels alot harder to achieve but a lot more sudden and therefore a lot harder to control and maintain a steady oversteer round a bend. I learned this in an empty car park. The 106 gti however was so easy that I could oversteer round roundabouts endlessly. It definitely flattered me in the sense that it made me out to be better than the beginner level drifter that I am. I'm wondering if it's because of the size of the wheels. The 106 gti is 15 inch IIRC? The swift sport is 17 inch. Notice how I am focusing on tyres since I have had to change one of mine lately and so I've been reading up about them. I'm probably ignoring other factors that determine understeer/oversteer like suspension set up perhaps?
Progressive traction loss as a function of the tyres is usually inversely proportional to the amount of grip; realistically it's inversely proportional to the sidewall height. The flexing of the sidewall pulls at the contact patch, and that's what gives the progression. Crossplies, which have the floppiest sidewalls, also have the most progressive breakaway. But the handling setup of the car will have at least as much of an impact on the breakaway.So am I right in saying you 2 disagree about whether lower profile tyres are better or worse for progressive traction loss?
Got another question:
4. How are you suppose to control a RWD car that is prone to lift off oversteer? In a FWD car you power out of it, but in a RWD car aren't you just going to add power oversteer to it and make it worse?
Got another question:
4. How are you suppose to control a RWD car that is prone to lift off oversteer? In a FWD car you power out of it, but in a RWD car aren't you just going to add power oversteer to it and make it worse?
BadBanshee said:
So am I right in saying you 2 disagree about whether lower profile tyres are better or worse for progressive traction loss?
Got another question:
4. How are you suppose to control a RWD car that is prone to lift off oversteer? In a FWD car you power out of it, but in a RWD car aren't you just going to add power oversteer to it and make it worse?
I think we've written the same thing in a different way!Got another question:
4. How are you suppose to control a RWD car that is prone to lift off oversteer? In a FWD car you power out of it, but in a RWD car aren't you just going to add power oversteer to it and make it worse?
4. As ScoobyChris says, don't lift off in the first place! If you do find yourself in a position where it has happened, you need to smoothly find the balance on the throttle and wriggle your way out of it. It doesn't look particularly neat unless you're a driving god but hopefully you'll come out of the other side in one piece.
Lifting off mid-corner is usually going to be a bad idea in most cars, if you've gone in too fast then you're better off with neutral throttle to balance the car as much as possible. Unless you've gone in ridiculously fast, you'll usually be surprised at how well a balanced car will stick compared when it has been unbalanced by severe inputs.
BadBanshee said:
I have read loads on the topic and there are still a few things I don't really understand:
1. Why do MR cars oversteer more than FR cars to the point that it is easier to lose control of an MR car? Surely less weight on the front wheels means less traction. Less traction on the wheels that are steering must mean more understeer. Also, a heavier back end must make the rear wheels have more traction and more traction on the rear wheels must mean minimal oversteer. Once it does oversteer though, I understand that the extra weight means more sideways momentum and hence why oversteer in an MR is more prolonged. But an MR should be more difficult to initiate oversteer shouldn't it?
Yup, FR cars (and FF) tend to have a more forward weight distribution - let's say 60:40 F:R. MR cars tend to be more even - I believe mine's about 45:55(ish). So, as you say, there's more 'weight' on the front wheels of an FR car, so they do provide a bit more grip. However, the reason there's more weight on the front because the centre of *mass* is nearer the front. ('weight' is just mass accelerated by gravity). When you go around a corner (and we'll keep it at a steady speed for now), you're accelerating the car sideways to achieve the curve - the mass wants to continue going as it was (newton's first law). THAT also has to act through the centre of mass, so you NEED more grip from the fronts anyway - offsetting the advantage of the extra 'weight'.1. Why do MR cars oversteer more than FR cars to the point that it is easier to lose control of an MR car? Surely less weight on the front wheels means less traction. Less traction on the wheels that are steering must mean more understeer. Also, a heavier back end must make the rear wheels have more traction and more traction on the rear wheels must mean minimal oversteer. Once it does oversteer though, I understand that the extra weight means more sideways momentum and hence why oversteer in an MR is more prolonged. But an MR should be more difficult to initiate oversteer shouldn't it?
When you start accelerating and decelerating, you change the relative 'weights' on the tyres because the accelerating/decelerating acts at the contact patch (floor level), but the centre of mass is some way up - thus the car wants to pitch (and roll). lifting off will tend to tuck the front in - the car tries to carry on over the top of the wheel, the nose dips and that adds grip at the front without the centre of mass moving (appreciably). In the opposite sense (light) acceleration will tend to make the car push wide/understeer as you're adding grip at the rear - to a point. Once you start asking for too much acceleration, you're using up more of the grip for traction than the weight is adding to the grip and so you start to run out of sideways capability at the rear, and you get oversteer.
That's it in a nutshell - understanding the weight transfer and why can really help you drive (fast).
Most FR cars have similar tyre sizes front and back. In many MR cars you have more tyre on the rear than the front - you're asking the rear to take more of the cornering load AND provide traction, so it needs some help. If you take it to the extreme and look at a 911 (RR) they tend to have markedly larger rears and quite little fronts. Great for traction out of corners once you've got it all hooked up, but in some circumstances, that's going to make it quite prone to understeer.
Personally I wouldn't say it's 'easier' to loose control of an MR car; I think it depends quite a bit on the car - in my limited experience of comparing (MX5, Z3, Boxster -mostly on track) the FR cars are more inclined to move around under acceleration, on the way out of the corner, whereas the Boxster's really planted on the way out and will generally wag the tail on the way in. I suspect that for regular driving on the public road, most folks aren't going to be coming out of corners wagging things around, but they're far more likely to catch themselves out with a corner that's tighter than they expected and get in too hot. Mostly the Boxster's fairly easy to catch so long as you're willing to jump off the brakes - put some weight back on the rears. I reckon you catch it with your feet more than your hands - that's not necessarily intuitive.
One of the factors that makes things mid engined cars more tricky is the polar moment of inertia - basically if you put two weights on opposite ends of a broomstick and try to spin it, it's quite hard - move both to the middle and it's pretty easy. The same goes with cars; mid engined cars tend to keep the mass very centralised, that means it finds it easier to rotate, whereas an FR car spreads the mass out over the length of the car more. That helps the FR car to do things a little less suddenly, whereas the mid engined car will rotate more quickly for the given input. Of course that's a wonderful thing when you're trying to get it turned in - swings and roundabouts..
Beyond that, you start getting into things like roll-centres and the way the suspension geometry changes the contact patch as things roll, pitch and squat - which is a bit beyond me.
As for FF cars, they're not that different to FR in mass / weight terms, but you're asking the fronts to provide traction too, so it's going to have a propensity to power understeer - on throttle you're transferring weight to the rears, giving them more grip, and asking the fronts to pull forwards as well as sideways. Bit of a disaster, so you set the car up to combat that and rob the rear of some of it's grip (e.g. with a stiff anti roll bar). Now it's going to be oversteer city when you're off the power, the fronts aren't trying to pull and the weight is thrown forwards.
Hope that's of some help.
In light of the last couple of posts:
1) yes, I believe that (as a massive generalisation) low profile tyres tend to 'let go' in a less progressive manner than taller ones.
2) it takes more power than you think to get 'power' oversteer. A neutral throttle to light acceleration is fine. 99% of the time, the best way to fix what the car is doing (e.g. lift off oversteer) is to undo whatever you just did (which almost certainly caused the problem!)
Edited by upsidedownmark on Sunday 17th February 22:41
The change in grip of the driven wheels is due to the tyres having a finite well of grip which can be used to steer brake or acclerate. If you google "tyre slip angle" you should be able to spend an instructiv hour or two. The oversteer of mid to rear engine cars is due to the aft Cofg causing more weight transfer at the rear axle. This means loss of grip that end as one tyre is unloaded. I had a fantastic period picture of a 964 with one front wheel in the air on a track, this happens as to get that car nuetral-iish you need a strong front ARB to even up weight transfer. At MGCC car club meets you see the opposite effect in the MG Metro races, they corner with one rear wheel off the ground.
yorkshireegg said:
I think we've written the same thing in a different way!
4. As ScoobyChris says, don't lift off in the first place! If you do find yourself in a position where it has happened, you need to smoothly find the balance on the throttle and wriggle your way out of it. It doesn't look particularly neat unless you're a driving god but hopefully you'll come out of the other side in one piece.
Lifting off mid-corner is usually going to be a bad idea in most cars, if you've gone in too fast then you're better off with neutral throttle to balance the car as much as possible. Unless you've gone in ridiculously fast, you'll usually be surprised at how well a balanced car will stick compared when it has been unbalanced by severe inputs.
So low profile tyres lose traction progressively, not suddenly? So how come my Swift Sport is harder to oversteer than my previous 106 gti?4. As ScoobyChris says, don't lift off in the first place! If you do find yourself in a position where it has happened, you need to smoothly find the balance on the throttle and wriggle your way out of it. It doesn't look particularly neat unless you're a driving god but hopefully you'll come out of the other side in one piece.
Lifting off mid-corner is usually going to be a bad idea in most cars, if you've gone in too fast then you're better off with neutral throttle to balance the car as much as possible. Unless you've gone in ridiculously fast, you'll usually be surprised at how well a balanced car will stick compared when it has been unbalanced by severe inputs.
About RWD loo: so if you lift the throttle, you will spin, if you floor the throttle, you will spin. The trick is to put a medium amount of throttle in?
BadBanshee said:
I've set up this thread really to discuss oversteer and understeer largely in the context of FWD cars. Reason is, my understanding of how a vehicle behaves when steering has changed quite a bit since the days of Gran Turismo for the Playstation.
I used to think it was a rule of physics that FWD cars understeer and RWD cars oversteer. When I started hearing about RWD cars understeering, doubt had entered my mind.. When I started oversteering in my FWD car, I had to rethink my purpose in life.
The physics rule is that front wheel drive will understeer when wheelspin is occuring and rear wheel drive will oversteer when wheel spin is occuring. This is what has lead to the common beliefs that you had. I used to think it was a rule of physics that FWD cars understeer and RWD cars oversteer. When I started hearing about RWD cars understeering, doubt had entered my mind.. When I started oversteering in my FWD car, I had to rethink my purpose in life.
BadBanshee said:
I have read loads on the topic and there are still a few things I don't really understand:
Don't blame yourself that you don't understand. That's because most of what is written about it is simply a misinformed rehash of the rules written above about understeer and oversteer. Much of it is written by those who don't understand.I have had a respected motoring journalist write a whole article in reply to a letter of mine when I pointed out their misunderstanding over five years ago. In the article Newton's Laws of motion were called into question and the journalist stated they didn't believe that explained what was going on. This is the first time I've written on the subject on a public forum since then.
BadBanshee said:
1. Why do MR cars oversteer more than FR cars to the point that it is easier to lose control of an MR car? Surely less weight on the front wheels means less traction. Less traction on the wheels that are steering must mean more understeer. Also, a heavier back end must make the rear wheels have more traction and more traction on the rear wheels must mean minimal oversteer. Once it does oversteer though, I understand that the extra weight means more sideways momentum and hence why oversteer in an MR is more prolonged. But an MR should be more difficult to initiate oversteer shouldn't it?
The main problem is that most works on the subject are confused by the words grip and traction. Less weight on the wheels means a lesser force in needed to push static tyres sideways (or, in pedantic mode) in any direction. Conversely more weight means a greater force to push a static tyre sideways. Therefore what you understand is true when the car is stationary. However the car is not static, it is a dynamic object and other factors come into play. The momentum of the car will be in the instantaneous direction of travel and not sideways. It is only sideways subjectively to those in the car and this is not a defining factor in the duration of oversteer.
Initiation of oversteer is not necessarily more difficult to initiate in a MR car.
BadBanshee said:
2. Why does increased rear tyre tread/decreased front tyre tread and increased front tyre pressure/decreased rear tyre pressure result in increased oversteer/decreased understeer? I would think the opposite was true. For oversteer to happen the back wheels need to let go and the front wheels need to stay put in order to tell the vehicle to steer. If rear wheels have more grip though that means more traction and if front wheels have less grip that means less traction, which is surely a recipe for understeer. With regards to tyre pressure, it's well known that you let some air out to give the car more grip since there will be more surface area touching the road. This is a widely practised strategy in poor weather conditions. Again, if the rear tyres have less pressure that means they have more grip which means less oversteer, and vice versa for the front tyres.
Here you are repeating what most of the uniformed regurgitated texts on the matter write. It seems common sense what you say, but what only very few people understand is that it is not the grip at the road surface that we are interested in but the slip angle, which is mainly influenced by sidewall flex. That is what defines understeer and oversteer until at least one set of tyres are sliding. By lowering the pressure at the rear the sidewall flexes more, thus increasing the slip angle and therefore increasing oversteer.Similarly all the nonsense that is written about braking on turn in to put weight on the nose exasperates me. It is not extra weight that improves turn in, that increases understeer. The effect that braking has is that it slows the car down, and unless the front tyres skid, this allows the car to turn better as a slower car needs less force to make it change its direction of travel in a given distance than a faster car.
BadBanshee said:
3. I've heard of the concept of tyres that lose traction progressively rather than suddenly. My 106 gti felt like it would lose grip progressively rather than suddenly. Is this directly correlational to grip level in general or can you actually get a set up that is any combination of easy/hard to lose grip and progressively/suddenly lose grip? The grip level on my Swift Sport certainly doesn't feel that great, yet when I try to oversteer in it, it feels alot harder to achieve but a lot more sudden and therefore a lot harder to control and maintain a steady oversteer round a bend. I learned this in an empty car park. The 106 gti however was so easy that I could oversteer round roundabouts endlessly. It definitely flattered me in the sense that it made me out to be better than the beginner level drifter that I am. I'm wondering if it's because of the size of the wheels. The 106 gti is 15 inch IIRC? The swift sport is 17 inch. Notice how I am focusing on tyres since I have had to change one of mine lately and so I've been reading up about them. I'm probably ignoring other factors that determine understeer/oversteer like suspension set up perhaps?
Anyway, that's all for now but there are loads more questions I'm just struggling to remember them all at the moment, plus I'm typing all this out on my phone which is getting tiring. Thanks for any input in advance!
If you want to know a bit more try hereAnyway, that's all for now but there are loads more questions I'm just struggling to remember them all at the moment, plus I'm typing all this out on my phone which is getting tiring. Thanks for any input in advance!
http://www.road-and-track.co.uk/expert-driving/cor...
johnao said:
Can someone enlighten me as to what MR and FR means, please?
I know it'll be obvious when it's explained but at the moment my brain hurts and I can't work it out for myself!
Edited to add: and FF?
Mid engine, Rear wheel driveI know it'll be obvious when it's explained but at the moment my brain hurts and I can't work it out for myself!
Edited to add: and FF?
Front engine, Rear wheel drive
I blame Gran Turismo.
Martin A said:
(...)
Similarly all the nonsense that is written about braking on turn in to put weight on the nose exasperates me. It is not extra weight that improves turn in, that increases understeer. The effect that braking has is that it slows the car down, and unless the front tyres skid, this allows the car to turn better as a slower car needs less force to make it change its direction of travel in a given distance than a faster car.
(...)
I DO agree that the use of many terms (e.g. weight) is misleading and confusing. Equally, much of what you say I might not entirely agree with - for instance, I would suggest that slip angle is a resultant rather than causal, but don't have any way of proving that. Similarly all the nonsense that is written about braking on turn in to put weight on the nose exasperates me. It is not extra weight that improves turn in, that increases understeer. The effect that braking has is that it slows the car down, and unless the front tyres skid, this allows the car to turn better as a slower car needs less force to make it change its direction of travel in a given distance than a faster car.
(...)
WRT the quote above however, you're wrong - I know that empirically: slowing down will allow the car to go more easily around the corner yes, but trail braking adjusts the balance of the car. If I simply drive round the corner more slowly, the car will go where I point it, if i'm trail braking, it will most likely try to swap ends.
Weight is a complete misnomer, it is a shorthand for mass, accelerated by gravity. It's a force, no different to any other. When you're trail braking there's a couple between the centre of mass and the point where the braking forces are acting, with the centre of mass being above above the braking plane.
That couple tries to rotate the car nose-down, and is resisted by the tyres/suspension, causing more 'downward' force to be applied to the front wheels contact patch, and less to the rears. Friction is a function of the reaction force between the two surfaces (download), so there's now more friction available at the front than the rear. Meantime, as the actual forward 'rotation' of the car is minimal, the actual movement of the centre of mass is minimal. The momentum which causes the car to want to go straight on is acting at the centre of mass. You've redistributed the 'grip', but not the mass. Ergo the car will tend to oversteer.
'weight' (appart from not really existing), and being ill defined hasn't moved anywhere. It's probably even more complicated still, and I'm just presenting my understanding of why; in any case, a few hundred laps round various tracks leaves me in no doubt as to the effect. Trail braking is very very different to simply going around more slowly. Probably one of the biggest things I've learned on track is how much faster the car will go around things if you use "weight" transfer properly.
[edit for spelling and grammar]
Edited by upsidedownmark on Monday 18th February 11:04
BadBanshee said:
So low profile tyres lose traction progressively, not suddenly? So how come my Swift Sport is harder to oversteer than my previous 106 gti?
Other way round! High profile tyres are more progressive, the elastic bands you see on most modern cars are very sudden. Think of the sidewall flex as giving you feedback and warning of the impending slip. Suspension set up will be the major factor in what you've experienced between the two cars, a 106 has torsion bar suspension on the rear, the swift will likely have independent rear suspension. In essence, the 106 is set up to be tail happy.
Martin A said:
Some very interesting and well presented stuff
Finally, someone who understands some vehicle dynamics on the Internet! There is a whole subject matter of roll centers vs CofG etc that we haven't touched upon. This is driven by the type of suspension and geometry of it. Although there are trends influenced by overall car layout, its far more complex and is part of the reason why cars of similar layout don't always behave the same.
Weight transfer and application of power / breaks then also have a huge influence. This is before we go into differential type etc. The generics that we're lead to believe really cant be trusted, you have to take each car on merit.
Generally, the most progressive tyres for losing grip are higher profile, skinny and harder compound, the total opposite of what we put on our performance cars.
Weight transfer and application of power / breaks then also have a huge influence. This is before we go into differential type etc. The generics that we're lead to believe really cant be trusted, you have to take each car on merit.
Generally, the most progressive tyres for losing grip are higher profile, skinny and harder compound, the total opposite of what we put on our performance cars.
Although it is an old American book, and mostly about rwd cars, I found this an interesting read:
FWIW, I do believe in trail braking
Weight (ie. A Force in newtons) transfer is fundamental to understanding vehicle dynamics.
A slightly contrived idea, but I've tried to visualise the of fore:aft weight transfer being 'linked' to the throttle pedal:
ie. 'Pushing' against the pedal 'pushed' the driver (me) and the weight backwards, and vice versa. Smoothly, of course.
(It's more apparent on a motorbike when operating the twist-grip) It's served me well.
If you have a go on a skidpan or skidcar, you test out the theories at low speeds -They do work.
From what I observed of other people when I had a go, they didn't understand the concept of weight transfer, only applying and removing power.
FWIW, I do believe in trail braking
Weight (ie. A Force in newtons) transfer is fundamental to understanding vehicle dynamics.
A slightly contrived idea, but I've tried to visualise the of fore:aft weight transfer being 'linked' to the throttle pedal:
ie. 'Pushing' against the pedal 'pushed' the driver (me) and the weight backwards, and vice versa. Smoothly, of course.
(It's more apparent on a motorbike when operating the twist-grip) It's served me well.
If you have a go on a skidpan or skidcar, you test out the theories at low speeds -They do work.
From what I observed of other people when I had a go, they didn't understand the concept of weight transfer, only applying and removing power.
Edited by MC Bodge on Tuesday 19th February 13:27
) In RWD though weight transfer isn't as important to understand power oversteer I don't think.BadBanshee said:
Weight transfer is something big in my mind atm especially since I'm wanting to drift in a FWD car
When you say drifting, do you just mean lift off oversteer or something else? I'd always understood that drifting required RWD and a good dollop of power oversteer?Chris
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