Front axle weight and vehicle dynamics question
Discussion
New year greetings 
The more I think about this, the more I realise I'm totally lost.
I have an old rear engined car, a Renault 8. Its handling is not great, most likely due to its unbalanced weight distribution and a suspension that's poor by design (too much camber change with suspension travel is probably the worst of its shortcomings).
I'd like to focus on weight distribution and the lack of weight on this car's front axle.
All RWD cars tend to oversteer if you give them a lot of throttle in a bend. This particular car is no exception, but let's not focus on that. The thing is that this car also tends to understeer in those very same conditions. Basically, due to weight transfer to the rear, the front axle is all floaty and you lose your steering.
And this is where I realise how clueless I am.
Because front axle weight is precisely what makes front wheel drive cars understeer, right? As you turn the steering wheel to enter a bend, they "want" to continue straight due to the engine weight at the front.
Then, what's the deal? Too much weight at the front and you get understeer, too little weight at the front and you get understeer?
I thought weight is a bad thing. In the sense that you want your track day car to be light. among other things, this allows you to go through bends faster. If weight is a bad thing, then there can't be such thing as too little weight!
But there is. To the point that some people who can't afford to modify these cars and relocate some weight (battery, fuel tank, radiator) simply add weight to the front in order to improve its grip. I find this extremely counterintuitive, which bugs the engineer in me. But when I drive my own car I totally get why they do it.
BTW, overall I really enjoy driving this car, it's a light car and that makes it a lot of fun by itself. Either axle can break loose (or even both simultaneously), but that doesn't spoil the fun at all. Also it looks lovely to me
The more I think about this, the more I realise I'm totally lost.
I have an old rear engined car, a Renault 8. Its handling is not great, most likely due to its unbalanced weight distribution and a suspension that's poor by design (too much camber change with suspension travel is probably the worst of its shortcomings).
I'd like to focus on weight distribution and the lack of weight on this car's front axle.
All RWD cars tend to oversteer if you give them a lot of throttle in a bend. This particular car is no exception, but let's not focus on that. The thing is that this car also tends to understeer in those very same conditions. Basically, due to weight transfer to the rear, the front axle is all floaty and you lose your steering.
And this is where I realise how clueless I am.
Because front axle weight is precisely what makes front wheel drive cars understeer, right? As you turn the steering wheel to enter a bend, they "want" to continue straight due to the engine weight at the front.
Then, what's the deal? Too much weight at the front and you get understeer, too little weight at the front and you get understeer?
I thought weight is a bad thing. In the sense that you want your track day car to be light. among other things, this allows you to go through bends faster. If weight is a bad thing, then there can't be such thing as too little weight!
But there is. To the point that some people who can't afford to modify these cars and relocate some weight (battery, fuel tank, radiator) simply add weight to the front in order to improve its grip. I find this extremely counterintuitive, which bugs the engineer in me. But when I drive my own car I totally get why they do it.
BTW, overall I really enjoy driving this car, it's a light car and that makes it a lot of fun by itself. Either axle can break loose (or even both simultaneously), but that doesn't spoil the fun at all. Also it looks lovely to me
I'm not sure that "front axle weight" is what makes FWD cars understeer. I think it is the combination of asking the front wheels to drive and also steer. If you go into a corner on a trailing throttle in a FWD car, i think that the amount of understeer you get is due to chassis tuning for a particular intended characteristic, rather than a fundamental FWD characteristic.
Weight can be a factor but it’s more distribution than ultimate mass. If the engine is hanging out in front of the wheels then inertia works against the tyres.
Which is why front mid became the favoured layout for sporty cars, ie the whole engine is inside the wheelbase but still in front for the driver.
A bit of weight on the wheels can help with handling. I’ve seen people experiment with flat weights on a spindle to find the right balance.
Which is why front mid became the favoured layout for sporty cars, ie the whole engine is inside the wheelbase but still in front for the driver.
A bit of weight on the wheels can help with handling. I’ve seen people experiment with flat weights on a spindle to find the right balance.
Mave said:
I'm not sure that "front axle weight" is what makes FWD cars understeer. I think it is the combination of asking the front wheels to drive and also steer. If you go into a corner on a trailing throttle in a FWD car, i think that the amount of understeer you get is due to chassis tuning for a particular intended characteristic, rather than a fundamental FWD characteristic.
Absolutely. Overall a fwd car is typically more likely to oversteer on turn-in in than a rwd. Mid-corner when you go back on the throttle a fwd will then be more likely to understeer whereas a rwd may do either depending on grip/balance/driver technique. Thank you for the insightful comments, everyone. Top quality stuff and excellent food for thought.
I guess I'm somehow pondering if throwing money at this car will ultimately be worth it in terms of overall fun increase. A subjective thing of course.
But I really wanted to understand what's happening from a physics perspective, if only out of curiosity.
I like boxy cars and I think the R8 in particular has an excellent looks/value ratio. Even today you can find a fair spec one in great condition (zero rust) for 5 grand.
And it will show me the amount of improvement I can expect from relocating the fuel tank.
Yes, people do that to their R8s too. The two cars are very similar overall. A friend of mine is keeping an eye on Hillman Imps and Sunbeam Stilettos for sale these days
I guess I'm somehow pondering if throwing money at this car will ultimately be worth it in terms of overall fun increase. A subjective thing of course.
But I really wanted to understand what's happening from a physics perspective, if only out of curiosity.
mac96 said:
Really cool car by the way!
Thanks a lot I like boxy cars and I think the R8 in particular has an excellent looks/value ratio. Even today you can find a fair spec one in great condition (zero rust) for 5 grand.mac96 said:
Mate of mine years ago had a rear engined Hillman Imp which hadsimilar issues. Normal solution was a paving stone in the front boot. Might work for R8?
This is something I'm going to do because the corrective measure takes 10 secs to implement and 10 secs to revert And it will show me the amount of improvement I can expect from relocating the fuel tank.Yes, people do that to their R8s too. The two cars are very similar overall. A friend of mine is keeping an eye on Hillman Imps and Sunbeam Stilettos for sale these days
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