Lower unsprung weight, what is the advantage?

[Tyre Smoke]

Okay, so you hear the expression '...to lower the unsprung weight' particularly with alloy wheels over steel ones. But apart from a slight weight advantage, what does lowering the unsprung weight really do? I know lighter equals faster, but surely this applies to sprung weight as well as unsprung?

Ta.

[kambites]

If the wheels (and other unsprung bits) have less inertia, the suspension reacts faster to bumps meaning the wheels stay in contact with the road better.

[Psimpson7]

ride quality is one thing it can have a big affect on.

Edited by Psimpson7 on Friday 3rd October 11:31

[Gad-Westy]

Slightly off topic, but with regard to wheels, you're also talking about rotational mass and the less of that the better when it comes to acceleration.

[Tyre Smoke]

Psimpson7 said:

ride quality is one thing it can have a big affect on.

Edited by Psimpson7 on Friday 3rd October 11:31

I was thinking more of track cars, particularly the beautiful big Healy that I had the pleasure of fitting a clutch and gearbox into at Spa last weekend.

[kambites]

Gad-Westy said:

Slightly off topic, but with regard to wheels, you're also talking about rotational mass and the less of that the better when it comes to acceleration.

Is it measurable? I would have thought the rotational kinetic energy of the wheels paled into insignificance in comparison to the linear kinetic energy of the car.

[snotrag]

Weight any where is a bad thing. But lowering unsrpung weight has extra effects.

Unsprung weight is effectivwly everything on the wheel side of your spring.

For instance, on a caterham,



Such as the bottom half of the shock absorber, (Thats why its best to fit shocks/coilovers with the damper end at the top, suspended end) the wheel, the upright, the brake assembly, the mudguard, etc etc

All this extra weight increases the forces going through your shock absorber.

By reducing this, you can reduce the damping forces needed, making the shocks more sensitive, and reduce the need for such tight seals, which reduced friction/stiction.

ETA - no expert, just my views. Bound to be other reasons aswell...

Edited by snotrag on Friday 3rd October 11:37

[Callan.T89]

As said, quicker reaction from the suspension thus keeping the tyres on the road better so more grip, also sharper steering reaction at the front and better feel. Reducing the mass of the wheels also reduces there "giro" effect, which is basically always trying to keep the car going in a straight line so improves turn in.

[Gad-Westy]

kambites said:

Gad-Westy said:

Slightly off topic, but with regard to wheels, you're also talking about rotational mass and the less of that the better when it comes to acceleration.

Is it measurable? I would have thought the rotational kinetic energy of the wheels paled into insignificance in comparison to the linear kinetic energy of the car.

Well, it is measurable to extent and all these things are relative but its the same reason why one might fit a lightweight flywheel or a carbon propshaft. The cost are very high of course but the benefits are more significant than saving the weight elsewhere in terms of pure accelaeration.

[kambites]

Gad-Westy said:

kambites said:

Gad-Westy said:

Slightly off topic, but with regard to wheels, you're also talking about rotational mass and the less of that the better when it comes to acceleration.

Is it measurable? I would have thought the rotational kinetic energy of the wheels paled into insignificance in comparison to the linear kinetic energy of the car.

Well, it is measurable to extent and all these things are relative but its the same reason why one might fit a lightweight flywheel or a carbon propshaft. The cost are very high of course but the benefits are more significant than saving the weight elsewhere in terms of pure accelaeration.

I thought a lightened flywheel was used more to increase throttle response than to increase outright acceleration?

Edited by kambites on Friday 3rd October 11:42

[snotrag]

Same thing - its rotational weight.

So sat in neutral, you blip the throttle, the engine has less work to do to spin up to high revs.

But in first gear when you launch it, the engine also has (marginally) less weight to spin up too, so can accelerate the car faster.

[Gad-Westy]

Callan.T89 said:

As said, quicker reaction from the suspension thus keeping the tyres on the road better so more grip, also sharper steering reaction at the front and better feel. Reducing the mass of the wheels also reduces there "giro" effect, which is basically always trying to keep the car going in a straight line so improves turn in.

Its odd isn't it, the more you get into it the more adavantages you can see for fitting smaller wheels/tyres and yet they're always getting bigger. Partly this is down to mammouth braking systems that are fitted to cars but they're only neccessary becasue the cars weigh so much. In Caterfield circles, those serious about racing or trackdays nearly always go for 13" wheels.

When I swapped wheels and tyres on my Westfield from 15" with Toyo T1's down to 13" minilites with 048 tyres, I also changed brakes for something lighter and saved nearly 12 kg per corner. The difference was massive.

[Gad-Westy]

kambites said:

Gad-Westy said:

kambites said:

Gad-Westy said:

Slightly off topic, but with regard to wheels, you're also talking about rotational mass and the less of that the better when it comes to acceleration.

Is it measurable? I would have thought the rotational kinetic energy of the wheels paled into insignificance in comparison to the linear kinetic energy of the car.

Well, it is measurable to extent and all these things are relative but its the same reason why one might fit a lightweight flywheel or a carbon propshaft. The cost are very high of course but the benefits are more significant than saving the weight elsewhere in terms of pure accelaeration.

I thought a lightened flywheel was used more to increase throttle response than to increase outright acceleration?

Edited by kambites on Friday 3rd October 11:42

Was going to reply but Snotrag has said more or less what I would have said. The differences will be small of course but all these things add up.

[ewenm]

Gad-Westy said:

Callan.T89 said:

As said, quicker reaction from the suspension thus keeping the tyres on the road better so more grip, also sharper steering reaction at the front and better feel. Reducing the mass of the wheels also reduces there "giro" effect, which is basically always trying to keep the car going in a straight line so improves turn in.

Its odd isn't it, the more you get into it the more adavantages you can see for fitting smaller wheels/tyres and yet they're always getting bigger. Partly this is down to mammouth braking systems that are fitted to cars but they're only neccessary becasue the cars weigh so much. In Caterfield circles, those serious about racing or trackdays nearly always go for 13" wheels.

When I swapped wheels and tyres on my Westfield from 15" with Toyo T1's down to 13" minilites with 048 tyres, I also changed brakes for something lighter and saved nearly 12 kg per corner. The difference was massive.

I've got 15s on my Caterham for road (extra inch or so of ground clearance ) and a set of 13s for track.

[snotrag]

Gad-Westy said:

Stuff about wheel sizes.

The difference in acceleration between my GTi-6 with 195/50/R15s and one with 17" Halfords wheels is very noticeable. But the, I do have to sit at 4000 odd revs on the motorway, Bwoooooaaaaaaaaaaaaarrrrrpp....(Ear bleeding) etc etc.

[Gad-Westy]

snotrag said:

Gad-Westy said:

Stuff about wheel sizes.

The difference in acceleration between my GTi-6 with 195/50/R15s and one with 17" Halfords wheels is very noticeable. But the, I do have to sit at 4000 odd revs on the motorway, Bwoooooaaaaaaaaaaaaarrrrrpp....(Ear bleeding) etc etc.

Reading between the line there, I assume the 17's have a larger rolling radius with tyres fitted and so you have effectively two sets of gear ratios? I can see that could give very noticable changes in acceleration.

Edited by Gad-Westy on Friday 3rd October 12:04

[spikeyhead]

It's worth remembering that most cheap alloy wheels wheigh as much as steel ones.

[snotrag]

Yup, 15" 55's is stock. I've got 15" 50 profiles.

Tried a car with 17" wheels, not sure what tyres but they are bigger overall. So yes, effectively changing the gearing.

[HiRich]

The principle advantage of lowering unsprung weight (at least the one most people are talking about) is about how the wheel assembly deals with imperfections in the road surface. On a mirror-smooth surface you wouldn't notice, but even on the Silverstone circuit there are little dips and bumps that have to be managed. Then consider grazing those sawtooth kerbs.

The critical area is the contact patch - rubber to tarmac. You want to keep this as constant as possible. If it suddenly spikes higher, you are not going to be able to react fast enough to use the extra grip generated (and you might be damaging the tyre). If it drops suddenly, you're going to lose grip and you're not going to be able to react to that (even if it's brief and minor, it's going to unsettle the car and you're going to waste time doing something about it). So the less it varies, the closer you can run to the optimum average grip available. With more grip you are going to be faster, and the car is going to remain more settled.

So how does this work?

• First of all, remember that as you hit bumps, dips and kerbs, there's a displacement input - the road surface rises (or drops), your tyre has to move with it, and everything else has to sort itself out to deal with the new location of the road surface.
• Secondly, basic physics tells us that how quickly the system deals with this is determined by the stiffness of the spring (higher means faster), and the mass at the other end (less means faster).
• And the third point is to remember that we have several springs and masses. First, the tread area of the tyre (we could even separate the tread blocks from the rest of the carcass if we wanted to get really technical). Secondly the tyre sidewalls (and remember that each of these has mass as well as elasticity), then the major unsprung mass of the wheel and hub. Then the actual spring/damper. And finally the mass of the bodyshell.

Now it helps here is you have done the O-level science experiment with springs and masses in series. If you have, you'd know that at the intermediate points between springs, all sorts of weird effects happen. Those points can move in ways that seem to bear no relation to the input (road surface) and far end (bodyshell).

The main point is, though, if we consider the first stage from contact patch to unsprung mass (major part: wheel & hub), reducing the unsprung mass means that mass can move more quickly, and the load/pressure variation can be much smaller. The pressure variation at the contact patch is smaller, so the change in grip is smaller, so it's more predictable, so you can run closer to the limit (ie corner faster). And that's the principle benefit of reducing unsprung weight.

Other benefits include:

• Damper settings: the damper works not only to control the movement of the bodyshell, but also to control the movement of the wheel/tyre. Reduce the unsprung mass, and you can probably reduce the amount of damping you need. This also (effectively) softens the whole springing system, amplifying the main benefit
• Noise/vibration/harshness: Think of a Range Rover crossing a sleeping policeman at decent speed. As the tyre hits the ramp, that huge mass of wheel gets hammered up into the air. The spring/damper tries to manage that load, but there's a huge load spike into the chassis that will be felt by the driver as a bang & thump. But if we replaced the wheel and hub with lightweight plastic ones, the input load to the spring/damper will be less (as this is now a force input), and we also have the possibility of siftening the damper setting. As a result, the load spike into the chassis is less, the bump/thump is less, and might even be reduced to that situation of the chassis gliding over the obstacle with the driver barely noticing.
• Chassis loads: As above, smaller load spikes are created at the chassis mounting points. That means less risk of failure, and particularly of fatigue failure. Good news for durability on a road car, but on a racecar it means you could save a bit of weight with a smaller mount.
• Rotational inertia: I'm always a bit skeptical of this, but I've heard several reports of it being noticeable. Rotational inertia is a product of the mass, and how far it is from the the axis of rotation. So using the rotational equivalent of F=ma, less rotational mass means less force for the same rotational acceleration. Of the total force (what the engine or brakes are providing), less is being spent on building rotational inertia, so more can be spent on accelerating the vehicle. Which means fitting carbon fibre wheel rims would provide an increase in vehicle acceleration.

But most important of all is it's effect on how the suspension copes with variations in the road surface, and how little variation that induces in grip.

[Tyre Smoke]

Brilliant reply, mate. Clearly explained!