tyre pressures and understeer

[dogvet]

Why is it generally accepted that increasing the front tyre pressures reduces understeer?

[Paul Ranson]

Because it does, but perhaps it's not generally accepted.....

 

FWIW it's generally accepted that increasing front roll stiffness increases understeer (and vice-versa). However on a 7 the converse is frequently true. What works, works. And sometimes the tyres on a car like a 7 are not pushed hard enough to get them into the 'generally accepted' zone.

 

Paul

[jonhill]

Hmmm, I thought it was generally accepted that *lowering* front tyre pressure reduced understeer ?? I've always thought there has to be a point where the pressure gives the optimum grip, and either increasing or decreasing pressure will give less. It then depends where you are relative to this pressure which determines whether you need to increase or decrease for added grip.

 

Having recently played with the front roll resistance on my car, the results conformed to my expectation that increasing the roll stiffness generated more understeer, and softening the front gave (a lot) less. Changes to the rear arb also gave the expected (conventional) results.

 

Paul, can you explain why the opposite might be true for a Seven, as someone else also suggested this, but IME it's not the case?

 

Jon

[captain chaos]

Hi Jonhill, hope you are keeping well.

 

A question for you and Paul.

Does ARB adjustment at front have an independant or more dramatic effect on understeer than spring rate. ie, can you reduce understeer by reducing the front ARB even though the spring rate has increased from 250 to 300lb?

 

I'm already on the softest rear arb setting.

Comments please.

[EFA]

Gary, See the ARB thread for some odd setttings I've put on my car.

 

I think you will find that any kind of stiffening to the front end causes an increase in understeer, but to varying levels.

 

ARB stiffness has a greater effect than springs - my Juno adjustable ARB only has to have a slight change made for a significant change to turn in sharpness/roll understeer.

 

Springs are less noticable but 50lbs is quite a bit.

 

Many people who had Sevens of early Bilstein Di Dion vintage will notice Caterham used to increase the ARB thickness to overcome the roll effect the soft springs gave the car. People corrected the softness by uprating the springs (meant the car didn't then hit the bump stops on bumpy roads) but the result was massive understeer. This is where the VX gets its reputation for understering from.

 

In general 300lbs is hard for a std track car, 340lbs hard for a Wide track (increase in poundage as the damper angles change). In slower corners the cart will have an understeer tendency with a stiff ARB (say 18mm) Reduce the ARB though (the Juno bar is ideal for this), and assuming you have a VX engie this is a pretty good setup.

 

On a lighter engine such as a K 260/300 would give similar setup characteristics.

 

Opoosite ned of the range is achieved using spring rates about 70lbs less - a soft car, as per the 1996 std spec with the thin ARB.

 

 

 

 

Tyre pressures have a different effect on understeer.

 

Tyre pressures need to be set so you get an even tyre pressure across the tyre width. Over inflating increases understeer because it reduces the size of the contact patch.

 

Under inflating does the same but to a lesser extent (until the tyre is distorting) as the contact patch is larger on an under inflated tyre in relative terms.

 

Best way to measure tyre pressures across the width is to use a pyrometer.

 

 

Fat Arn

The NOW PROVEN R500 Eaterid=red>

See the Lotus Seven Club 4 Counties Area Website hereid=green>

 

[jonhill]

Hi Gary,

 

Yes I'm fine thanks, and you?

 

I was experiencing excessive understeer, so using the rule of thumb about needing to soften the front or stiffen the rear to correct this, I was pleasantly surprised to find the rule seemed to do exactly what it said on the tin. Without knowing too much of the theory behind this, it is a useful tool, which is why I was interested in Paul's comment about this rule of thumb not always applying.

 

I'd agree with Arnie in that the bigger front springs (from 150 to 250) had much less of an effect than changing the front roll bar from 18mm to 13mm (which is a big change, granted), as the overall effect was softer (in roll). Are you having US problems? You say you already have the rear arb on its softest setting, you could tweak that up a notch or two to start with.

 

Another thing to consider is the effect of the Watts Link on my car. This removes the car's sensitivity to rear ride height changes. If you have radius arms only, you can raise the rear ride height, and get the same effect on understeer as stiffening the rear. Peter C seems to use this approach, as according to the Low Flying handling test, he has 250 + 18mm on the front and std rear progressives. This is a recipe for mega understeer on my car, but his was voted the best handling car.

 

Rear tyre size must have an impact too. Has anyone ever put different compounds of the same tyre on the front and rear to achieve balance? And of course tyre pressure, which is where we came in. You could over inflate the rears the kill the US, but I tend to think you want the maximum contact patch possible, and use other adjustment available to get the balance right.

 

I'm told that for a track car, it is preferable to have bigger springs and thinner arbs to achieve the desirable roll resistance, as you then benefit from resistance to dive and squat too. On a road car, this would make the ride too harsh, so softer springs are used, with thicker arbs to give the roll resistance as they (arbs) have no effect until the car tries to roll. A downside to this approach is that an arb is an *undamped* spring, which isn't a good thing.

 

Arnie, how easy is it to return accurately to a known setting with the Juno bar? I'd like to be able to soften the front off for the road, but don't want to lose a setting it may have taken half a track day to achieve.

 

Jon

[Fast Westie]

Increasing front tyre pressures reduces understeer because it reduces the tyre slip angle.

 

Increasing the front suspension stiffness would normally increase understeer. However, the Seven suffers from roll understeer, so by increasing roll stiffness you reduce roll and hence understeer.

 

The car in front is a Westfie1d wink.gif

[Paul Ranson]

When a car corners an amount of load is removed from the inside wheels and transferred to the outer, this total amount is defined by the track, the height of the centre of gravity, and the lateral acceleration. The amount of sideways force that has to be generated at the front and rear is defined by the front/rear location of the centre of gravity and the lateral acceleration.

 

The basic theory of changing the roll stiffness at one end to affect the handling is that when you push down harder on a tyre its co-efficient of friction changes, and consequently the slip angle it runs at to generate a given sideways force changes.

 

So if the co-efficient of friction reduces with extra load you will get increased understeer with increased front roll stiffness. This is the 'generally accepted' case.

 

On the front of a Caterham, particularly one with a light engine, you have relatively low vertical loads, maybe as low as 100kg. If you were running tyres engineered to cope with saloons or hatchbacks, (Yokohamas for example), then the tyres are designed to work with 300-400kg loads. It can be the case that 'pushing harder' can actually increase the co-efficient of friction, hence reducing apparent understeer.

 

All theory and my imagination, no warranty, assumes steady state cornering, and tyre companies are notoriously cagey or ignorant about the actual characteristics of their tyres.

 

And of course if your car rolls sufficiently to hit the bump stops, or rolling reduces the size of the outside tyre's contact patch, then reducing the roll will reduce understeer regardless of how the reduction is achieved.....

 

Paul

[Graham Perry]

JonHill, yes I am regularly playing round with tyres. I am currently using a set-up with different compounds front and rear. Although only on 1A road rubber, I find that with Yoko 510's or Avon Zv's all round I get tight corner understeer even though all settings are about optimum. But if I put 510's on the back and ZV's on the front the car then becomes neutral.

 

Edited by - Graham Perry on 5 Dec 2001 16:07:35

[captain chaos]

Dogvet,

I am sorry for hijacking your thread!

 

The reason I asked was that I wanted to try and reduce the understeer on my car for next year...but just recently I have uprated the dampers/springs(250 up to 300lb) on my car. The new set up will reduce my overall ride height and considering the new set up is stiffer I wanted to reduce ARB effect in the hope of controlling understeer. I am pleased to hear that ARB has more efeect than springs because in making the set up stiffer I was hoping not to have made the understeer worse. Reducing the overall ride height should also help aswell..shouldn't it?

 

Thanks for the interesting replies so far.

[Swiss_Tony]

Reducing the ride height may make the problem worse.

It depends where the roll centres are.

If the front roll centre is too low this can induce understeer!

As for tyre pressures there is only one correct pressure!

The one that gives you even temperature across the whole tread. This is made a bit confusing by running a lot of camber or toe in/out etc.

Also its worth remembering that you can set the suspension too stiff. This can make it slower round corners and "let go" suddenly.

Just my 2p worth.

 

James

 

[andygriffiths]

I thought as Captain said lowering the front end reduced understeer and increased oversteer.The reason why 10-15 mm less at the front on a VX is a happy medium.

Andy G

 

GRIFF id=red>

[Swiss_Tony]

It depends where you are starting from!