Reducing Understeer By Increasing Tyre Pressures, But To What Psi?

Hi all - I've always found my turbo has too much understeer at low speeds for my liking. I have no spare cash to modify the car in anyway, but read that increasing the front tyre pressures may help reduce the understeer? If that's the case, does anybody have any recommendations as to what PSI? the car runs on standard size wheel/tyres and the PSi normally is 26. Cheers.

Increasing the psi will make your tyre rounder, which is lesser grip, thus more understeer, not less.
The cheapest way to get more oversteer is to change the geo of your car.
By simply removing some camberplates you can get a much better feeling with the car.

Our cars are set around 0 degree camber at the front.

As you can see an exige goes up to 0.5 negative, and so does the 340 road.
That gives you enough of a difference to notice, and no it will not chew up your tyres.

In the front the change is 0.25 dergee per 1mm plate, and rear is 0.30 degree per 1mm plate.
 

The next step would be to get wheels that can fit wider tyres like a 205 to get rid of even more understeer.
Keep in mind that Vauxhall was overprotective with these rwd cars. If you start fiddeling with this, take your time to know the car again.
 

Edited by smiley, 02 July 2019 - 11:48 AM.

On the geo side, if you want less understeer get a fast road geo for the road.  Most specialist places will have one of these in their arsenal for our cars.

Personally I run a slightly more aggressive 340R geo for track work.  I wouldn't recommend it for the road though as makes the car track in grooves really badly.  The attitude of the VX is completely different on this geo from the standard set up, and is quite oversteer heavy, but if you have decent feel and experience of driving a track car on the throttle it's great

Tyre pressure adjustment is free.  Try it.  You won't die

Tyre pressure adjustment is free.  Try it.  You won't die

What he says!  Set all your pressures, F and R, to what you normally use and then go up by 2psi at the front, note the effect on a known roundabout or wherever is safe, repeat and note again, you will never remember what you did if you don't keep notes.  Then go down by 2psi, note and repeat.  Set the front to whatever you liked best and then do the same at the rear.  As has already been pointed out reducing pressure increases wear on the shoulders of the tread and increasing pressures increases wear in the centre of the tread as well as reducing grip in the wet.  If you end up running lower pressures than previously then keep a check on tyre temperatures until you are happy that they are not running noticeably hotter than previously, if they are then increase all round by 2psi.

Thanks all - tyre pressue fun here we come! Up on the fronts first, as (I know smiley disagrees!) that seems to be the accepted quickest route for reducing understeer if not eliminating it, which does require altering the geo.

Let us know the conclusions of your experiment.

Smiley is correct. The rule is, make it softer to increase grip. The problem with using tyre pressures for balance is that it is a dynamic variable, you should aim to get hot pressure balance correct which will probably make it understeer more when cold. If you are not driving on circuit it's a bit moot though as you will (or should) never see real hot pressures on the road.

yeah, defo not going to be on the track, road usage only - particularly at slow speeds I want a more pointy front end. I used to be privileged enough to go through the Alps every year and it always seemed to understeer and it does not have the power (standard engine) and I the skill/bravery to get the rear moving at slow speeds to compensate.

DIY geo check first as many cars have ignored or badly set this up over the years, won't cost you anything, just some string and your time.

Edited by Nev, 04 July 2019 - 06:00 AM.

I assume you have standard suspension. If not that could be adjusted perhaps.

What tyres do you have (and sizes), as different ones use different pressures and they need to be correct. Typical for a VX on things like AD08R is 27psi rear 25fr, Toyo 888's 26 rear, 24 front measured when cold.

As mentioned pretty well every car made by Lotus is designed with under steer and the best mod everyone does it get the geo changed.

Typically to get more weight on the front, I personally reduce pressure as this widens the patch on the road so will increase grip and reduce understeer. This is a simple method often used on trackdays by many if its raining and you want to get just a bit less understeer from standard pressures.

Good luck.

Martin S

DIY geo check first as many cars have ignored or badly set this up over the years, won't cost you anything, just some string and your time.

I think the big thing with home geo is to take care to be as accurate as possible. Measure, measure and measure again 🙂

if you have ever driven a new factory Lotus exige , they have more understeer  

I assume you have standard suspension. If not that could be adjusted perhaps.

What tyres do you have (and sizes), as different ones use different pressures and they need to be correct. Typical for a VX on things like AD08R is 27psi rear 25fr, Toyo 888's 26 rear, 24 front measured when cold.

 

As mentioned pretty well every car made by Lotus is designed with under steer and the best mod everyone does it get the geo changed.

Typically to get more weight on the front, I personally reduce pressure as this widens the patch on the road so will increase grip and reduce understeer. This is a simple method often used on trackdays by many if its raining and you want to get just a bit less understeer from standard pressures.

 

Good luck.

 

Martin S

Whatever you do dont put a whacking great front ARB on it!  

short answer.. on a MR layout you would have slightly lower pressures on the front and chances are that lowering your front tire pressures will resolve "under steer" and it would grip well, but good grip doesn't necessarily mean good handling..  

under steer isn't just a simple thing to solve by just 1 adjustment, car handling is highly dynamic.. for instance, your under steering at low speeds, how "low" are the speeds in reality? also consider your inputs, are you jerking the steering wheel, does it do it under throttle or under braking, are you just going too fast or are turning in a lot more than needed.. there are many variables you need to consider.. lets take for instance, sudden hard steering, or high steering angle, the car weight transfer might be so much that you are rolling over your tires lifting the inside of the tire.. if that was the case than you would need more camber, or stronger anti roll resistance, and there is certainly nothing wrong with putting a larger front ARB on if you have identified that its needed or have the means to adjust for it. 

How old are your tires? are they cracking or hard? what make are the tyres? 

It could just be the car needs a geo and its 10x better than as is, not that i am a leading authority but i would recommend you get the car to somewhere which can do you a full geo, dont bother with corner weighting or that.. just a 4 wheel alignment will be fine to make sure everything is pointing where it should be then put your pressures to 24/26 and see how you go.. 

Edited by chris_uk, 05 July 2019 - 11:35 AM.

Thanks all - car is absoloutley boggo standard - it's alignment was done in 2015 at the same time it had a thorough check of bushes etc by a Neil Garner motorsport and all is good. Wheels are bog standard 17" and the tyres believe it or not are the Bridgestones - RE040 I think from memory - all standard and all less than 2 years old. I've had the car since 2005 and have always wanted less understeer in the (very) slow corners (think Stevlo pass etc). Some very good things to pursue from above, thanks again, really appreciated 

Thanks all - car is absoloutley boggo standard - it's alignment was done in 2015 at the same time it had a thorough check of bushes etc by a Neil Garner motorsport and all is good....

Do you have the allignment data?

Alternatively you can do a quick rough camber check on a flat level surface with a (digital) angle meter to your wheels. (or use a level app for your phone...)