[jeho]

I currently have 18x8.5 and 18x9.5 Advan RS...

I am wondering would it work if i put 245/40/18 all around so it would be a box setup for less understeer for HPDE?
or will the extra 1inch stretch in the rear on the 245 make it the same as me running 235/265 setup

[uhaulball]

i have 245 on my front 18x8.5, but 275 on my rear. i think u may have a smaller diameter in the rear if u stretch it.

[S4to335]

I dont know if you would get the benefit of the wheels..if the narrow tire was on the back.

[jeho]

so better off sticking with the 235/265 setup unless i get another pair of 18x8.5 for the rear to match with the 245s?

[VMRWheels]

Yea, I'd stick with your staggered setup. But, if you're going to get another pair of wheels, I'd try to fit 9.5s up front with 265s too.

[jbass524]

Originally Posted by VMRWheels

Yea, I'd stick with your staggered setup. But, if you're going to get another pair of wheels, I'd try to fit 9.5s up front with 265s too.

+1000

[jeho]

Running 9.5 and 265 on front on e92 will neeeeeed soooooo much work......i wish..id definately need to order camber plates haha...

[jbass524]

255s fit easily. I use 10mm spacers in front and 5mm in rear. No rubbing on th inside or outside. I have camber plates and I could run 265s easily.

[jeho]

^^ ON e92??!?!?!?!?!?!? WTF...
I rub the rears with 18x9.5 +35 265/35/18 Direzza Z1 Star Spec as they are much wider/thicker than your falken fk452...
I know falken fk452 and toyo t1r 265 I would not rub but the direzza's are beefy as shit

[scollins]

Originally Posted by jeffE92TT

^^ ON e92??!?!?!?!?!?!? WTF...
I rub the rears with 18x9.5 +35 265/35/18 Direzza Z1 Star Spec as they are much wider/thicker than your falken fk452...
I know falken fk452 and toyo t1r 265 I would not rub but the direzza's are beefy as shit

Jeff, are you rubbing on the inside or the outside of the wheel well? I've got a crazy idea of trying to stuff 255 Z1's up front with 275's in the back (same wheel and specs as you, but on an E90.)

[jeho]

ok with..

18x8.5 +32 on 235 direzza z1 SS fronts with -1 camber (i dont have camber plates yet) it doesnt rub

but my rears with 18x9.5 +35 on 265 direzza z1 SS with -2 camber, it rubs on hard corners, i've hammered in the fenders already so its good for now...

the fronts would definately rub with 245 or 255 cuz its very very close alrdy

and the Direzza z1 SS are THICK as i mentioned...I place the 235 direzza Z1 SS compared to my Pirelli PZero Nero 255, they are not far apart..

[Orb]

I just love to know how non staggered setup is better. Can anyone please tell me who made this up or is this on some site? It is time to debunk this garbage.

The fact is a wider front tire will cause more under steer and will in the first few pages on tires in any vehicle dynamic book. Wider tire in the front will deflect less and since it is in series with your springs is be behaves accordingly…..the wheel pair that is stiffer losses traction first. It is a big enough difference to increase your total load transfer to the front by 1-3% on street car and much 3-6% on a track car. The number is subject since it depends on your tire profile but it all means under steer!!!!!.

There a lot more to this topic but this is the big one.

Orb

[Josephkao]

wider front tires reduce under-steering, not increase.
I have 245 front and 255 rear.

[leftcoastman]

I don't have an equation to show that wider front tires decrease understeer.

The best I have, from an equation point of view is that wider tires, if the wheels are wide enough, generally have more traction. So it would make sense that if the front tires are wider and all else equal, the car would have more traction in front....reducing understeer.

From an experiential point of view, I've run the stock runflat 255/225 setup on track. I then borrowed someone's 255 runflat "rears" and put them on my fronts, so I was running a 255 runflat square setup. Understeer decreased dramatically.

On the opposite end, I now run rcomps on track. 255 square setup. I corded a front tire and my buddy with his 245 square setup had two spare tires that had roughly the same amount of use on them. He gave them to me, I mounted them on my front wheels (gotta love Walmart on Sundays) and went back out on track. Low and behold, more understeer.

Since I was melting his 245 front tires from all the understeer going on, they were about to cord. so I put them on the rear and played around. Low and behold, with 255s in front and 245s in back, I had less understeer.

[Orb]

Originally Posted by Josephkao

wider front tires reduce under-steering, not increase.
I have 245 front and 255 rear.

You heard this from....is there any reason why you think this is so. Do you have a single referance or anything to support this.

Originally Posted by leftcoastman

I don't have an equation to show that wider front tires decrease understeer.

The best I have, from an equation point of view is that wider tires, if the wheels are wide enough, generally have more traction. So it would make sense that if the front tires are wider and all else equal, the car would have more traction in front....reducing understeer.

From an experiential point of view, I've run the stock runflat 255/225 setup on track. I then borrowed someone's 255 runflat "rears" and put them on my fronts, so I was running a 255 runflat square setup. Understeer decreased dramatically.

On the opposite end, I now run rcomps on track. 255 square setup. I corded a front tire and my buddy with his 245 square setup had two spare tires that had roughly the same amount of use on them. He gave them to me, I mounted them on my front wheels (gotta love Walmart on Sundays) and went back out on track. Low and behold, more understeer.

Since I was melting his 245 front tires from all the understeer going on, they were about to cord. so I put them on the rear and played around. Low and behold, with 255s in front and 245s in back, I had less understeer.

I would say your experiment can now continue as you correct you load transfer imbalance with a larger tire. The question is why was it corrected which you need to know to close the loop on your theory. One is no good without the other.

Now that you have 255 in front that can actually work with a normal friction range based on your load transfer for give corner a certain speed. The difference now is that the friction curve will not change drastically when compared to tire stiffness. Put on some 275 or 285 tire is the rear and the car will start to over steer more. The question is what is the dominate change. The tire normal force, load transfer (other than the rear tires influence), and speed around the corer is the same so the tires are not producing more traction. The only real change is the tire spring rate and the bigger tire is stiffer so you loss traction not gain. Traction can only be increased with changes in friction and normal forces…there is nothing else.

Keep your test controlled for repeatability and you see a big 4% change in load transfer to and surprising bonus of faster exit speed that doesn’t want to over steer because of the changes in normal forces exploit the bigger tire traction capabilities.

Nonetheless, I will continue to wait for a least one credible source that can substantiate that a non staggered setup reduces under steer.

Orb

[nicknaz]

Originally Posted by Orb

The only real change is the tire spring rate and the bigger tire is stiffer so you loss traction not gain. Traction can only be increased with changes in friction and normal forces…there is nothing else.

wider tire = more contact patch = more force of friction? Could you please explain why that is not true? I'm just not seeing it conceptually, and it goes against what I've experienced in real life (ie going from 205 wide to 225 wide tires in my old WRX resulted in a dramatic increase in traction)

Perhaps you can go to a track with a bunch of tires and a transponder and post up some times with different tire setups to put this topic to rest?

At the risk of adding fuel to the fire, I'm going to add my take to this 'staggered/non stagger' debate with my own qualitative observation... I find that wider front tire allows for higher turn in speed and wider rear tire allows for more throttle earlier in the corner

[Orb]

Originally Posted by Nick Nazareno

wider tire = more contact patch = more force of friction? Could you please explain why that is not true? I'm just not seeing it conceptually, and it goes against what I've experienced in real life (ie going from 205 wide to 225 wide tires in my old WRX resulted in a dramatic increase in traction)

Perhaps you can go to a track with a bunch of tires and a transponder and post up some times with different tire setups to put this topic to rest?

At the risk of adding fuel to the fire, I'm going to add my take to this 'staggered/non stagger' debate with my own qualitative observation... I find that wider front tire allows for higher turn in speed and wider rear tire allows for more throttle earlier in the corner

I not sure what you are referring to with “force of friction”. The basic equation is F=uN

F = Grip in either lateral or longitude
u = friction
N = Normal force

One of these variables will need to change in order to increase or decrease grip. The additional piece of information is that you need to consider the pneumatic tire which is a spring and does affect load transfer considerably of track cars. Also, I didn’t say wider tires don’t give you more lateral grip. I’m saying that it effects load transfer and will change the balance of the car due to the stiffness. The tire is another springs in series with your main spring for each wheel and really start to become a factor as the tire spring rates decrease with load. Do a quick search on tire spring rates vs deflection.

Just to throw thing on a bit of a tangent. I would suggest that no track car in this forum has high enough suspension frequency in there rear to even get close to average grip with R compound tires….they need 2.2 Hz for peak performance. I can say the motion ratio for the rear is around 0.59^2….I’m sure someone will figure this out sooner or later.

[LOLrance]

How are you guys running 255 or even 265's on your front?!?!

Im using my stock 189's on the track right now. Setup is as follow 18x8 +38 on NT05 245/40/18 and im rubbing on sharp turns. The car is not even dropped, just stock ZSP suspension.

i have a set of non staggered Volk LE28's coming in and im really not sure what tires to put. If im rubbing on my stock 189 front, then whats going to happen when i try to put a 245/40/18 on my LE that is 18x8.5 +30

[nicknaz]

Originally Posted by Orb

I not sure what you are referring to with “force of friction”. The basic equation is F=uN

Yea, I am not so great with precise physics terms.

When I went from 205 wide Kumho MX to 225 wide Kumho MX on my WRX I went around the track faster. Based on your equation this change of tire width shouldn't have mattered ( mu and normal force didn't change) but in reality, it mattered.

Seems like that equation is not a good model for this situation (maybe because it is an approximation for 'ideal conditions' which don't occur very often in the real world?)

Based on what you are arguing you are saying if I went with 205 front and 225 rear tire I would have less understeer and go around the track even faster than 225 front and rear? Because of the slight difference in spring rates contributed by the tire? (as in, with that 205/225 i would essentially be stiffening up the rear spring system relative to the front?) am I understanding it right?

interesting thought. i think the real world is too complex for the spring and friction approximations this thread is discussing.

Does anyone have a set of style 161 wheels with 255 OEM run flats and a transponder they want to lend me? I don't mind trying to pick up some data regarding 225/255 versus 255/255 as the 'faster' way around the track.

[Orb]

Originally Posted by Nick Nazareno

Yea, I am not so great with precise physics terms.

When I went from 205 wide Kumho MX to 225 wide Kumho MX on my WRX I went around the track faster. Based on your equation this change of tire width shouldn't have mattered ( mu and normal force didn't change) but in reality, it mattered.

Seems like that equation is not a good model for this situation (maybe because it is an approximation for 'ideal conditions' which don't occur very often in the real world?)

Based on what you are arguing you are saying if I went with 205 front and 225 rear tire I would have less understeer and go around the track even faster than 225 front and rear? Because of the slight difference in spring rates contributed by the tire? (as in, with that 205/225 i would essentially be stiffening up the rear spring system relative to the front?) am I understanding it right?

interesting thought. i think the real world is too complex for the spring and friction approximations this thread is discussing.

Does anyone have a set of style 161 wheels with 255 OEM run flats and a transponder they want to lend me? I don't mind trying to pick up some data regarding 225/255 versus 255/255 as the 'faster' way around the track.

The equation I mention is obviously simple but it has the basic for all of them. Keep in mind most of these equation are formulated from empirical data and testing so they are very precise. The complexity with tires dynamics really hurts my head more than any other subject.

The case here is we can make a good distinction between tire stiffness and grip with reasonable degree accuracy and the general trend is what I mentioned. As we see high loads on the tire the wheel spring rates actually drop about 10% which also affects the suspension frequency. If you start considering these numbers in the load transfer calculation you can see why they make a difference. The expectations are when there is already an existing load transfer problem or something else that need to be explained first. I can provide some calculation to support this if you wish.

The 205/225 tire combo my not be the best. You need to find the best front tire size for you car and setup ( just like leftcostman has) which might be a 225 then if you have the power the bigger rear will get you more grip like 245. The bigger rear tends to keep a neutral handling car more stable on comer exit. There is really a reason why high end car have staggered setup. You understood it right you will now be biased to over steer due to tire stiffness given you don’t have any abnormal handling problems.

Finding the right tire size for this car is not difficult and if you are perfectly balanced you can get away with 235 up front. The 255 will support the 1900-2000 lb load transfer on the front at under 1 g for an unbalanced car.

Orb

[nicknaz]

Originally Posted by Orb

The 205/225 tire combo my not be the best. You need to find the best front tire size for you car and setup ( just like leftcostman has) which might be a 225 then if you have the power the bigger rear will get you more grip like 245. The bigger rear tends to keep a neutral handling car more stable on comer exit. There is really a reason why high end car have staggered setup. You understood it right you will now be biased to over steer due to tire stiffness given you don’t have any abnormal handling problems.

ya can you please post some sources i can read about the tech on how load affects the tire's contribution to the wheel rate? interesting stuff

I always thought the fat rear tires were just there to make the expensive car look better

[Ramos]

The fat rears are there to intentionaly create understeer as it is deemed safer for inexperienced drivers to deal with than oversteer..

What we have here is one person claming the opposite of everybody else on every forum and everyone I have ever spoken with about this..
I think the burden of the proof with empirical data is on you since you are
being devil's advocate trying to refute an established theory that wider fronts will in fact reduce understeer.