Understeer reduction via tire size change

fasteract · #1 06-15-2003, 05:52 PM

Has anyone tried mounting 245/45's on the front only with 225/50's stock on the rear as an attempt to reduce understeer on their SVX ? The 2 sizes are dimensionally the same except for tread width and Paisan et. al. confirm 245's fit w/o rub.

BTW, I put Prothane swaybar bushings (3/4"-17mm) on the rear on mine and have noticed definitely less push in the corners and faster turn in...at limit in wet conditions a bit more ready to throw the tail around. Nice cheap mod...you need to use their metal clamps with new bolt/nuts since the OEM bushing clamp is too loose.

Craig

Earthworm · #2 10-29-2003, 08:04 PM

Old thread revival begins!

stock 225/50's are 24.86" tall
245/45's are 24.68" tall

difference = 0.18" or 0.7%

That would definately reduce understeer however I would not want to drive like that all the time. It should be safe for a few auto-x runs but I would switch back to 4 equal tires immediately after.

for reference 205/55's are 24.88" tall and would be safer to run with stock size.

There's probably safer (to the drivetrain) ways to reduce understeer than different size tires on AWD vehicles.

#3 10-30-2003, 06:15 AM

Why not adjust/play with your tire pressures?

You'd be amazed what tire pressure changes alone can do for the way your car will handle. You can tune in over/understeer with tire pressures & a different alignment without upgrading a single thing... assuming you have tires with some kind of grip.

Mr. Pockets · #4 10-30-2003, 10:05 AM

Quote:

Originally posted by omahasubaru
Why not adjust/play with your tire pressures?

You'd be amazed what tire pressure changes alone can do for the way your car will handle. You can tune in over/understeer with tire pressures & a different alignment without upgrading a single thing... assuming you have tires with some kind of grip.

This would have the same effect as running different size tires - it would change the difference in rolling diameter between front and rear.

I personally don't condone adjusting the tire pressure. You're just too likely to affect the rolling diameter and wear the transfer clutch assembly far more quickly.

#5 10-31-2003, 09:18 AM

Quote:

Originally posted by Mr. Pockets

This would have the same effect as running different size tires - it would change the difference in rolling diameter between front and rear.

I personally don't condone adjusting the tire pressure. You're just too likely to affect the rolling diameter and wear the transfer clutch assembly far more quickly.

You've got to be kidding me. There is no way that tire pressure is even close to changing tire sizes to be mixed and matched.

I could run 20 Psi front and 40 psi rear and the diameter of my tires wouldn't be that much different at all.

I'm talking about minor adjustments as well.

From the factory Recommended tire pressures front to rear are different by 2 Psi IIRC.

You really should learn a bit more about tires & tire pressure and how they affect handling before you say that it's 'drastically changing the tire diameter' Who are you kidding?

Phast SVX · #6 10-31-2003, 10:19 AM

Quote:

Originally posted by Mr. Pockets

This would have the same effect as running different size tires - it would change the difference in rolling diameter between front and rear.

I personally don't condone adjusting the tire pressure. You're just too likely to affect the rolling diameter and wear the transfer clutch assembly far more quickly.

glad i dont have one of those to worry about anymore

, now i just have a viscous diff.....
phil

Mr. Pockets · #7 10-31-2003, 12:40 PM

Quote:

Originally posted by omahasubaru
You really should learn a bit more about tires & tire pressure and how they affect handling before you say that it's 'drastically changing the tire diameter' Who are you kidding?

Then please feel free to educate me. The only way I can see tire pressure having 'no effect' on rolling diameter is if the sidewall was totally rigid. It's not.

Quote:

Originally posted by omahasubaru
I could run 20 Psi front and 40 psi rear and the diameter of my tires wouldn't be that much different at all.

I will be more than happy to test this over the weekend. I will record my results. My wife will, undoubtably, give me strange looks, but in that way it'll be no different than any other weekend.

Quote:

Originally posted by omahasubaru
From the factory Recommended tire pressures front to rear are different by 2 Psi IIRC.

I just went out to my car and checked. The door plaque says 33 front, 29 rear.

#8 11-03-2003, 07:08 AM

Ok... so 33 front 29 rear is even more drastic, does that worry you?

If your sidewall aren't stiff, you need some new tires... what are you on... 1950's radials?

So how did your tire pressure testing this weekend go?
What kind of tires are you on?
How often do you check your tires?
What do you usually run them at?

May I recommend this site to you for learning 'stuff'? http://www.howstuffworks.com They cover tires, tire pressures, pretty much anything you might need to know.

Back to the original person having understeer issues...
What tire pressure are you running?
What tires do you have on your car (size, brand, speed rating)?
Any other modifications that would affect handling?
How good are you struts (if stock & high mileage)?
* I bet I can give you some different tire pressure settings to try that will help.

Thanks - Jon

Mr. Pockets · #9 11-03-2003, 08:53 AM

Quote:

Originally posted by omahasubaru
Ok... so 33 front 29 rear is even more drastic, does that worry you?

If your sidewall aren't stiff, you need some new tires... what are you on... 1950's radials?

So how did your tire pressure testing this weekend go?
What kind of tires are you on?
How often do you check your tires?
What do you usually run them at?

May I recommend this site to you for learning 'stuff'? http://www.howstuffworks.com They cover tires, tire pressures, pretty much anything you might need to know.

Back to the original person having understeer issues...
What tire pressure are you running?
What tires do you have on your car (size, brand, speed rating)?
Any other modifications that would affect handling?
How good are you struts (if stock & high mileage)?
* I bet I can give you some different tire pressure settings to try that will help.

Thanks - Jon

I have Dunlop D60A2s. I have no idea how they compare to other tires in terms of stiffness.

I've been to 'howstuffworks' many, many times. I don't think that tire pressure, as it relates to rolling diameter, has been covered on the site.

As for Subaru's recommended tire pressure, I don't understand what you mean by 'does that worry' me. No, it doesn't worry me - I simply inflate my tires to those pressures. And, to answer your question, I check the pressure regularly and also rotate the tires every time I do an oil change.

I was not able to get to the tire experiment this weekend. I was disappointed, because now I'm really curious to see the results.

svxsubaru1 · #10 11-03-2003, 09:51 AM

The only reason subaru has more air preasure in the front than the rear is because the extra weight afects the rolling diameter. If you had 20PSIin the front and 40inthe rear there would be a measurable difrence probley more then there spose to be on the AWD. side walls flex a decent amount, or else there wouldbe no change to the handling by air preasure if they dident.

Mr. Pockets · #11 11-03-2003, 10:03 AM

Quote:

Originally posted by svxsubaru1
The only reason subaru has more air preasure in the front than the rear is because the extra weight afects the rolling diameter. If you had 20PSIin the front and 40inthe rear there would be a measurable difrence probley more then there spose to be on the AWD. side walls flex a decent amount, or else there wouldbe no change to the handling by air preasure if they dident.

Right - that's my argument exactly. But I'd like to test it with the proposed experiment, because I've seen no actual documentation on how tire pressure affects rolling diameter - specifically as it relates to AWD systems.

Sonar · #12 11-03-2003, 10:18 AM

Hi folks, sorry if I intrude....
But...
Tire pressure HAS effect on the rolling diameter...., maybe not much, but it has effect!

I have owned as many as 6 Subarus with "diff-lock" systems, on those I usually "tuned" the rolling diameter with approx. 0,2bars more pressure in front to reduce the "diff-disengage" noise. (it builds up force when rolling diameter is not equal).
When tyres are matched equally in this way, there is a totally noise-free disengage of the diff-lock.

The effect of different rolling diameter is that there is a heat build-up in the clutch (plates,visco, whatever) because of the slipperage. (but not on "locked" systems)
This heat can be more or less depending on the system used, and the amount of slipperage, but could cause the clutch to fail if enough heat builds up.

To you who are tuning and tweaking your cars for over/under steer, check out the attached html file.

Sonar · #13 11-03-2003, 10:21 AM

-Sorry, here's the file....

Save and rename it to chassie.htm and doubleclick it....

Sonar · #14 11-03-2003, 10:25 AM

I made an PDF of it....

Mr. Pockets · #15 11-03-2003, 10:35 AM

Quote:

Originally posted by Sonar
Hi folks, sorry if I intrude....
But...
Tire pressure HAS effect on the rolling diameter...., maybe not much, but it has effect!

I have owned as many as 6 Subarus with "diff-lock" systems, on those I usually "tuned" the rolling diameter with approx. 0,2bars more pressure in front to reduce the "diff-disengage" noise. (it builds up force when rolling diameter is not equal).
When tyres are matched equally in this way, there is a totally noise-free disengage of the diff-lock.

The effect of different rolling diameter is that there is a heat build-up in the clutch (plates,visco, whatever) because of the slipperage. (but not on "locked" systems)
This heat can be more or less depending on the system used, and the amount of slipperage, but could cause the clutch to fail if enough heat builds up.

To you who are tuning and tweaking your cars for over/under steer, check out the attached html file.

Thank you for lending us your experience and knowledge, Sonar. I appreciate it.

The tires' rolling diameters (and the difference between them) effect on the transaxle is exactly why I'm concerned about how tire pressure affects rolling diameter. Your conclusion is what I have believed - that tire pressure doesn't affect rolling diameter much - but it does affect it. To what extent different tire pressures affect the rolling diameter I hope to find out.

This additional heat caused by more slippage in the transfer clutch is of concern to me because, as we all know, this transmission is particularly sensitive to heat.