Understeer reduction via tire size change - Page 2

Chicane · #16 11-03-2003, 10:46 AM

http://www.subaru-svx.net/forum/show...5&pagenumber=1

There was another thread I was involved in that sparked a 'heated' debate. Bottom line is that tire pressure affects rolling diameter.

- Rob

SVXRide · #17 11-03-2003, 03:22 PM

Rob,
Just a measured piece of info to add to this discussion - when I was running in the MB autocross this Sunday I started with 39 psi in the back tires and 44 in the front to minimize the chance of the sidewalls rolling. After 4 runs the front tires had heated up to 49.5 psi while the back tires were still at 39. I used a digital tire gage for all measurements. It would have been interesting to have measured the O.D. of the tires...
-Bill
p.s. Harry (newsvx) and Mike (Rufus) also drove in the same autocross. Both of them did great -- I continue to learn from watching and listening to both of them.

Mr. Pockets · #18 11-03-2003, 03:42 PM

Quote:

Originally posted by SVXRide
Rob,
Just a measured piece of info to add to this discussion - when I was running in the MB autocross this Sunday I started with 39 psi in the back tires and 44 in the front to minimize the chance of the sidewalls rolling. After 4 runs the front tires had heated up to 49.5 psi while the back tires were still at 39. I used a digital tire gage for all measurements. It would have been interesting to have measured the O.D. of the tires...
-Bill
p.s. Harry (newsvx) and Mike (Rufus) also drove in the same autocross. Both of them did great -- I continue to learn from watching and listening to both of them.

See, in that case, you'd be less concerned about a small detail like tire rolling diameter. Autocrossing is already pretty abusive, so why sweat the small stuff?

You know, Bill, I'll be entering my first autocross in less than two weeks. I can't friggin' wait!

Andy · #19 11-03-2003, 03:44 PM

Quote:

Originally posted by fasteract
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.

Craig

Understeer?

What's that??

Sonar · #20 11-03-2003, 03:46 PM

-Would it be possible to use bicycle-computers (speedos) to measure each wheel (only small magnetos attached to the rim)?

SVXRide · #21 11-03-2003, 03:59 PM

Quote:

Originally posted by Mr. Pockets

See, in that case, you'd be less concerned about a small detail like tire rolling diameter. Autocrossing is already pretty abusive, so why sweat the small stuff?

You know, Bill, I'll be entering my first autocross in less than two weeks. I can't friggin' wait!

Pockets,
Enjoy! Harry (newsvx) got me hooked on it. It will chew on your tires, so make sure they're in good shape. Actually had the car semi-drifting around the 270 cones - definitely a rush. Now, if I can only get my brain to listen to Harry's advice that "slower is faster"....
-Bill

Mr. Pockets · #22 11-03-2003, 04:02 PM

Quote:

Originally posted by Sonar
-Would it be possible to use bicycle-computers (speedos) to measure each wheel (only small magnetos attached to the rim)?

I don't know what you could hope to learn from these, unless you put two on the car - one on a front wheel and one on a rear. Cycling computers tend to want the circumference of a tire. So if you figured out what the circumference was supposed to be, then plugged that number into both, you could drive for a bit and see what kind of discrepancy there was between the measurments the computers recorded from the front and rear wheels.

Of course, if you take any turns, you're going to see different numbers between those readings, anyway.

I still think that the simplest way to test how tire pressure affects rolling diameter is to measure the distance between the center of the hub and the ground at different tire pressures.

As much as I'd like to attach some sort of laser-rangefinder to the center of my SVX's wheels and see if the number changed at different pressures, I lack such an instrument. So I'll be using something low-tech. Like a stick.

This is unquestionably less precise, but without a doubt cheaper.

Sonar · #23 11-03-2003, 04:20 PM

-I had all 4 corners in mind

.. and a loooong straight....

Then you can set different pressures and measure in "your" way, after that drive 1 mile or so (straight) and check the "counters" on the displays!

Mr. Pockets · #24 11-03-2003, 04:33 PM

Quote:

Originally posted by Sonar
-I had all 4 corners in mind .. and a loooong straight....
Then you can set different pressures and measure in "your" way, after that drive 1 mile or so (straight) and check the "counters" on the displays!

You would, indeed, need a long straight for that.

mbtoloczko · #25 11-03-2003, 05:02 PM

Quote:

Originally posted by Sonar
-I had all 4 corners in mind .. and a loooong straight....
Then you can set different pressures and measure in "your" way, after that drive 1 mile or so (straight) and check the "counters" on the displays!

Use white-out on the sidewalls. Its not high-tech, but it works, and its cheap. Put a mark at the bottom of all your tires on the sidewall, drive straight for 1/2 mile, and look at the difference in revolutions between the front and rear tires. If your tires all have the same amount of tread, and there is at least a few psi more in the fronts than the rears, there will be less than 1 revolution difference between the front and rear after 1/2 mile. Try it with a variety air pressures in the front and rear. Its kind of a fun experiment. I did it a while back when I got new tires. It took a 37.5 psi in the front and 30.5 psi in the rear before the front and rear had the same rolling circumference for my 225/45-17 tires. If you do these measurements and also determine the rolling circumference, then you can find out exactly how much of a difference in rolling circumference there is between the front and rear.

grnSVX96 · #26 07-25-2004, 05:22 PM

mbtoloczko [or anyone else who'd like to chime in..ta]

I read with interest your tyre experiment, in my case with my rim/tyre combination 265/35x9x18 rear and 235/40x8x18 front, according to tyre calc programs my front tyre has a larger diameter by 0.3inches to the rear tyre, but with weight over the front...which way will the air pressure gap likely go wider front to back or lesser.

I will do the experiment after my car comes off the hoist, but if I can estimate the air pressure gaps to start it will give me something to work with..[yours with same tyre/rim f to b was a 7 psi difference..but with my front tyre being slightly 'longer' but with the weight of the engine over it ...do you see the psi gap even wider or narrower..??]

Also how do you compensate for heat load.. perhaps getting it right for when the tyres are cold wont this change when say the front tyres have heat in them more than the rears..

Thanks

dave

mbtoloczko · #27 07-26-2004, 12:14 AM

When I do the tire diameter calculations, I get that the dia of the rear is 0.1" less than the dia of the front. Since the tire diameters are a bit different, keep an eye on the difference in revolutions as you move the car forward. You might want to check every 1/4 or 1/8 of a mile.

Sonar · #28 07-26-2004, 12:44 PM

I wonder if it is possible to "tap" into the ABS signal sensors, maybe in the ABS control box, and build a simple frequency counter to monitor all 4 wheels

Maybe I'll check into that l8er

/Sonny

Earthworm · #29 07-29-2004, 03:00 PM

Quote:

Originally posted by Sonar
I wonder if it is possible to "tap" into the ABS signal sensors, maybe in the ABS control box, and build a simple frequency counter to monitor all 4 wheels

Maybe I'll check into that l8er

I'm sure you could do that but I think chalk would be that much easier.