analogmike

No, if you don't mess with the suspension like Gary did, the corner balance will be perfect. Even if the car's rake changes by a few inches the corner balancing will still be on. It would be nice to lower the car though if you have more clearance with the smaller tires, then you had best corner balance.

paradisenb

15 x 40, 45 or 50 may not require R&R of TBs. I guess the 35 leaves the car higher than optimum. What I want is to be able to run 15s in historic racing (Toyo tires) and 15s in PCA or NASA (Ho Hos) w/o having to change ride height or anything. Just want a bolt on solution. Will this compromise the setup with Hoosier 15s?

winders

Mike,

If you lower the front of the car 2" more than the rear with a tire size change, does that not change how the weight is distributed? Maybe not a lot, but it does change, right?

Scott

Astroman

I think it changes the front/rear percentages but not the cross weight percentages.

KaiB

Might want to check with Kibort on this one.

winders

The cross weight percentages would change as well unless each wheel carried 50% of the weight on its end of the car.

It's not much of a change, but it is a change. Not likely enough to worry about.

Scott

Astroman

The cross weights would change but the percentage should not.

For example, lets make up a perfectly balanced 2000 lb car with 500 lbs on each wheel:
F 500 500
R 500 500

Then change the ride height and rake to shift weight towards the rear:
F 400 400
R 600 600

The cross weight percentage in both examples is still 50% of the total weight.

winders

Your example is the exception I mentioned in my previous post.

Pick a car with these weights:

F: 500 470
R: 750 720

Change the rake evenly on both sides so 100 lbs is shifted to the front wheels. What happens with the cross weights which were both at 1220 lbs.? The weight transferred to the front is split based on the front percentage (51.5/48.5) and removed from the rear based on the rear percentage (51.0/49.0).

F: 551.5 518.5
R: 699 671

LF-RR: 1222.5
RF-LR: 1217.5

Subtle, but different....

Scott

analogmike

If you raise the front of the car by a foot it won't barely change the ft/rear distribution. It WILL change the handling, but not because of weight transfer, because of geometry. You can do the math (I have, just calculate the C of G position and see how it changes by raising one end) or try it with scales and wood.

So the answer is, no you don't have to corner balance your car when changing wheel sizes. Gary, you should be able to find a good compromise to run with both setups though you CAN just crank the TBs a certain amount the same right and left and be close enough. I do that when I remove ballast for vintage racing.

winders

Mike,

I understand and agree. I actually hadn't thought about how much you would have to raise or lower one end to get a significant change. I only got as far as thinking that changing the rake would change the weight distribution so it would change the cross weights.

Scott

analogmike

Most people think that, even some really good pros.

paradisenb

Interest analysis. I will try this when I test mount a set of 15 inch Fuchs. Switching from 17s to 15s and back. Only draw back is the tires on the 15s are Toyos 235-50-15 I think. I won't be able to test with 275-35-15s.

sig_a

[QUOTE=Astroman;8897534]I have wondered the same thing. However I hope that I'll only require 5th gear (with 15"s) in the same locations that I currently require 5th on 17"s (back straight Atlanta, back straight M-O, back straight at the Glen, all 3 straights at Road America). I may need 5th on the front straight at Putnam on 15"s (which would be no big deal).

I was recently at Putnam with KYPCA on 15" v710 kumho 225x50 at 23.8" diameter. I up shift to 4 at tower, up shift to 5 at bottom of dip. Downshift 5 to 3 between 200' and 100'. Relearning shift points changes approach over entire track. Square setup thrilling, and could use more tire at rear.