OK guys been doing some calculations.

In 2010 I was running 245/40/17 rear tires (24.5" diameter).
In 2011 I plan on using 275/35/15 rear tires (23.0" diamter).

My calculations indicate...... in theory I should gain about 6% in MAX Torque.

Can anybody verify my calculations?

Does that change in size really makes a huge difference like that?

I was planning on having 16 inches, since I could buy 16 inches barrels for my Jongbloed's (replacing the 17's).

Hoosier size that I would use is 245/45/16 at all 4 corners which is 24.7" in diameter. Or keep my 17 inches wheels and use 245/40/17 which is 24.5".

Otherwise, I would have to buy 15" cookie cutters. So maximum size would be 225/45/15 (22.9" dimeter).

If I use a non LSD set-up, I need more tires. If I do have an LSD, then the cookie cutters is the most economic solution...

Don't know what to do...

Charles,

Assuming that the Engine HP and Flywheel Torque have not changed and you only change the Outside Diameter of the rear tires...

LTF = Linear Tire Force (Lbs) ---- Force imparted to the ground to accelerate the car
ATQ = Axle Torque (Ft-Lbs)
TRADIUS = Tire Radius (Feet)

LTF = ATQ / TRADIUS

You can calculate your car's existing LTF and then the calculate the change depending upon the change in rear wheel diameter. Now, if you are at a track with a very long straight you may need to make sure that you do not run out of RPM in Top Gear or you may be willing to give up a few MPH to gain acceleration every where else. That's what make car setup so much fun:)

If my info is incorrect...I'm sure somebody will chime in.

Ok.

Let's say I'm at 142hp and 144 TQ with tires of 24.5" of diameter. If I switch to 23" tires, then I'm not legal, right? So these 23" tires will only help cars with less hp/tq.

Am I understanding correctly?

Or is it only the fact that I don't want to pay again thousands of dollars for wheels that makes me ask questions again?

That does not change dyno readings but it does translate to more power on the ground?

Thanks

c.

n.b. I on't want to hijack your thread Tim, sorry for that!

This is ALL in theory.

However I am confident about the following:

The smaller wheels are lighter (Lower Unsprung Weight, Smaller Contact Patch)
The smaller diameter tires will accelerate faster (Always Good)
The smaller tires will limit top end speed
The dyno uses Rear Wheel Torque (RWTQ)to calculate Rear Wheel HP (RWHP)
The Torque curve will be steeper and reach maximum Torque and HP at lower RPM

However, I to have the following question:

Will the MAX RWTQ change which would also result in MAX RWHP change or will the Torque normalize to the same MAX RWTQ regardless or tire diameter? The engine only has so much torque.

Regardless, I will spend my $200 US at the DYNO and find the answer.

Let us know what the dyno tells!

Tx.

c.

Maybe not. Depends on whether there was ever enough power to reach max velocity with the bigger tires.

The dyno, in theory, should normalize every thing and the motor numbers don't change. The power source stays the same. Only the lever (arms) change.

Charles,

Two other thoughts:

1) Smaller diameter tires will lower the CG of the cars weight and improve cornering stability and hopefully cornering speed.

2) The tire diameter is just the last factor (Transaxle Gear Ratio, R&P, Tire Diameter) in determing the RPM that your engine is turning in a certain gear. Depending upon the gearing, going to a larger diameter tire may allow you to take advantage of the acceleration you can gain by going down to 2nd gear versus 3rd gear coming out of a corner.

Happy New Year!

Right.

The quickest cars are all on 15's... And very low too.

Happy new year to you too!

Regards

c.

New R6 Hoosiers came in yesterday and I weighed them all.

Data:
Rear 245-40-17 = 21 lbs.
Front 225-40-17 = 19 lbs.
Rear 275-35-15 = 20 lbs.
Front 225-45-15 = 18 lbs.

That is a reduction of 1 pound on each corner and the small diameter is closer to the center of rotation.

Wheels should ship late next week.