gearing

[Bill Verburg]

I've been playing w/ some ss illustrating the affect of gearing on available forward motivation(thrust). This was sparked by a not too recent discussion on Rennlist.

This chart shows a stock US 3.2 Carrera in blue, vs. a typical short geared 915 in the same car in red.

1^st and 3^rd are the same in both boxes.

yaxis is available forward thrust in lbs.

I find this fascinating because you can compare the effect of changing tires, gearing, or engines.

If you need to set up gearing for a particular track this stuff helps immeasurably.

[Jack Olsen]

Very cool.

Uh, I choose red.

[Chuck Moreland]

Something doesn't seem right.

Red has the same thrust profile as blue, as expected, corresponding to the torque curve of the motor. It's just higher due the the reduction in ratio.

However, I expect the red curve to be shifted to the left in an amount proportional to the reduction in gearing. Instead they line right up.

Also, it appears that it is 1st and 3rd that are the same.

What am I missing?

[Bill Verburg]

Chuck, Right you are! There is a shift I just picked the wrong inputs in the original chart. Which I have since ammended

Jack, You did chose the red(that's your trans)

Do you want one for your car as it sits? I haven't done a 964 engine yet. I just need to know what tires are in the back, and the all up weight if you want potential acceleration curves. As I said I did this for my car as it sits, fascinating stuff!(to me)

[Bill Verburg]

This one shows the shift more clearly. I wanted to see the effect of a different tire.

[Jack Olsen]

My car weighs 2390 pounds, gassed up and ready to go. Add my 160 pounds. Rear wheels are 315/35's

[Bill Verburg]

Ok I'll work on it. BFG KDW? I love rainy weekends

[Bill Verburg]

Ok Jack here it is, assumptions

• 964RS hp and torque curve
• No air resistance
• 2590 lbs all up weight

[Jack Olsen]

Cool! Nice and close on the graph, which is how it feels when you're winding through the gears.