Semi-Useful Calculators

[reddevilsix]

Copied from Dix's site

I modified a couple calculators that I've been using for a while now.. One is useful for calculating jackshaft speed, clutch shift ratios, & such... and the other is a simple "speed" calculator modified into a "Gear Selector Calculator" that can be used to figure out what gear you may want to try if you're going to a new track & have a good idea of what your predicted top speed may be, or what you may want to try for a gear change based on what you'd like your clutches top shift ratio to be.

The Gear Selection Calculator is pretty straightforward & simple to understand.

There are instructions on the Speed/RPM calculator to show you how to figure your jackshaft speed & shift ratio if you only have a speed output to deal with & don't have a jackshaft RPM input (or software that will do the math for you... like Digitron)

Simply download the ".ZIP" file attached to this post & unzip it wherever you'd like to have it saved on your laptop... no internet connection required... they are simple javascript calculators that will run in your web browser... just double-click the one you want to open... & there are links on each that will let you swap back & forth between the two.

~GEAR_CALC.htm - The Gear Ratio Selector
~SPEED-RPM.htm - The Speed/RPM Calculator

The SPEED/RPM calculator has 10 different "transmission ratio" inputs... you don't have to fill them all in, you only need to input one ratio for the calculator to generate an output.... naturally 1:1 is likely to be the one you'll use most.

wmmsg101.ZIP

(8.6 KiB) Downloaded 628 times

For the benefit of some of those new to the class... when using a typical "gear calculator" such as this one, remember that belt driven sled clutches aren't transmission gears... there WILL be a certain amount of slippage... nature of the beast... you need to have some slip or the clutches wouldn't function... therefore always "over gear" some if you're unsure of exactly what gear you may want to run.

For example... at NHMS just before you enter turn 1 you'll be knocking on the door of 110MPH... now, if I do the calculations for 8200 RPM on 19.5" tires & 110 MPH at 0.9:1 full overdrive I get a gear ratio of 4.81:1.

If I actually USED that gear ratio I'd likely run out of shift at about the start/finish line... due to slippage.

Calcualting for 1.125:1 & 1:1 comes in with a gearing of 3.84 & 4.32:1 respectively... which is probably in the ballpark of what I'll end up using, now that I know I should be shooting for 8200 rather than 8000 with my primary setup.

I was geared pretty conservitive last year... did my calculations for 8000 RPM & 1.125:1... which comes in at 3.75:1 final drive... closest set I had gave me 3.78:1.... Jackshaft RPM said I got to 1:1 with the clutches at 108 MPH... but the math says I should have been doing 123.

So there was about 15MPH worth of slippage going on.

Just as an FYI for those who don't know... since I was having a recent conversaion where the subject came up....

The way the math of the formula works, you can actually also use the gearing calculator give you an idea of what ratio your clutches were in for a given speed & engine RPM by substituting your rear axle drive ratio for the "transmission ratio" input.

For example... your data says you were doing 125 MPH at the end of a long straight, engine was at 8000 RPM, you were on 19.5" tires, & you had a gearset on the jackshaft & rear axle that was 3.25:1.

Plugging all that in gives you a theoretical shift ratio of 1.14:1 that your clutches would have been in at the time.

But remember, that's only if there was no belt slip in the clutches... which we know is not the case... allowing for 10-12% belt slip you can figure that they were actually pretty close to 1:1 at the time.