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Paul V.
10-20-2004, 09:47 AM
Hey everyone,
I have a quick question for everyone about sprocket sizing. I need longitudinal slip ratio data with load for the goodyear tires to calculate rear sprocket. Does anyone know where I can find it? I couldn't find it in the tire data book.

Just curious, how do ya'll size your sprockets? Do you just use stock ones off of a bike?, is there a good reference for this? and are there any preferred tricks to getting a caliper to set deep enough to clear the teeth around the sprocket?



Just trying to keep from re-inventing the wheel.
Paul Vaughan
Univ. of Alabama Huntsville FSAE

Paul V.
10-20-2004, 09:47 AM
Hey everyone,
I have a quick question for everyone about sprocket sizing. I need longitudinal slip ratio data with load for the goodyear tires to calculate rear sprocket. Does anyone know where I can find it? I couldn't find it in the tire data book.

Just curious, how do ya'll size your sprockets? Do you just use stock ones off of a bike?, is there a good reference for this? and are there any preferred tricks to getting a caliper to set deep enough to clear the teeth around the sprocket?



Just trying to keep from re-inventing the wheel.
Paul Vaughan
Univ. of Alabama Huntsville FSAE

Lyn Labahn UW-Madison
10-20-2004, 01:41 PM
Well as far as an analytical method goes, you do have the dimensions of the tightest corners, largest corners and longest straights on the autocross and endurance. If you have a general idea of your lateral acceleration and longitudinal acceleration capabilities you will be able to get a reasonable estimate of your min and max speeds to expect on the course. Figure out how many gears you want to use (here is a hint, I would use a max of 3, you might be tempted to use more, but this is generally not a good idea) and space them accordingly.

That said, there is room for error of course, so I would make sure that your car can accomidate a variety of rear sprocket sizes, cause no matter what your calcs get you, it would be in your best interest test some combos in the spring to see what is fastest! Even a set of two fronts and two rears would give you a reasonbly large tuning range.

Angry Joe
10-20-2004, 05:18 PM
We did some quick calculations for our desired top in sixth gear and sized the sprocket that way - nothing fancy with tire deflection though.

We got blank sprockets from Sprocket Specialists and machined the center bolt pattern and pockets ourselves.

Charlie
10-20-2004, 09:55 PM
How did you decide your 6th gear top speed goal? Surely you aren't using it in competition.

Eddie Martin
10-20-2004, 11:06 PM
I'd work out a final drive ratio that you want to use to launch in and use on slow speed corners when the car is in 1st or 2nd gear in the stock box. The top gears don't matter at all.
At both the last OZ and US comps our slowest speeds were about 35 - 40 km/h and our top speeds were about 85 - 90 km/h. We only used 2 gears.
In the acceleration you will hit over 100 km/h.

Frank
10-21-2004, 05:34 AM
for traction control cars

design so that 2nd gear has (just) enough torque to break traction
1st gear has plenty more

for us its about 735Nm to break traction

so 13.5:1 total reduction in second gear

this spreadsheet might help

http://www.uq.edu.au/fsae/traction.xls

Paul V.
10-21-2004, 09:16 AM
Wow,
As usual more help than I was expecting. Thank you very much guys.

Paul Vaughan
UAH FSAE

Angry Joe
10-21-2004, 02:11 PM
<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR>Originally posted by Charlie:
How did you decide your 6th gear top speed goal? Surely you aren't using it in competition. <HR></BLOCKQUOTE>

We geared it to a little over 100 mph, if I recall. We reasoned this would allow 3-4 gears to be useful at competition, while allowing for the higher speeds of SCCA solo 2 courses.

After testing we stopped using first gear altogether, and probably would have switched to a larger sprocket had there been time...