Final gear ratio issues

[Juan Piñeiro]

I have some trouble deciding which final drive ratio to use in our car. Presently, we're stuck in a 4:1 ratio due to budget issues, we have to import the sprockets, etc..

The problem is, in my accel simulation spreadsheet I'm getting max values of 1.6g in 1st gear and for what I've read before in the forums that's way too much for the tires to handle.

I wanted to know if you guys could help me, or where I can find some info about how to find out the max longitudinal force that the tire can handle (depending on weight transfer, etc)

Perhaps we could just launch at 2nd gear http://fsae.com/groupee_common/emoticons/icon_wink.gif

Juan Piñeiro
Drivetrain and Brakes Team Leader
FSAE-LUZ

[Juan Piñeiro]

I have some trouble deciding which final drive ratio to use in our car. Presently, we're stuck in a 4:1 ratio due to budget issues, we have to import the sprockets, etc..

The problem is, in my accel simulation spreadsheet I'm getting max values of 1.6g in 1st gear and for what I've read before in the forums that's way too much for the tires to handle.

I wanted to know if you guys could help me, or where I can find some info about how to find out the max longitudinal force that the tire can handle (depending on weight transfer, etc)

Perhaps we could just launch at 2nd gear http://fsae.com/groupee_common/emoticons/icon_wink.gif

Juan Piñeiro
Drivetrain and Brakes Team Leader
FSAE-LUZ

[Logan S.]

Do you have any measured acceleration values from a previous car?
Our school will not help us buy the TTC tire data (which may or may not have longitudinal fricion coefficients) so we used some accelerometer data instead.
After competition this year, we took our car out and did some hard launches (8,000 RPM clutch drops). We sheared our axle in just a few runs (unable to heat treat-design flaw). We then analyzed that data and saw that at our peak through all 4 runs, we were pulling 0.81-0.84 g's of acceleration.
Using your wheelbase and CG height, you can calculate your weight transfer onto your rear wheels given that acceleration value. Using this, you can calculate a rough coefficient of friction value based on thrust required to achieve that acceleration value and the weight on the rear tires.
For us, this correlated into a coefficient of roughly 1.25. In our spreadsheet, we use a series of "IF" statements to use either what the tires can handle based on rear wheel weight, Coef. of Friction and thrust at the wheels (whichever is lower). This works well for us and we have been able to calculate our 0-75 m time to within about 0.1 seconds.
Good luck. Hope this helps.

[John Grego]

Our team tries to get our gear ratios set so that we can launch from 2nd gear. This prevents us from accidentally shifting into neutral and losing a bunch of time.

We weren't able to do this with our smaller, less powerful engines we've been using the past 3 years, but with the yzf 600 we were able to run a 4.07 acceleration in Detroit in 2005.

[murpia]

Juan, try flipping your analysis around and deciding on a gear ratio that can handle a range of grips say from Mu=1.0 to 1.8

That way you won't get caught out by track conditions on the day and 'over optimising' your gearing.

But, starting in 2nd gear sounds good to me...

Regards, Ian

Regards, Ian

[Thrainer]

There are different approaches at finding a good gear ratio. Depending on shift times, you can decide how many gear shifts you want during acceleration and in what gear you want to cross the line (~100-120 km/h). With slow gear shifts, you should shift less often. Of course, the torque curve of your engine if important as well.

We've been launching in 2nd gear this year.

I recommend keeping some different sprockets and pinions as options. Sometimes your gear ratio just doesn't fit the autocross or endurance track, so you want to change it slightly.

[Wesley]

Tuning your final drive for acceleration is probably not going to net you the most points.

Generally, we use track data to figure out our average corner exit speed, and make sure that with our given power band we generate enough thrust to equal the tractive capacity of the tires.

It'll take logged track data, a tire tractive force graph, and a power graph, but you want to look at how much power you can put down as you exit a corner, because that has a huge bearing on how fast you're going to go on track.

This year we've also messed with the idea of interchangeable sprockets for accel/skidpad, and autox/endurance, because one drive ratio is far better suited for one event than the other.

[J. Vinella]

Charlie Ping did a paper on this back in 2004 that should give you a few more ideas and variables to consider.

SAE Paper Number: 2004-01-3554

[nickerss]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Wesley:

This year we've also messed with the idea of interchangeable sprockets for accel/skidpad, and autox/endurance, because one drive ratio is far better suited for one event than the other. </div></BLOCKQUOTE>

Isn't that illegal?

[D Collins Jr]

Far from it ross, it's a "tuning" change, outlined as one of the allowable changes after tech.

[Pete Marsh]

3.7.3 Modifications and Repairs
(A) Once the vehicle has been presented for judging in the Cost or Design Events,
or submitted for Technical Inspection, and until the vehicle is approved to compete
in the dynamic events, i.e. all the inspection stickers are awarded, the only
modifications permitted to the vehicle are those directed by the Inspector(s) and
noted on the Inspection Form.
(B) Once the vehicle is approved to compete in the dynamic events, the ONLY
modifications permitted to the vehicle are:
a) Adjustment of belts and chains
b) Adjustment of brake bias
c) Adjustment of the driver restraint system, seat and pedal assembly
d) Adjustment to engine operating parameters, e.g. fuel mixture and ignition
timing
e) Adjustment of mirrors
f) Adjustment of the suspension where no part substitution is required, (except
that springs, sway bars and shims may be changed)
g) Adjustment of tire pressure
h) Adjustment of wing angle
i) Replenishment of fluids
j) Replacement of worn tires or brake pads

Unless all the sprockets are on the car and your changing them over like a bicycle I reckon its illegal by this rule. I'm sure "adjustment" refers to tension.

Pete

[D Collins Jr]

Good point Pete. I'm now glad that I didn't try to point at a rule that said I was right. I'm also glad that I remember that our thinking was to use multiple sprockets in testing to try and maximize our on track performance with the most versatile sprocket.

[Pete M]

I've never understood why it was illegal. If you're allowed to change suspension springs, why aren't you allowed to change sprockets? Changing gearing to suit a track is extremely common in motorsport. And if you're allowed to nearly completely change your suspension setup between acceleration and autocross, why not your final drive? I mean, i can understand not allowing swapping a torsen for a spool mid comp, but surely sprocket ratios should be allowable tuning.

[Wesley]

I agree with you Pete, and I think my misunderstanding probably stemmed from a conversation I had with Charlie Ping at the California competition. He mentioned it, and how it would probably net some gains, but I hadn't looked at the rules.

Either way, it makes a lot of sense to do some actual testing with sprockets - you might find it affects the driver more, even, which can make it more than beneficial, even if the car is theoretically slower.

[Pete M]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Wesley:
Either way, it makes a lot of sense to do some actual testing with sprockets - you might find it affects the driver more, even, which can make it more than beneficial, even if the car is theoretically slower. </div></BLOCKQUOTE>

Yep, we've done extensive final drive testing over the last couple of years and ended up with a radically different final drive to what we were running just a few years ago...

[TMG24]

What kinds of final drive ratios are you guys running?