Rear sprocket design - Max speed or max torque?

[nvpF1crazy]

I have come across a few posts and reports in which the number of teeth on the rear sprocket and hence the final drive ratio is decided by the target top speed. But shouldn't the maximum required torque at the wheels be the deciding factor? After all the gear ratios are torque multipliers..

[Tom Wettenhall]

Neither.

You want to set your ratios so you spend as much time in the right part of the rev range (i.e. the bit that makes you go forward quickly) as possible. That means looking at the whole rev range, not just peak torque or maximum speed.

First up, in which gears do you want to spend most of your time? Some people don't like to skip between first and second too much as you run the risk of hitting neutral in between, on a bike engine. The longer and more crap your shift linkage, the easier that is. Also, making the final drive shorter will bring the gear ratios closer together, good for keeping the engine on the boil.

Second up, where in those gears do you want to be? You need to know two things most of all: What does the track look like (Check out last year's track) and what does your torque curve look like? Avoid both ends of the powerband - bogging and the rev limiter, especially the limiter.

[nvpF1crazy]

Currently we are using the stock gearbox of our engine, as in the case of almost all other teams. So the final drive is being thought upon.

If its max speed, simply select a target and based on the tyre size calculate the rotational velocity at the rear sprocket and we should get a fair estimate of its size. But I don't see how it takes into account whether or not there will be wheelspin and how much traction will be available at launch.

I have formulated a method to start off with the torque but haven't heard anyone else doing so. Hence not sure.

[wagemd]

Pay very little attention to top speed, so long as the gearing you want allows at LEAST that speed. What you should probably be looking at is maximizing the power [hint: force x velocity] at the wheels.

[rjwoods77]

Do a search into tractive effort diagrams. The question you are asking opens up into other areas and they eventually all kinda end up being discovered/worked out through that diagram.

[Sormaz]

Originally posted by nvpF1crazy:
Currently we are using the stock gearbox of our engine, as in the case of almost all other teams. So the final drive is being thought upon.

If its max speed, simply select a target and based on the tyre size calculate the rotational velocity at the rear sprocket and we should get a fair estimate of its size. But I don't see how it takes into account whether or not there will be wheelspin and how much traction will be available at launch.

I have formulated a method to start off with the torque but haven't heard anyone else doing so. Hence not sure.

Trust me, if you chose a final drive that will gear limit you to the course top speed, you will have plenty of wheel spinning action.
But, do you really want to be flipping through 6 gears? can you really fit such a large sprocket on your car?

Maybe there is a different trade off that you should be looking at

[nowhere fast]

Originally posted by Tom Wettenhall:
Some people don't like to skip between first and second too much as you run the risk of hitting neutral in between, on a bike engine. The longer and more crap your shift linkage, the easier that is.

If your shift technique is bad enough that you are hitting neutral between 1st and 2nd; then you are also at risk of hitting a false neutral between 2nd and 3rd, or any other gear.

[Tom Wettenhall]

Not really. You can't really go wrong with the CBR 'box, I've never seen anyone get a false neutral. It's just that the bloke who was going to design our shifter never showed, so we've got a 'placeholder' shifter which flexes a bit, and allows you to hit neutral on a downshift if you stuff it up. I suppose technique is a big part of the problem, though.

[Sormaz]

Originally posted by nowhere fast:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Tom Wettenhall:
Some people don't like to skip between first and second too much as you run the risk of hitting neutral in between, on a bike engine. The longer and more crap your shift linkage, the easier that is.

If your shift technique is bad enough that you are hitting neutral between 1st and 2nd; then you are also at risk of hitting a false neutral between 2nd and 3rd, or any other gear. </div></BLOCKQUOTE>
Bold statement
Have you ever looked at the star on the end of a shift drum? Oh hey, neutral sits in a stable position, just like the rest of the gears.
What's that? false neutrals are unstable?

Has a little to do with technique and practice, sure - but a lot more to do with how "long and how much crap is in your shift linkage"

Only the relentless few get to spend 7 years in the driver's seat. Most FSAE drivers are lucky to drive at even 1 or 2 competitions. Design your car to be easier to drive, and hey! it's easier to drive