Gearbox

[Silente]

I was wondering why a lot of fast teams use just 3 or 4 gears gearboxes.

Anyone can help me to understand?

[Silente]

I was wondering why a lot of fast teams use just 3 or 4 gears gearboxes.

Anyone can help me to understand?

[Thrainer]

What is the top speed of a motorcycle?
What is the top speed of an FSAE car during competition?

One fast acceleration run is 4 seconds. How much of that time do you want to spend with gear shifting?

[Silente]

yeah, ok, but you can adjust final ratio and for sure you have a smaller wheel then a bike. SO i thought that if you choose a short final ratio you can use all your gears and keep the engine always in the right zone...

[Marshall Grice]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Silente:
yeah, ok, but you can adjust final ratio and for sure you have a smaller wheel then a bike. SO i thought that if you choose a short final ratio you can use all your gears and keep the engine always in the right zone... </div></BLOCKQUOTE>

you can adjust the final drive ratio. you keep going shorter and shorter with the gearing and eventually you run out of traction and/or spend all your time shifting. That point happens before you effectively use all 6 gears.

[Silente]

but so how you would adjust gearboxes to keep gears away?

would you just put the last two gears away?

[jdstuff]

Do you really NEED to "keep the gears away?" How about simply not shifting into 4th-5th-6th? Seems to work for most teams out there.

If you are really interested, however, in 2005 The Univ. Akron had a pretty extensive engine development program. In that case we removed the unwanted gears from the gearbox, and replaced them with appropriate "blanks." There were non-trivial performance gains achieved in the reduction of rotating mass.

[VFR750R]

If you got the engine apart anyway, maybe take one or two out, but for the gains to be truly significant would require modifications throughout the whole system. Which shafts are spinning the fastest(also would then be accelerating the fastest), and which parts in the system have the highest inertia x their shaft speeds.
The answer to both of those questions are neither the transmission shafts or the gears, so although they shouldn't be ignored, they would be part of a much broader list of modifications.

[PBR-keith]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by jdstuff:
Do you really NEED to "keep the gears away?" How about simply not shifting into 4th-5th-6th? Seems to work for most teams out there.

</div></BLOCKQUOTE>

Why leave them in if your not going to ever use them. to be a competitive team in design event you need to hit 450 lbs. every bit counts

[JamesWolak]

I know this is a little off subject but in 2007 we ran two motors on the dyno within a week of each other on the same exact car with the same compression, intake, cal, exhaust, ect. The only differences were that one had titanium rods and a knifed edged/lightened crankshaft and the other had stock rods/crankshaft. There was no noticeable differences.

I just think things like this aren't worth the effort in FSAE. Besides the information that we gained we would have been better off spending that time/money on calibration, testing, or driver training. Our current power curve embarrasses our previous ones that had all the lightened rotating components. And those gains weren't hiding in some trick unknown location.

But i must admit last year I had a design Judge call us out for not removing mass from our crank/gears/rods. He stated that other teams had claims of notable gains from it. I replied with the information from above but i don't think he bought it.

I also agree with Keith it does help lower the overall weight of the car which is huge in design. But in most cases in FSAE the motors sits soo close to the ground that you might be negatively effecting the CG of the car (nothing worth noting though).

[VFR750R]

The neat thing about inertial power is it is a simple sophomore level math calculation to prove it either is worth the time and effort or it is not. The hardest part is modeling the parts, but someone with reasonable cad skills could make a representative crankshaft in 5 hours or less. Clutch and gears would take even less time.

Power data from a dyno is good, but matching theoretical results knocks the point home.

Did you tell the judge your results, or SHOW him your results?

Compared to calculating power through porting, or manifold changes, it's pretty straight forward.

[flavorPacket]

To be competitive in the design event, you need to know everything about your car. We were 457 in 07 and made finals. Don't justify a design decision based on the design event, do it based on sound project management and engineering principles!

In a competition where 50% of the cars don't finish the event due to mostly powertrain failures, why do you want to risk additional powertrain failures? You might gain a pound and 3 hp. What % of a lap are you traction limited again? How good are your drivers?

[JamesWolak]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by VFR750R:
Did you tell the judge your results, or SHOW him your results?
</div></BLOCKQUOTE>

Of course I didn’t have the data that’s where I failed and why I know he didn’t believed me. My team is an unorganized mess prepping for design consisted of sitting in the trailer and printing stuff off two hours before comp (which was good enough for 15th place). But that’s besides the fact.
The dyno data didn’t even look real. I wouldn’t have believed it myself. It looked like we just ran two dyno runs in a row. Only reason we did the test is because our dyno motor we were using didn’t have the lightened components and the comp motor did.
They did the calculations in 06, it was something stupid high like 3HP. The calculations were based of the rough shape of the crankshaft so I take that with a large grain of salt.

[LCT]

You shouldn't look at removing rotating mass from your gearbox until you've taken a look at your flywheel first. The calculations are a bit difficult if you've never done them before, but depending on your engine, you may be able to remove a significant amount of mass from your flywheel. When I did the calculations for our engine, I found that with our overbalanced crank on our B&S V-Twin we could use a flywheel that was less than half the weight of the 18lb steel one that we were using. Of course there are other things to consider when changing a flywheel (starter, stator magnets, etc...) But if you're trying to reduce rotating mass, I would look at the flywheel before trying to pull gears.

[Wesley]

Don't forget that a dyno run with a slow enough rev ramp rate will decrease the effects of inertial mass on power output.

T = I cross alpha!