axle ratio

[wesh]

hi guys i have a ques of how i can determine the proper axle ratio of the vehicle (final drive ratio) i know that higher ratio means good acceleration but less fuel economy and lower ratio means bad acc
but i want to know the best way to determine the proper ratio (i have the gear box reduction ratio and the engin spec that i can use )
thx >>>... http://fsae.com/groupee_common/emoticons/icon_confused.gif

[kcapitano]

Final drive ratio is a heck of a thing, and will require lots of data analysis and working with the driver to get just right. That being said there are numerous ways you can get a good value to start off with. This year my team used a chart to match predicted road speeds with RPM in different sections of the track. Tune to Win chapter 12 is an excellent place to start.
It would also be wise to look at any data you have from previous testing. Using a trace with RPM and road speed is good. My team has an on board camera that allowed us to overlay speed and RPM with the video. Somewhat subjective but very useful.

[nowhere fast]

Don't forget the primary reduction ratio.

As a very basic starting point for picking the final reduction; remember that there is only a limited amount of acceleration your tires can provide.

[Owen Thomas]

The BEST way is through proper testing, but to do that you need a current and fully built vehicle. There are ways to get in the ballpark with lap simulations and simple calculations (you need to know your lateral acceleration limits, though).

Our team is currently working on something to help find our final drive, it will probably end up being an Excel sheet with a few input variables. We're optimizing it to stay within our powerband during skid pad, and be at the bottom of our torque curve coming out of the smallest radius corner at competition. You can find track specs (but not a map) in the 2012 FSAE rulebook to help with this.

I should mention that while a higher final drive ratio does in fact give you better accel, there are performance limits due to shift time and issues with driver control. The last thing you want is to have your driver drop the hammer at the apex and just spin out immediately. It's all based on your teams setup and what you're looking to optimize with your drive ratio.

[Gruntguru]

Originally posted by Owen Thomas:
We're optimizing it to stay within our powerband during skid pad
You need to be in the power band during skid pad?

[wesh]

Originally posted by nowhere fast:
Don't forget the primary reduction ratio.

As a very basic starting point for picking the final reduction; remember that there is only a limited amount of acceleration your tires can provide.

i can know that from that eqation
1) FTmax=temax*ig*ip*(id)/r

2) FTmax=wr*adhesion coeff

wr is the rear load on the drive axle but when i determine the load i would determine it in the static condition or in the dynamic ?????????(load transfer will happened) taking into account that i start the motion from rest on the first gear???????
thx