Modeling Traction Limited Accel

[Spetsnazos]

Just trying to check to see if I am doing this correctly. Any input would be great!

Using this equation to determine the maximum tractive force, I get a value of 531.97 lbf, which results in a maximum acceleration of about 1.06g's

http://i295.photobucket.com/albums/mm133/liquidoleg/tract_limit_1.jpg

I am unsure how to model this in the program in the case that the vehicle produces a higher tractive force and ultimately a higher acceleration.

http://i295.photobucket.com/albums/mm133/liquidoleg/tract_limit_2.jpg

For example,

In first gear, my maximum tractive force is about 660lbf with a peak acceleration(at maximum Torque) of about 1.32g's.

Should I program so that the tractive force is always capped at the maximum traction limit of 531 lbf?

[Gruntguru]

Originally posted by Spetsnazos:
In first gear, my maximum tractive force is about 660lbf with a peak acceleration(at maximum Torque) of about 1.32g's.

Should I program so that the tractive force is always capped at the maximum traction limit of 531 lbf?
Yes. BTW all the gearing calcs etc to obtain tractive force are a lot easier if you know engine power at every road speed. Then F = P/v (Force = Power/Velocity.) If P is at the crankshaft you will need a driveline efficiency factor thrown in as well.

For FSAE engines the power is often fairly constant around the peak power rpm so F = P/v can be modelled very simply for a quick and dirty calculation (which will nevertheless yield acceleration times within a few percent of the best simulation)

[Demon Of Speed]

Things it looks like you are forgetting to take into account:

weight transfer
rolling resistance
wind resistance
inertia
inertia of rotational parts (which is a function of what gear you are in)
efficiency (which is also a function of what gear you are in

In total there should be at least 23 vehicle (or that is how many my "accel sim" has) parameters that will define a vehicles strait line acceleration.

[exFSAE]

Not to mention the challenge of track-realistic tire data

[Gruntguru]

Originally posted by Demon Of Speed:
Things it looks like you are forgetting to take into account:

weight transfer
rolling resistance
wind resistance
inertia
inertia of rotational parts (which is a function of what gear you are in)
efficiency (which is also a function of what gear you are in

In total there should be at least 23 vehicle (or that is how many my "accel sim" has) parameters that will define a vehicles strait line acceleration. If you take a proper look at the OP you will see that several of those terms are in fact accounted for.

I question the need for 23 terms when you can get within 1% of the same result with perhaps 5 or 6 variables. Many of the terms (rotational inertia for example) have absolutely no effect on "traction limited" acceleration.

[Demon Of Speed]

Originally posted by Gruntguru:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Demon Of Speed:
Things it looks like you are forgetting to take into account:

weight transfer
rolling resistance
wind resistance
inertia
inertia of rotational parts (which is a function of what gear you are in)
efficiency (which is also a function of what gear you are in

In total there should be at least 23 vehicle (or that is how many my "accel sim" has) parameters that will define a vehicles strait line acceleration. If you take a proper look at the OP you will see that several of those terms are in fact accounted for.

I question the need for 23 terms when you can get within 1% of the same result with perhaps 5 or 6 variables. Many of the terms (rotational inertia for example) have absolutely no effect on "traction limited" acceleration. </div></BLOCKQUOTE>

I will agree with you that you can get close without taking some of those things into account.

Some of my variables are air temp. and whatnot to be able to calculate air density for wind resistance as an example. I broke them up into things that I could easily measure, not calculate and put in like air density.

As for rotational inertia having no effect on traction limited acceleration, you are wrong. The vehicles effective mass drops around 10% (more for vehicles without a primary reduction gear) changing from first to second gear. This is a massive amount, enough that you can be back in the traction limited area of acceleration.

[Gruntguru]

Originally posted by Demon Of Speed:
As for rotational inertia having no effect on traction limited acceleration, you are wrong. The vehicles effective mass drops around 10% (more for vehicles without a primary reduction gear) changing from first to second gear. This is a massive amount, enough that you can be back in the traction limited area of acceleration. True, rotational inertia has a large effect on acceleration and some effect on determining whether the vehicle is traction limited or not. It has no effect on traction limited acceleration.

How much difference does primary reduction make? We are only talking about an increase in the effective inertia of the gearbox internals.

[exFSAE]

Again.. your results will be extremely questionable without knowing what to do with the tire data.

And no use of the actual car's torque curve??

Who cares about peak acceleration. You want to manipulate things to have the highest average acceleration over the run.

[Crispy]

Originally posted by exFSAE:
Who cares about peak acceleration. You want to manipulate things to have the highest average acceleration over the run.

The highest average velocity is what you want. Acceleration early in the run is by far the most important.

[exFSAE]

True, good call. I misspoke. And yes, starting line hook is key.

Just seems like this is really missing a lot of critical stuff

[Edward M. Kasprzak]

Two thoughts:

1. The original poster should have referenced the source of scans.

2. As for the thread, here's a reminder that a discussion of how things work in the real world is somewhat different from a discussion of what equations should be used to describe that reality. As engineers we always make simplifying assumptions, even in very detailed models. How much or how little detail a model contains is your call. The engineer simply needs to understand the assumptions and the limitations of the chosen model/equations. Of course, more detailed models with more effects give greater accuracy but are also harder to populate and use. There's a place for simple "back of the envelope" calculations, too, and sometimes that's good enough for a given purpose.

I agree--there's a lot more that could be in those original equations. They make some big assumptions and are missing many effects, but (given the list of assumptions) they might suffice for the original poster's application.