Hi all,

This thread is applicable to LFS, so I'll put it in an LFS section, but I am intending to use it for real life.

The question is - should one gear a car for racing so that at top speed the engine is revving at peak power, or shortly after peak power, or (unlikely) shortly before peak power. The actual numbers of power, speed and gearing aren't important, as the theory should apply to any car.

It would be grateful if someone could somehow 'prove' which is best via the wonders of [simple] mathematics, rather than just people's opinions.

Edit: This suggests something that makes sense, but I'd like to see a bit of 'proof' before I consider it as fact.

Many thanks,

Tristan

i would say shortly after, but you run the risk of hitting the small gap between peak rpm and the redline.

sorry, i don't have any math to back that up.

I've always tended to err shortly after - somewhere between peak power and the rev limiter, on the basis that you still have a lot of tractive effort available after peak power that you might as well use, but sometimes you want to stretch a gear (say if you get a slipstream).

But I wanted to be sure that I'm not just throwing lap time down the drain by not gearing top speed = peak power rpm.

Here are some hopefully simple thoughts:
- Optimizing for maximum speed reduces acceleration, optimizing acceleration reduces maximum speed
- If the track was an incredibly long straight the ideal solution would be very close to maximum speed. You'll make up for what you lose in acceleration if the straight is long enough.
- If the track was a series of extremely short straights the ideal solution would lean towards strong acceleration. You get an advantage at the start of acceleration and don't suffer the drawback because the straights are so short.
- We conclude that at the highest speed on the track you should be beyond peak power for good acceleration on your way to highest velocity.
- And very, very likely you should be below the rev limiter to achieve an acceptable speed.

If you want to solve it mathematically calculate covered distance by time for a given power output. You'll see that for every given distance there is a best starting acceleration. The longer the specified distance the lower the starting acceleration. Thus the arguments as above.

Vain

I can run some numbers that should prove the difference, if you want to base your opinions on that. I'd say you're doing it correctly at the moment.

at top (terminal forward) speed the force of drag will be at its peak and thus the work F*s to cover any distance s will be at its maximum as well
power is the rate at which work is done so to maximise the work your car will/can do in any give timeframe you have to go for maximum power

so to cover a certain distance doing the most work (ie overcoming the most drag and thus traveling at the highest speed) in the shortest amount of time you want to go go for max power

Does that apply even if top speed is well below terminal velocity? I see the logic - never thought about it in terms of work done.

Thanks.

The more top speed tends to be terminal speed, the more you need to shoot for peak power instead of higher revs.

In VHPA, XRG with hard track gearbox, I played only with the differential gear. Peak HP is at 5990 rpm. what I did was setting up the diff ratio until I got the quickest 0-whatever time.


To reach 120 kph the quickest you need to reach that speed at 6450 rpm
202 kph - 6286 rpm
211 kph - 6135 rpm (that is almost terminal speed)

Edit : duh... this is exactly what the advanced setup guide says. :facepalm. Well...maybe those numbers prove the point.

On tracks where the top speed is also the vehicles top speed, you would want to reach top speed at the same rpm at which the engine produces peak power. As the highest attainable speed for the track lowers in proportion to the top speed of the vehicle, you would want the engine revs to be going further and further beyond the peak power rpm.

Edit 2 : Also...what shotglass says is right considering an "infinite ratio" gearbox

Since we have to settle for one(1) fixed gear that will cover at least some amount of wheelspeed difference from the moment you get in it to the moment you reach top speed the ideal point will always be at least a liiiittle beyond peak hp.

I suggest getting down Snetterton and doing some testing mate

How long does it take you to change top gear ratio or final drive?

Slightly after peak power, since you need the peak power to get to top speed.