Jersey Tom wrote:Edit - Was originally gonna include some thoughts on the claims of some engines to "give tires time to rest between cylinder pulses and regain grip" but I'm saving that for another day.
Belatti wrote:BTW, Tom, can you tell us something about this?
Jersey Tom wrote:Belatti wrote:BTW, Tom, can you tell us something about this?
gioma wrote:• Torque is what makes acceleration possible.
• Power is what makes possible to run at a certain speed without accelerating (not increasing the speed).
Or just assume 'all else being equal'. Say you have an rx-8:
232 hp @ 8500 rpm
159 lb-ft @ 5500 rpm
Put it on 1st gear. Is it more likely to break traction at 8.5k or 5.5k rpm? Of course you'll say, it'll spin the tyres in both cases, and you'd probably be right.
So say you start putting weight over the rear axle, 100 Kg at a time, will it stop spinning the tyres at 8.5 or 5.5 first?
And yes, assume you put new tyres and clutch every run, and engine temp, and air pressure, and any other potential variables remain the same. Driver is a robot of course.
engineguru00 wrote:Figure I would actually register to answer this, I have been lurking around for a while. I did a good amount of work with wheel slip when gearing an FSAE car last year around a CVT transmission.
The amount of longitudinal force relies on a few things. Tires produce their maximum force at a certain slip ratio, which is not to be confused with runaway slip. Tires also vary their longitudinal thrust with load, although it isn't a linear relationship. Temperature and inclination angle both play a big role as well and neither are constant. Most SR vs LF vs Load plots you see are on a belt setup which doesn't accurately predict tire capacity in real life during operation, so you normally have to add a correction factor to take this into account and scale down all the values. A good driver can modulate the throttle to keep you at this peak of force, so you take that multiplied by your correction factor to get your maximum longitudinal force.
We all know torque is force times the distance it is being applied. With tires this distance is the loaded tire radius, which is not a constant value as it changes with load transfer. You now know how much torque you need to provide. You then work back through the half-shafts, differential, and transmission and take into account all the gear ratios and transmission losses to get the amount of engine torque required to get this longitudinal acceleration.
Now that all that ground work is laid out, to the question. The answer is "Yes". Your SR vs LF vs Load plot looks like a mountain on most tires, so once you exceed the peak force you actually have less tractive capacity and the wheel will spin faster and easier while actually providing less force. This is because you have no transitioned from static friction to kinetic friction at the contact patch. If you notice through all of this I have said nothing about HP and that is because it has nothing to do with wheel spin. You could spin the tires at peak torque or peak horsepower if the car has enough torque to pass the peak on your tire plot. Sorry if its a bit of a long winded answer, but thats the engineering behind it.
"Horsepower may get the car off the showroom floor, but torque is what gets it off the line" -C.S.
EDIT: I didn't go into the whole inertial side in this, but thats another story for another day. I just got out of my class on system modeling and don't really want to go through that again.
autogyro wrote:I think going through all the complex Math is exactly what the question was posted for. It remains impossible to answer.
Jersey Tom wrote:autogyro wrote:I think going through all the complex Math is exactly what the question was posted for. It remains impossible to answer.
...I'm still goin with plain and simple 'torque.'
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