Physics question for Nerf

[ComradeAsh]

Hey Nerf, can you solve the following question with the variables that I have, or do you need more information?

If a car, in top gear, can do 127kph @ 2000RPM,

How many RPM is it doing at 200kph?

[Professor_Skullsworth]

By my calculations, it looks like you'd need to turn 3150 rpm

[ComradeAsh]

Originally Posted by Professor_Skullsworth

By my calculations, it looks like you'd need to turn 3150 rpm

Does doing a straight errm...whatever the function is work, though?

Isn't there some rule about more power required at higher speed?

That said, I would assume the lines are moving a little too fast at 200kph to want to take your eyes off to look at the tacho and maybe the fuel consumption gauge for bonus points.

[Professor_Skullsworth]

Yeah my calc was in an ideal situation (didnt have the numbers/wouldnt know what to do with them for any other factors) I suppose drag coefficient would have and effect, and perhaps a tires ability to supply traction at high speeds.

Honestly I cant think of too many things bedsides that having a tangible effect, however, im just a wrench, im no scientician

[Animal Farm Pig]

Yeah, it should be 3150 RPM's.

Unless your clutch is slipping, this is a fixed ratio. Think of it this way-- due to gearing, tire size, etc., your car moves at ~63 MPH for every 1000 RPM's the engine turns in top gear. Change the gearing or tire size, and it will be different.

[BoilingLeadBath]

Mmmm... effective tire diameter will change. At higher stable speeds, the torque@tires required to keep the car going that fast will cause a slight slip of the tires.

http://www.drivingfast.net/new/image...ency-graph.gif

Not very damn much, mind you... wind resistance at 200 kph is only a few hundred pounds, which requires (300lbs/2500lbs*.8) = 10% of maximal breaking force.
Which, if the graph is correct, suggests [slip]<1%

Well, maybe. This is all hypothetical.

***********

In other words:
1) The guys above are correct
2) I'm wasting time waiting for things to happen.

[HLC]

Originally Posted by Professor_Skullsworth

Yeah my calc was in an ideal situation (didnt have the numbers/wouldnt know what to do with them for any other factors) I suppose drag coefficient would have and effect, and perhaps a tires ability to supply traction at high speeds...

As mentioned earlier, the rpm of the tires doesn't change with drag and such.
those factors will only affect the horsepower needed to get to that rpm.

[Nereth]

The ratios are fixed (ignoring clutch slippage) in a manual, so the immediate answer is the speeds are proportional to the RPM, and that would give you a solid answer. I think the people above me probably did this calculation just fine.


But if you want to be more accurate, tyres have a slip ratio, that is, in order to provide grip they actually turn faster than you would expect them to to match with the road, they are in fact slipping, slightly, over the road. The magnitude of that effect could be anywhere up to about 20% of road speed but depends highly on the tyre.

If you intend to take that into account you need a *lot* more data because the magnitude of slip is proportional (roughly linearly, if you are not near or beyond peak traction, I believe) to the drive force the tyre is given out, and knowing that also tends to require a lot of data. So here is the new list

-Tyre slip ratio data, damn near impossible to get, but can be estimated to about 20% accuracy based on rules of thumb, I imagine.

-Tyre rolling friction data, probably not so hard to get and more easily estimated, also perhaps not so important

-Car aerodynamic data, quite tough to get but not too hard to guestimate

-Car CGH and wheelbase, reasonably easy to get and easy to approximate

-Gear ratios, if you want to have a more accurate measurement, because you cant deduce them from the low speed run because this to, is effected by the tyres slip ratio. You could run the maths for the low speed run as well offcourse but things tend to get dodgy when you get data from other data.

-Road grade (slope) for obvious reasons, and perhaps surface quality.

All things considered it will probably be easier to just measure this.