Discussion

power bhp = (rpm x torque) / 5252

so you'll need the rpm of the wheels (which you can get from vehicle speed and the tyre's circumference)

torque can be made by gears, but power is torque with respect to the rpm, so won't change (significantly, anyway)

by that, it means a 45bhp fiat Uno can put out more torque at the wheels than a Scania, but it's take it a day to move 6 inches...

As a result, torque means nothing if it isn't linked to an rpm, and when it is linked to an RPM, then it's BHP anyway!

TheEnd said:

power bhp = (rpm x torque) / 5252

so you'll need the rpm of the wheels (which you can get from vehicle speed and the tyre's circumference)

torque can be made by gears, but power is torque with respect to the rpm, so won't change (significantly, anyway)

by that, it means a 45bhp fiat Uno can put out more torque at the wheels than a Scania, but it's take it a day to move 6 inches...

As a result, torque means nothing if it isn't linked to an rpm, and when it is linked to an RPM, then it's BHP anyway!

If you want to know wheel horsepower, test at the wheels, if you want engine horsepower test at the engine.

With many years experience some people can make a pretty good estimate but that is all it is, an estimate.

Obviously, if you have the raw numbers from a rolling road, you already have this, but mostly what people have are either manufacturer's flywheel figures or estimates of flywheel figures based on rolling road or chassis dynos.

Ignoring transmission losses, if we know that a car has 200lbft @ 5000rpm at the flywheel, and 2nd is geared to 10mph per 1000rpm, we should be able to calculate the torque at the rear wheels at 50mph in second. That gives the maximum theoretical torque, we can then argue about what the transmission losses are

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otolith said:

Those numbers look a bit high! About 10x too big in fact.....2 ways to do it....

Torque at engine x gear ratio x final drive ratio x transmission loss (0.85 (85%) ish for normal FWD)

(Power at engine x transmission loss) / angular speed of wheels

In kW / Nm / radians per second

Edited by blank on Tuesday 11th November 18:29

otolith said:

blank said:

Torque at engine x gear ratio x final drive ratio x transmission loss (0.85 (85%) ish for normal FWD)

Is that not leaving the wheel out of the system?The scale on the graph may well be out by an order of magnitude, it's not my graph and beggars can't be choosers

1) Measure it yourself.

2) Guess it from your vehicle specifications.

3) Get someone else to measure it for you.

The methods:

1) Okay, this is only going to be an estimate, but:

a) Time how long it takes you to accelerate from 10 to 30 mph in first gear.

b) Divide 8.94 by this time (in seconds) to calculate your ACCELERATION in meters per second squared.

c) Multiply your acceleration calculated in b) by the gross mass of your vehicle (ie including driver, fuel, junk in boot) which must be in Kg. This will give you your accelerative FORCE.

d) Measure the radius of your wheel in meters (or calculate it from the data on the sidewall {Width in mm x Aspect Ratio / 100,000} + {Rim Diameter x 0.0127}).

e) Multiply the force calculated in c) by the radius calculated in d) to get your wheel torque in NEWTON METRES.

f) Divide this figure by 1.36 to convert to ft lb.

g) Multiply by top speed in first gear and divide the answer by top speed in other gears to get the figure for those other gears.

2) Take your engine torque figure and multiply by drivetrain efficiency (approx 0.8), gear ratio and final drive ratio.

3) Get it dyno'd. A dyno actually measures torque and speed, from which it calculates power.

liner33 said:

http://www.pumaracing.co.uk/trans.htm

also

http://www.pumaracing.co.uk/coastdwn.htm

I have to find the logic for the above raw data, to get 1.5 order wheel torque value, which is close to resulted value as attached (Result_1.5 order torque value as attached).

I am working on this, I request all members of research gate forum to help me on this to create a logic/formula.

If the above information is not clear enough, please ask me

Many thanks,

CKS

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