TheEnd
14,372 posts
75 months

well, if you want to work out wheel torque, you need a wheel power figure, and reverse the calculation of
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!


RobCrezz
7,854 posts
95 months

TheEnd said: well, if you want to work out wheel torque, you need a wheel power figure, and reverse the calculation of
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! Exactly this.


Callan.T89
8,422 posts
80 months

Deep Breath.... It's actually impossible to completely calculate transmission loss unless you test exactly the same engine on an engine Dyno and then on a chassis dyno (an then it's only a measurement not a formula). You can't do a reverse calculation, there are just far too many variables. I studied it at uni' and it is a subject way too compliacted for an internet forum.
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.


otolith
26,460 posts
91 months

I had the impression that what the OP wanted to know was given a torque or power curve for the engine (either measured at the wheels and already "corrected" to the flywheel or measured at the flywheel), and the gearing of the car, how do you estimate torque at the wheels to get a graph like this: 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


RubberDucky
223 posts
92 months

dr.pepper said: As per the topic title, i am trying to figure out how to calculate the actual amount of torque a car produces at the wheels. Mass of vehicle x Acceleration x Radius of wheel. Easy!

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blank
2,260 posts
75 months

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


otolith
26,460 posts
91 months

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


blank
2,260 posts
75 months

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 Torque at the wheels will be the same as coming out of the diff as they're turning together on (essentially) the same shaft. You only need to bring the wheels into it if you want the force on the road.


RubberDucky
223 posts
92 months

Okay, I'm dragging up this topic again but, having thought about it, there's 3 ways to get a torque at the wheels figure: 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
4,303 posts
89 months

I dont buy this idea that transmission losses are a percentage of horsepower made that would mean a 1000hp horsepower car has ten times the trans losses of a 100hp car and whilst i will accept that as the horsepower goes up the transmission losses can also go up the effect will be minimal


otolith
26,460 posts
91 months

liner33 said: I dont buy this idea that transmission losses are a percentage of horsepower made that would mean a 1000hp horsepower car has ten times the trans losses of a 100hp car and whilst i will accept that as the horsepower goes up the transmission losses can also go up the effect will be minimal Good article: http://www.pumaracing.co.uk/trans.htmalso http://www.pumaracing.co.uk/coastdwn.htm

