Rear wheel HP

[jackh707]

Does anyone know if when caterham quote a 125 or 140, if they are talking about crank HP or rear wheel HP?

Just curious as what to call ours, we have 125 at the rear wheels (hub dynoed) , and were advised by our friendly dyno guy that you'd expect about a 37hp crank to rear wheel loss.

So is ours a a '125' or a '160'?

How much power have you got?

[kenno78]

Crank. All manufacturers quote crank figures.

Did they not give you an estimated crank figure when they gave you the wheel power?

[jackh707]

Yes and no, he said he reckoned 37hp on top. But anywhere between 30hp and 50hp is a typical loss for a rear wheel drive car.

But he didn't use the dyno to measure the losses because he said they are really inaccurate and its a bit pointless. As it's wheel HP that counts, I agree.
He new his stuff, and was talking to me about published papers on transmission losses in rear wheel drive cars, and that it's almost impossible to accurately measure with a dyno.

[kenno78]

I wouldn't say they were really inaccurate. I've used a number of dyno's now and they've all been quite close to the mark. (As in standard cars calculated as running near the manufacturers crank figures).

If crank figures don't matter, be happy with your 125 WHP

[DVandrews]

If it's taken on a rolling road then it isn't really wheel horsepower, but something akin. The losses on a rolling road are far higher than you would experience on the road as the tyre has unnatural compression from two small diameter rollers. Tyre pressures will have a profound affect on the 'at the rollers' figure too.

Dave

[jackh707]

It was hub dynoed, like I say in my first post so I don't think there will be huge losses at the wheels.

[DVandrews]

Sorry,missed that.. losses will be fairly minimal. The major loss is normally the tyres.

Dave

[EFA]

WHP is a meaningless figure unless you actually know your losses through causes such as tyres as Dave mentions (this varies enormously depending on tyre type/compound/tightness and location of straps holding car down etc, not to mention varying levels of internal friction.

Losses vary on a case by case basis. No two cars are alike. Losses should also be expressed as a percentage, as you would not have a 37bhp loss across all engine speeds.

If your rolling road operator is worth is salt, he will knock the car into neutral and perform a run down test after the power run. Your engines power is the delta between the power recorded and the losses.

[BertBert]

discussing whether either is meaningful or meaningless is meaningless unless you define the purpose for which you want the figure.
Bert

[Pumaracing]

jackh707 said:

Does anyone know if when caterham quote a 125 or 140, if they are talking about crank HP or rear wheel HP?

Just curious as what to call ours, we have 125 at the rear wheels (hub dynoed) , and were advised by our friendly dyno guy that you'd expect about a 37hp crank to rear wheel loss.

So is ours a a '125' or a '160'?

How much power have you got?

The losses on hub dynos have been well quantified by the Swedish Rotatest Institute. For fwd cars the hub figure is about 93% of the crank bhp and for RWD cars about 91%.

Your flywheel bhp is therefore 125 / 0.91 = 137 bhp give or take a bit.

Your "friendly dyno guy" clearly doesn't really understand his equipment if he's quoting the losses you mention.

[PeterBurgess]

We are 2 and a half years into runniung and learning our Dynocom rolling Road. This is best used as an inertia dyno as the runs are so repeatable and minute changes can be measured. We run in 4th gear (or whatever is direct drive) then do a coast down test to measure transmission losses. The figures when added together may or may not be 'genuine' flywheel figures but allow us to compare like for like cars and see if there are any transmission problems or variations from the norm for that setup.

It is a shame the culture of flywheel bhp guesstimates still obtains.

Hi Dave Andrews

Interesting your comments on tyre deflection for twin rollers. We have run a car on our twin roller Dynocom and on a single large roller Dynocom, same power curve so no extra tyre deflection. This was approx 175 bhp at the flywheel engine . Maybe when bhp gets into the 400 plus this may have an effect, will find out eventually I suppose. When I read LandSea and Airs criticism of twin rollers when I was in the process of modernising my equipment I was worried until I did a little more research for myself rather than believe what one or two Gurus or websites say. I made the choice to run twin rollers which I feel are intrinsically safer if there is a problem with tyres or transmission. Looking at disasters on YouTube the effects seem to be far worse on single roller dynos.

Hi Dave Baker

Rototest are guesstimates not measured I believe. We get about 5 or 6 bhp more losses for early IRS over a live axle. I do not think a % can be applied on any occasion. The losses we reckon are as Dave Andrews says, biggest culprits tyre speed, width and compound. As you know the tyre losses are exponential with speed.Tyre pressure is also vital, dropping from 31 to 18 psi cost 6 or so bhp, this is not a percentage. The diff does not seem to sap much power, a little more with slippy diff.

Peter

Edited by PeterBurgess on Monday 8th October 06:50

Edited by PeterBurgess on Monday 8th October 06:53

[DCL]

PeterBurgess said:

This is best used as an inertia dyno as the runs are so repeatable and minute changes can be measured. We run in 4th gear (or whatever is direct drive) then do a coast down test to measure transmission losses. The figures when added together may or may not be 'genuine' flywheel figures but allow us to compare like for like cars and see if there are any transmission problems or variations from the norm for that setup.

This has been my experience - a good operator and attention to detail does give surprisingly repeatable (and seemingly accurate) results. The coast down run returned figures that were spot on for our anticipated flywheel figure. Maybe just a coincidence but I do think you get what you pay for and a £50 power run isn't going to do much more than give a ballpark figure.

[BertBert]

DCL said:

This has been my experience - a good operator and attention to detail does give surprisingly repeatable (and seemingly accurate) results. The coast down run returned figures that were spot on for our anticipated flywheel figure. Maybe just a coincidence but I do think you get what you pay for and a £50 power run isn't going to do much more than give a ballpark figure.

So it's accurate because it gave the figure you guessed at to start with???????
Is there a DIN standard for guesswork?
Bert

[DCL]

Bert, Just saying it gave the same figure as quoted by Caterham. OK, their figures might be optimistic for a production line engine, but it's a good reference for an engine that had been carefully overhauled and set up - certainly not guesswork.

[PeterBurgess]

I realised when I said Rototest figures were guesstimates not measured I didnt explain myself clearly. Rototest have many test figures measured at the hubs with their equipment. To work out transmission losses they take whatever the manufacturer declares as the bhp/torque for 'that' engine then compare the results they get against what the manufacturers say they are. Would have been better if Rototest engine dynoed with their own equipment and then compared with hub figures from their own equipment, a little more scientific approach maybe? Trouble is it is very difficult to replicate engine dyno to fitted in engine bay.



Peter