Transmission losses

[itiejim]

Can anyone give me an idea of transmission losses on a Cerb with 255/35/18s on? I'm just playing around with an app on my new phone.

And yes, I do know that it's no substitute for a few hours on Joos dyno

[gruffalo]

It varies but Dom told me about 80 was average.

[itiejim]

Cheers, so about 20%. Gives me a starting point to work from.

[Redmist336]

You'll never get anywhere near 20% drive line losses. Energy isn't destroyed, just converted. As in you're talking of approx 80hp converted to heat and sound. You'll also have to remember that drive line losses aren't a percentage of power. There isn't a linea correlation between power and loss. It certainly increases with power but no linear correlation. For a silly approximation I'd guess more like 10-12% and I wouldn't be surprised if the difference between a engine dyno and wheel dyno is on the lower of that estimate.

[jammy_basturd]

I know definitely of other cars, 15-20% isn't unheard of.

[fatjon]

20% loss is excessive. All the losses are converted to heat and 20% of 400BHP is 60KW.

Let's picture that as 12 kettle heating elements in the gearbox and 12 in the diff housing. It would all be a molten pool on the floor of the dyno cell within 60 seconds, well not quite literally but after 100 seconds the gearbox oil would be smoking and the transmission would be a useless pile of scrap, the diff would be damn near catching fire and certainly stuffed as it would be approaching red heat. I have run mine on a dyno several times and can report that after 5 or 6 good runs the box and diff were only slightly warm to the touch. I would venture to suggest that the heat in the box was largely down to it's proximity to hot exhaust manifolds.


These silly loss figures are spouted by dyno operators and engine builders, the reality is probably nearer 5-7% maximum. You could actually measure the temp rise in the gearbox/diff and their oil mass to get a very accurate figure but it's not in the interest of most people giving out these numbers to do so.

[supersix]

Redmist336 said:

You'll never get anywhere near 20% drive line losses. Energy isn't destroyed, just converted. As in you're talking of approx 80hp converted to heat and sound.

Mine certainly generates a stload of both of those.....

[Vixpy1]

Your all forgetting that most of the power lost is not lost through the drivetrain, its lost through the tyres.

We've done some testing and it seems that about 9 or 10 % is lost through the transmission, the rest through the tyres.

[gruffalo]

supersix said:

Redmist336 said:

You'll never get anywhere near 20% drive line losses. Energy isn't destroyed, just converted. As in you're talking of approx 80hp converted to heat and sound.

Mine certainly generates a stload of both of those.....

+1 in France at the classic last year after a couple of hunderd miles run through the back roads the gear lever was to hot to touch for more than just the time needed for the quickest of gear changes.

[Redmist336]

So you're saying that transmission and tyre losses generate 60kw of heat? Have you ever sat near a 60kw heater? (edit: sorry for the repitition just read FatJon's almost identical post)

I've also never seen differences of 20% between chassis and engine dyno's in even 4wd vehicles.

Although I haven't engine dyno'ed any of my race cars they are all generated using very accurate computer modeling.

My Cosworth/Nissan offroader, with huge CV angles, massive torsen differential, straight cut gears, huge 35 inch tyres will generate substancially more driveline losses than my (or any other) Cerbera. The computer modeling puts the flyhweel hp at 700hp at 16psi of boost. It's generating 609hp at the wheels. That's a 15% driveline loss and CONSIDERABLY more than you'll see in a Cerbera. I believe both computer model and WHP to be very accurate.

These massive driveline loss percentages tend to be made up figures so that dyno operators can show that they have high HP figures coming from thier dyno. This in turn makes them the best tuner "I can generate more HP than any other tuner" I don't believe any of it.

Edited by Redmist336 on Thursday 13th January 02:29

[Redmist336]

gruffalo said:

supersix said:

Redmist336 said:

You'll never get anywhere near 20% drive line losses. Energy isn't destroyed, just converted. As in you're talking of approx 80hp converted to heat and sound.

Mine certainly generates a stload of both of those.....

+1 in France at the classic last year after a couple of hunderd miles run through the back roads the gear lever was to hot to touch for more than just the time needed for the quickest of gear changes.

So you would say 1/2 kw of power loss in heat? You certainly weren't hanging onto a 2kw bar heater were you? (if so might I suggest there is something seriously wrong with your gearbox).

I'm not suggesting that the gearbox is only loosing a 1/2 kw of energy. There is certainly a considerably bigger surface area to transfer heat to air, however I'm just trying to demonstrate the level of heat we are talking of. 60kw is huge!

[gruffalo]

I do not think that expressing the power loss as a percentage is the correct way of doing it as increasing the engine HP would not increase the drag through the gear box and the rest of the drive line, i believe it is more of a finite number at a certain speed and also depending on what gear is being used.

I may be wrong I have just mentioned a figure that I was told by a bloke who spends most of his time mapping cars on a rolling road. I asked him when I was on the phone to him ordering a chip for my Cerb ECU when I asked about what BHP I could expect the chip to release.

[fatjon]

I'm not sure you can really count tyre heat in the losses. It is a consequence of creating the traction and is uniform pretty much regardless of the engine output. In any case it is not up in the many kw range, if it was even into double figures the rubber would catch fire very quickly. Rubber has a specific heat capacity of 0.45 joules/kg/c which is half that of aluminium. With a weight of 5Kg each the two rear tyres would heat up 12 times faster than the transmission (assuming 30kg for that). That would put them at 150C in 15 seconds and on fire in 60 seconds. I think the bottom line is that if you look at the physics of the driveline losses claims they just don't stack up. The maths is a bit rough and ready but allowing even +/- 25% on the figures the loss claims are patently cobblers.

It's pretty easy to measure the driveline losses just by monitoring the temp rise in the gearbox and diff. My figure was based on the box being about 30Kg and the diff about 15Kg. I also assumed that the majority of this weight is ally which has a higher heat capacity that steel so if anything the numbers err on the low side of the heating effect, ie chuck 60Kw at it and it will actually get hotter much faster. The tyres have a low heat capacity and a small mass, it takes very little heat input to reduce them to ash, they cannot have any significant effect on losses, if they absorbing even 5% of the (cerbera) engine output (15Kw) they would cease to exist in seconds. A 1/2 mile Vmax run would be impossible as the tyres would get hit with 360Kj. Looking at the actual temperature rises in tyres on a dyno my best guess would be 2-3% when they are getting a lot of hammer and some wheel slip, rising rapidly when slip increases.

Edited by fatjon on Thursday 13th January 10:02

[FarmyardPants]

It's not a closed system, though, so one can't assume that ever joule of energy will heat the tyres up. The tyres are spinning in air (with a big fan) which is cooling them down, and the hotter they get the faster they dissipate their heat as the temp difference with the air increases. By the same logic a 1kW bar fire would melt after a few minutes

I do agree that the amount lost is exaggerated, though. But since everyone tends to use the the 15-20% range at least the figures are vaguely comparable. It's an unofficial standard, does it matter if it's not technically correct?

To the OP, mine was 342 atw and quoted at 420 at the fly if you're collecting samples