Power @ Hubs V power @ wheels

Hi ,

has anyone ever power run a car on a chassis dyno and hub dyno to compare power outputs .

We recently mapped a championship winning car on our Maha LPS 3000 dyno which produced 2% under the max power quoted in the regulations .

The regulations quote "power at the wheels" but the car was tested directly after winning its class on a hub dyno which showed 3 % over the stated at the wheels power

Just trying to build a case to dispute the fact that the regs say wheels not hubs

Thanks Dave

The simple answer is the chassis - wheel driven - dyno' has additional parasitic losses from the internal and external tyre friction.

If the rules say "at the wheels", what it's actually showing at the hubs means precisely NOTHING, because that isn't what the rules say.

If there is a protest of some sort that needs to be done on a certified wheel driven dyno', and even then there are potential issues. Heck, if I were the owner of the vehicle being tested, I'd be looking at the heaviest, softest, fully treaded tyres at the minimum pressure and, if it's being checked on an inertial dyno', I'd use the heaviest wheels I could find that met the rules, in the lowest acceptable gear - the greater the frictional and inertial losses the lower the "official" figure will be, and the greater the gain when using lightweight wheels, correctly inflated tyres, etc.

Personally, because the wheel/tyre combination can skew results so badly, I'd suggest you pushing for your hub dyno' to be the standard test machine, and you keep it up to date on certification. Is that your point?

The Maha LPS 3000 is a roller dyno so I believe Dave is saying he prepped on a roller dyno, the rules saw "power at the wheels" but the car was tested on a hub dyno for compliance.

You can absolutely make the case that power read higher since wheels/tires were off it, reducing losses from that rotating mass and friction. Whether they will go for that or not, I don't know.

In addition to the actual difference in the measurement method having the wheels/tires present or not, you also have the variance between dynos to contend with. Different brands of hub dyno read differently, different brands of roller dyno read differently. On top of that there are also settings which impact the outcome.

When I remote tune for a shop with a Maha roller dyno, the loading they choose has massive impact on not only engine operation, but also the HP and TQ results, so especially if you're prepping on that dyno, I believe you're going to get different values when testing at the track.

For the future, if it's an option to you, I suggest electing to test the car on the control dyno right before the race in case a small adjustment is needed to ensure the car is compliant when it goes on track.

Best of luck!

Hi ,

thanks for the replies ,we have discussed the issues directly with the company who carried out the hub dyno test and to say the goal posts are running around the pitch is an understatment :-

1 . They say they have built in a "conversion factor " from hubs to wheels but wont tell us what it is

2 .The vehicle was run in 4th gear 1.216 :1 , we ran in 5th 1.024 :1 as the regs say direct gear which we had previously asked for verification but never recieved any

3.All engine covers were removed due to their lack of cooling fan capacity on the hub dyno ,we mapped the car with 2 x Maha 26500 m³/h fans but in race conditions with all engine covers installed

The championship runs a control slick tyre which were fitted when we ran the car

We have discussed the dyno set up with Maha who have teamviewered in to our system and checked roller mass calibrations etc

The car was checked 1/2 way through the season on their hub dyno and then run on our rollers to confirm any increases in power

Unfortunately the customer has invested over £35000 in running costs to run in this "high profile" championship and we dont seem to be able to get any satisfactory explainations from the control dyno company who also ran a number of cars in the same championship and were outpaced by our customer

Well, personally, I would say 1/ is proof they're talking out their bottoms.

I would actually go as far as to say I'm almost 100% confident in saying cheating with their/their oppositions' vehicle's engine output figures - having the same company verify the vehicles that also has vehicles in the championship leaves SO much room for them to "adjust" things in their favour is why INDEPENDENT testing companies should be used, or at least the checking is done under supervision of a REPUTABLE race series official, with the engines sealed after testing.

About the only thing you, and your customer, can do is immediately after the vehicle is run on the hubs, re-run it on your rollers with the same running conditions and new tyres to correct pressures - the aim being to have a direct comparison between the different dyno' numbers*. It won't help you at this point, but in future you should know very close to what hp you can run on the rollers that won't exceed their hubs.

Seems the reg's are a bit slack, too - it should have said something like "In a direct 1:1 gearbox ratio, or the nearest gear to that if indirect"

*As Mike pointed out, different dyno's, test procedures, ambient conditions, correction methods - that's a biggie, and the like, make direct comparisons impossible - best you can do is to get a repeatable comparison/correction between them.

Dave was the car tested in 5th gear when they did the hub dyno check half way through the season?

If the car passed scrutineering then, and the procedure wasn't repeated properly for the final test, that seems to lend itself to the argument that the car didn't receive a power increase, the dyno procedure changed.

Hi ,

we are trying to find out how the hub dyno company have calculated this "conversion factor " from power @ hubs to Power @ wheels as it seems that some of the other cars racing in the same championship but different class have different gear ratios and tyre sizes but they apply the same conversion rate .

In my experience which is very limited on hub dynos we have found the hub power to be around 6% higher than wheel power ,but unfortunately the hub dyno wasnt using any temp or baro corrections and the ambient temps were different

Has anyone ever tested back to back hub v wheel on the same car