Best way to cheat a dyno? (using tyres)

[djones]

Just out of interest is it possible to slightly cheat how much power a rolling road says you have using different tyres or pressures?

For example I have two sets of tyres. A track day tyre and an average all year round tyre. Standard pressure is pretty much 30 PSI.

Would you be best off:

a. Track tyres - 50 PSI
b. Track tyres - 20 PSI
c. Road tyres - 50 PSI
d. Road tyres - 20 PSI

Presumably if everything is working correctly the 'coast down' bit should take these into account and show the same flywheel power though?

[Jersey Tom]

Depends how you're trying to cheat things. A track tire at low inflation (15psi maybe) will soak up a lot of power. A standard tire at 50psi will use up much less.

But, this should be able to be accounted for.

[bill shoe]

You might be able to cheat it a bit. I think this is "good" cheating, meaning cleverness as opposed to violating rules.

A chassis dyno uses rollers under the tires. The curvature of these rollers reduces the number of tread blocks making contact (compared to if the tire was on a flat ground surface). The curvature also changes how severely the tread blocks have to change angle as they go into and out of the contact patch. All this is lost energy.

You are right that this is partially compensated for with a coastdown run. My intuition is that a coastdown run with light load will not have a very hard time (will not lose a large proportion of power) due to this inefficiency between tread blocks and rollers. However, a highly loaded power run-up in a low gear is going to stress the heck out of the individual tread blocks as they get crammed through the awkward and small contact patch. I think this situation would lose a high proportion of power to the dyno rollers.

I have heard about extreme situations where large American trucks are put on chassis dynos. The dyno rollers are not very big. There can be as few as two rows of tread blocks per tire in contact with the rollers at any given time. Combine this with the large torque of a diesel engine and the large torque multiplication of a truck transmission and the tread blocks are very highly stressed (and thus inefficient). The tires will last for 100,000 miles on the highway but the overstressed and overheated tread blocks can start flying off the tires in a few seconds on the dyno.

Therefore my advice to get lowest possible power on a chassis dyno given your assumption of a coastdown run is:

1. Street tires with full/new tread and small tread blocks that will deform a lot and become inefficient under high load.

2. Very high air pressure to reduce the number of tread blocks in contact with the dyno at any one time, again to increase load on individual tread blocks.

3. Use the lowest possible gear (highest ratio overall). Can you decide what gear is used, or can you change out your gear ratios or final drive ratios? Again this is to increase torque multiplication through the drivetrain and thus increase stress and strain on the tread blocks.

4. If possible heat the tires up right before the dyno run. Ideally you would run them at say 80 mph for a few minutes. I guess tire warmers could also help. This will increase rolling resistance (bit counter-intuitive but true if the tires are warm enough). To help this process, you would want to reduce air pressure to something very low for the warm-up and then set it very high again for the dyno runs.

Again, my underlying assumption for all this is that the mechanical awkwardness of the tread blocks in the contact patch will not matter much proportionally during the mild coastdown run but it will matter a lot proportionally during the power-on acceleration run.

In order for you to determine if my advice is worth a crap you would have to do it my way and then do the opposite to see if there is any difference. If you have a chance to try this then please let me know the results. As a tire engineer this makes sense, but it's all theory and I've never tried it.

[djones]

Thanks guys.

Bill - So you think the low resistance tyre (ie normal road tyre) pumped up to say 50 PSI will yeild the best results?

If thats the case then I will try take my second set of tyres and do back to back runs.

Shall I run the track tyres at a low pressure to 'magnify' whats happening?

[bill shoe]

Yep, try that and see what happens.

[Belatti]

Its easier to give more bearing preload before de dyno run

[safeaschuck]

You could flatten the battery and jump start the vehicle prior to the run to maximise power losses... Do it secretly or just play dumb and pretend you accidentally left something on.
Overtightening the alternator alternator or power steering belts would also be a quick 'n' easy reversible process.
There are so many ways to do it the wrong way round!

[Fil]

i don't get it, isn't the point of such a dyno-run to see just where your car lays?
in the end you're only cheating yourself.

running a tyre with too little or too much pressure will result in your tyres slipping on the rollers, reducing chances of an accurate reading, in a negative manner.

if you want to maximise what your car has, then disconnecting ancillaries such as alternator/power steering etc will help.
remove your airfilter (whilst potentially risky, you have less-restrictive airflow)
want to get fancy? how about dry ice safely placed before your air intake?


by the way, what car is this for?

[Belatti]

We guess he may be trying to guess how to race in a series where the rules says 300HP max with a 380HP engine. We guess that in that series they control if the rulebook is being obeyed with a dyno run

BTW, with the safe as chuck tips you usually have to open the hood... and in that types of "dyno controlled series" the hood is usually sealed. The driven wheel bearing preload is something you can change with a sealed hood

[autogyro]

Of course you could measure the power train figures using a hub mounted dyno.
We used to use one imported from New Zealand.
It was also mobile, so you could take it in a van to test days and race meetings.
Good for one make series of course.