Dyno Question

[Peter Wright]

Having a Dastek UniChip fitted in two weeks time and as regards to the Dynograph results would it make any difference if I had my 15" or 17" wheels on

Pete   s:) s:)

[Slacey]

Good question Pete - I' not sure exactly how a rolling road works, but my G-Tech is dependent on the weight of the vehicle, meaning that as long as you compensate for heavier / lighter wheels, the result will be exactly the same. I think we need one of the techie guys on this one  s:oops: :oops: s:oops:

[Tem]

I think we need one of the techie guys on this one  s:oops: :oops: s:oops:

I'm not one of them, but here goes anyway  s8) s8)

Heavier wheels add to drivetrain loss, cause they need more power from the engine to make them roll, so you will get less rwhp with heavy wheels. If the dyno measures the drivetrain loss and uses it to calculate flywheel hp, the wheels don't matter. Over here just about any dyno does that, no idea about UK really.

If you know how different weight flywheels affect hp, wheels are pretty much the same.

[MRMike]

Yeah good question!  If i remember correctly Sean is right, if you set up the dyno properly taking into account any differences in size etc there will effectively be no difference to the power recorded on the dyno. However on the road, it is observable that a larger, heavier alloy will knock 0-60, 1/4 miles times etc as a result of an increase in unsprung mass, etc. The power at the flywheel has effectively not changed.

Here's a roundabout explanation of this..i think!

SPEED
at 10 kmph (6.1 Mph) you travel 10,000 metres per hour, or 166.67 metres (546 feet) per minute or 2.78 metres (9 feet) per second
26inch (Not saying put these on your MR2!! easier to calculate big no's!)has a tyre curcumferance of 1.990 metres (6.5 feet)
20inch has a tyre curcumferance of 1.275 metres (4.1 feet)

THEREFORE
a 26" wheel has to do 1.4 revelotions per second to do 10 Kmph
a 20" wheel has to do 2.2 revelotions per second to do 10 Kmph

But the tire is still traveling at the same speed (10 kmph), so
tyre size will not affect a tire driven dyno, if set up correctly.  I think thats right ! forgive me if im wrong!

The best tyre/performance comparison I have ever found is here

 m http://www.siscom.net/~welter/racing/rotation.html m

It's really good spreadhseet if you ever intend changing wheels etc..although sometimes the lure of shiny chrome wheels is too much (I had 18" chrome kahns I know i know..destroyed the performance of my little pug!

Mike

[Tem]

I was assuming the tyre diameter was same with both wheels...  s:? :? s:?

[GSB]

A rolling road dynamometer is NOT an accurate way of measuring specific power, due to there  being so many variables that can affect recorded power and so have to be compensated for.

However, it is an extremely good way of measuring changes in power, as long as when you are using a rolling road dyno you maintain all other variables bar the one you want to change between runs. things like air temps, fuel, tyre pressures etc...

i.e. As long as you dont switch betwwen 15"s and 17"s between runs they wont make a blind bit of difference, as any influence they have on the dyno will be be the same before and after fitting and will therefore be cancelled out...