Until I can get down to JKM for the rolling road day, I thought I'd give the butt dyno a go.

Went out on a session last night logging speed over time, including the decelleration parts.

Plugged all the values into the various spreadsheets, checking it's OK.

Put in approx weight of car + fuel + a lardy Xav

Results are way off, probably around 25%.

I've checked the calcs, but can't figure out if I'm doing something wrong or have combined wrong calculations, etc.

I've attached a spreadsheet with three tabs: first showing raw values, second with the decelleration calcs and the third with the values plugged in to produce the plots.

Any ideas where I'm going wrong?

xavdyno.xls

what gear and does it matter you did not exceed 4k rpm?

3rd gear, and did go up to around 4000 rpm. I don't think it should really matter. Ideally, I'd do it in fourth so it takes a little longer, providing more sampling points, but I'd need an unrestricted road for that!

Bump, anyone?

Not something Ive seen before but theoretically its principals reflect that of a rolling road. Just can't take into correct account for wind resistance, surface resistance and probably loads of other things!

It does take resistance into account because you're also measuring the decelleration.

But I just can't get reasonable figures out of it...