dyno test: do they shift the bikes?

Nevada Sixx · Nov 14, 2002

hi,,
looking at the curve of a dyno test in a magazine is interesting..
but do they shift the gears while running it, or do they just put it in first and let it rev up?

steve125 · Nov 14, 2002

Good question! When i have had my bikes dynoed it's always been a 4th gear roll on. The operator will run the bike thru the gears to make sure is cleared out and fully warmed up. Then he shifts down to 4th and lets the bike slow down on it's own still in gear and when the rpm's are at 4k(for a 125) he rolls the throttle on evenly untill it's wide open and lets the engine scream through peak and over rev(around13k) then slows the drum with his foot brake and repeats for the next run or shuts down for the next engine change and then starts back up all over again. There is such a thing as a timed acceleration program for a thru the gears test, but ive never seen it done or data on a run like that.

bclapham · Nov 14, 2002

steve: if i recall, HP=torque x RPM, so how much effect does the gearing have on the rear wheel HP. for example, if you added 2 teeth to the rear sproket for your second run what would happen to the HP curve? do you measure the RPM from the back wheel or the flywheel?

grass-digger · Nov 14, 2002

yes i was wondering that also but you would think that to get a acurate reading of all bikes they would hook the bike up to the flywheel somehow????????so there would be no gearing involved just the motor

steve125 · Nov 14, 2002

The rpm is measured at the plug lead. In the Dyno Jet software it makes the adjustment for the gearing change. It gets its data from the drum rotation sensor and the spark rpm. So a few teeth either way(or just the difference in the 4th gear ratios of different 125's) is not a factor in the results.

Nevada Sixx · Nov 14, 2002

are you saying that the chart would look the same if you changed the gearing?

steve125 · Nov 15, 2002

Yep thats what the techs at Dyno Jet say. 4th gear is used because it allows time for the data to be collected. A run from 4k to 13k only takes around 10 seconds max.

gonzodog · Nov 15, 2002

Has anyone ever seen any dyno results with throttle openings other than WOT? Most trail riders don't spend much time at WOT and it would be informative to see how much torque & power the motor was making at 1/8 and 1/4 throttle. I realize there would be problems with calibrating exactly how far open the throttle is, but, it could be done with a little careful measurement of throttle position. Wouldn't that be a better measure of responsiveness?

steve125 · Nov 15, 2002

The dyno run begins at just about closed throttle and then its a roll on to full throttle at peak rpm's. The idea is to look at the power curve bottom to top so the information you seek is at the lower end of the chart. You could do a 1/2 throttle run and it would look the same as a normal run untill the upper mid rpm's, then it would flatten out on peak power. Remember the dyno does not show responsiveness, just hp or torque at a given rpm.

bclapham · Nov 19, 2002

steve: can you think of a cheapo road tyre that would fit on a 19inch 250 rim?

steve125 · Nov 19, 2002

Bruce what you want to look for is a street bike front tire. They range in sizes around 3.50x19 to 4.00x19 either will work. I saw a cheng sux for about 30$ and a Dunlop for around 50$. Check out the tires here www.800-841-2960.com

David Trustrum · Nov 20, 2002

To add; it is totally possible to run through the gears on a dynojet (and most modern types of dyno). This is a useful test & can tell you a bit about how much the power falls off between gear changes. Influencing this is when you normally change, (too early/too late?).

Gearing won’t affect it much. However development should always be done in the same gear.

It is interesting to note that sometimes the power tails off in higher gears. Why is this? Well it may be that the bike wasn’t warmed up enough & the crankcase is getting hotter reducing the density of the charge.

Or maybe the venting or cooling in the dyno room isn’t adequate.

But also the load is changing & you can diagnose, for example; maybe a fuel starvation or frothing problem that starts to show when the bike is in higher gears & doesn’t quickly flicking through the lower ones.

Food for thought.

dyno test: do they shift the bikes?

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