Chassis Dyno. Am I missing figures in my calc? Torque looks high!

[bob8907]

Basically, for my own= information, I am looking at how chassis dyno torque is calculated. They all seem to come in below where the engine torque should be BUT there is ALWAYS a gear reduction to the final output which multiplies torque. For example, one vehicle I looked at was about a 3:1 in 2nd gear. Obviously torque would be multiplied but the RPM would also be reduced by that factor SO..... HP should remain the same as it is a product of torque and RPM.

However, the Torque to the rollers would be about 3X (assuming no Cf losses). Basically, you have a 100ft/lb motor, a 3:1 total ratio, and a 24" rear tire, you will get 300lb/ft to the rear axle. Obviously cars make MUCH more than this so thousands of lbs of torque would be seen at the load rollers. Am I missing something???

 

[Bribyk]

You have another gear ratio to consider, the difference between the diameter of the roller and the diameter of the tire.

Brian Bobyk - Hoerbiger Canada

 

[JSteve2]

Dyno torque has nothing to do with at-the-wheel torque (not in the sense you are thinking, anyway). Dyno torque is proportional to the amount of work being provided by each combustion event (or engine rotation, if you prefer). Dyno torque is calculated by the amount of force required to resist the engine rotation. Therefore, torque (dyno) * rpm / 5252 = horsepower (dyno). The 5252 is just a unit conversion (550 * 60)/(2*PI).

At-the-wheel torque does not appear in there. At-the-wheel torque would require you to have all gear ratios, current engine power, tire size, and current vehicle speed, but that would not be (directly) related to dyno torque. Of course, increasing dyno torque output, all other things held equal, increases the at-the-wheel torque.

 

[JSteve2]

Skimmed right over "chassis" - duh. Roller size, tire size, rear axle ratio, transmission gear ratio - did you make sure to account for all of those?

 

[TrackRat]

Chaasis dynos typically measure HP via mass acceleration and then try to convert it back to torque with varying results... There are also several types of chassis dynos which result in slightly different data.

 

[bob8907]

Yes, gentlemen, I was accounting for the roller, final ratios, etc. To look at it at an elementary level, If a car goes 40mph at 6000rpm in first gear with 24" tires, that is 6000/560 or 10.7:1. I am using a 24" tire to make things easy calculating torque to the ground.

I think the load roller is irrelevant at this point but lets just say a 24" roller to make things easy. This means that if the engine actually makes 250lb/ft of torque at 4000rpm, and 285HP at 6000rpm, at the load roller or rear tire, we would see 2675lb/ft of torque and 285HP (2675*560/5252).

Obviously there can be some back calculations for torque but it sure seems that nearly 3000lb/ft of torque would be seen at the load roller if 1st gear was used. I realize the goal is to run a test in a higher gear to reduce the torque level but it wold still show over 1000lb/ft of torque to the load roller.

Am I missing something??

 

[NormPeterson]

Sanity check: if you convert your rear wheel torque numbers to a tractive force at the ground, does the resulting acceleration (or the likelihood/certainty of wheelspin) make sense?


Norm

 

[bob8907]

This is the point of my question. I would like to think I have missed something BUT, would you agree that 100lb/ft through a 2:1 gear reduction will produce 200lb/ft but at a lower rpm?

I certainly hope I am missing something but I cannot figure it out thus far....

 

[NormPeterson]

100lb/ft through a 2:1 gear reduction will produce 200lb/ft but at a lower rpm

Yes. Less a little for losses.


Norm

 

[bob8907]

We are assuming perfect world here. No need to look at the Cd right now. For the sake of simple understanding, lets assume no drag anywhere.

 

[imcjoek]

What makes you think 3000 ft-lb at the drive wheels is unreasonable?

 

[bob8907]

Well, the math certainly works. However, I realize that testing in 1st gear is not advisable for this reason. Holding traction with 3000lbft of torque could prove an issue..

 

[JSteve2]

That engine could theoretically accelerate a 4,000 lb car at 0.67 g at the optimal operating point, less averaged through a range of engine operating points and increasing drag with speed. I don't think that's inherently unreasonable.

 

[bob8907]

Dido. I think we have finally put this one to bed and our math is correct. A lot of torque in 1st gear but the numbers do not lie.

 

[patprimmer]

One factor I missed from above, is if the transmission is an automatic, there is torque multiplication at the torque converter.



Regards
Pat
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