General Chassis Dyno Questions
04-30-2003, 11:47 PM
#1
Instructor
Thread Starter
General Chassis Dyno Questions
I understand what torque and horsepower are. I have a dyno question that I have yet to reason through, or get the answer.
Why have I never heard mention of gearing when people mention dyno results? I understand the tests are done in top gear, or maybe as close to 1:1 as possible, but doesn't the final drive ratio come into play somewhere? Doesn't the final drive ratio directly effect the vehicles ability to accelerate either the road surface, or the measurement drum, or spindle? Since the horsepower "aspect" has a time element in the equation, wouldn't a steeper set of gears provide a "false" horsepower reading? Or is there a formula somewhere that compensates for this? I am sure someone here has the answer.
Thanks
Why have I never heard mention of gearing when people mention dyno results? I understand the tests are done in top gear, or maybe as close to 1:1 as possible, but doesn't the final drive ratio come into play somewhere? Doesn't the final drive ratio directly effect the vehicles ability to accelerate either the road surface, or the measurement drum, or spindle? Since the horsepower "aspect" has a time element in the equation, wouldn't a steeper set of gears provide a "false" horsepower reading? Or is there a formula somewhere that compensates for this? I am sure someone here has the answer.
Thanks
05-01-2003, 08:20 PM
#2
</font><blockquote><font size="1" face="Verdana,Tahoma,Helvetica">quote:</font><hr /><font size="2" face="Verdana,Tahoma,Helvetica">Doesn't the final drive ratio directly effect the vehicles ability to accelerate either the road surface, or the measurement drum, or spindle? Since the horsepower "aspect" has a time element in the equation, wouldn't a steeper set of gears provide a "false" horsepower reading? Or is there a formula somewhere that compensates for this? I am sure someone here has the answer.</font><hr /></blockquote><font size="2" face="Verdana,Tahoma,Helvetica">Yes, and a competant(honest) dyno operator will include that figure to obtain the final HP and Torque figures. Some one that is out to artificially inflate their #s will not.
The raw #s will appear better in a lower gear than in a higher. Final drive ratio and tire radius are some other chassis related figures that can affect the data.
Of course some dynos do not go through the tires and are excetions to the above.
The raw #s will appear better in a lower gear than in a higher. Final drive ratio and tire radius are some other chassis related figures that can affect the data.
Of course some dynos do not go through the tires and are excetions to the above.
05-02-2003, 01:27 AM
#3
Race Director
"Why have I never heard mention of gearing when people mention dyno results? I understand the tests are done in top gear, or maybe as close to 1:1 as possible, but doesn't the final drive ratio come into play somewhere? Doesn't the final drive ratio directly effect the vehicles ability to accelerate either the road surface, or the measurement drum, or spindle?"
Ok, here's how a DynoJet works in a nutshell.
1. Known givens - are factors that have pre-determined such as the drum's diameter, mass, moment-of-intertia.
2. Time and RPM of drum - is then measured during the dyno-run. By measuring the RPM of the drum with a fast clock, its rate of acceleration against the known mass and intertia can be measured.
3. Torque at drum - can then be computed using the time and RPM data. Obviously, torque at the drum will be different for different gears, with lower gears like 1st gear which can lay down over 1000-lb•ft.
4. HP at wheels - can be computed from this torque figure because the drum's diameter (radius) is known as well as the RPM.
5. RPM of car - this is the secret ingredient! By calculating the ratio of the car's RPM vs. the drum's RPM, you can then calculate torque at the wheels. For example, 4th gear is typically a 1:1 ratio and there may not even be any gearing, just a lock to couple the input & output shafts together. This would then give the highest torque output because there's no friction through gear-reduction/multiplication.
After you run through the gear reductions in the tranny and the diameter of the tire, if the ratio of RPMs 5x (car's engine spins 5x faster than drum), the torque-at-the-drum is simply divided by 5 and the result is torque at the wheels.
Let's say you were to use 2nd gear instead which is typically 1/2 as tall as 4th gear and can generate twice as much torque. Due to the gearing, it will only be able to spin up the drum to 1/2 the speed of 4th gear. The ratio of the engine's RPM vs. the drum's RPM will be different (10:1) and dividing out the 2x higher torque-at-the-drum by this factor of 10 will result in the same torque-at-the-wheels figure as before in 4th gear. Probably a little less since you'll have more tire-slip in 2nd and more friction through the gearing, resulting in lower acceleration of the drum.
Another interesting note is that the only thing that is actually measured is torque at the drum, drum RPM and engine RPM. The car's HP and torque at the wheels are then calculated from that data.
Ok, here's how a DynoJet works in a nutshell.
1. Known givens - are factors that have pre-determined such as the drum's diameter, mass, moment-of-intertia.
2. Time and RPM of drum - is then measured during the dyno-run. By measuring the RPM of the drum with a fast clock, its rate of acceleration against the known mass and intertia can be measured.
3. Torque at drum - can then be computed using the time and RPM data. Obviously, torque at the drum will be different for different gears, with lower gears like 1st gear which can lay down over 1000-lb•ft.
4. HP at wheels - can be computed from this torque figure because the drum's diameter (radius) is known as well as the RPM.
5. RPM of car - this is the secret ingredient! By calculating the ratio of the car's RPM vs. the drum's RPM, you can then calculate torque at the wheels. For example, 4th gear is typically a 1:1 ratio and there may not even be any gearing, just a lock to couple the input & output shafts together. This would then give the highest torque output because there's no friction through gear-reduction/multiplication.
After you run through the gear reductions in the tranny and the diameter of the tire, if the ratio of RPMs 5x (car's engine spins 5x faster than drum), the torque-at-the-drum is simply divided by 5 and the result is torque at the wheels.
Let's say you were to use 2nd gear instead which is typically 1/2 as tall as 4th gear and can generate twice as much torque. Due to the gearing, it will only be able to spin up the drum to 1/2 the speed of 4th gear. The ratio of the engine's RPM vs. the drum's RPM will be different (10:1) and dividing out the 2x higher torque-at-the-drum by this factor of 10 will result in the same torque-at-the-wheels figure as before in 4th gear. Probably a little less since you'll have more tire-slip in 2nd and more friction through the gearing, resulting in lower acceleration of the drum.
Another interesting note is that the only thing that is actually measured is torque at the drum, drum RPM and engine RPM. The car's HP and torque at the wheels are then calculated from that data.
05-03-2003, 09:01 AM
#4
Instructor
Thread Starter
Ok thanks. I would assume from your response,Danno,that the "corrections" are done automatically by the program. Does the Dynojet system allow for the entering of the "guestimate",(or measurement, for that matter)for tire slippage. I guess what I am asking is tell tale signs that the output is being "biased".
Thanks
Thanks
05-05-2003, 03:03 PM
#5
Race Director
" Does the Dynojet system allow for the entering of the "guestimate",(or measurement, for that matter)for tire slippage. I guess what I am asking is tell tale signs that the output is being "biased"."
Hmmm, it depends upon the version of the software and the model DynoJet you've used. The later ones don't require manual input of atmospheric conditions, which was a prime source of manually tampering with the results. These have built-in sealed and calibrated weather stations that provides automatic corrections to SAE figures.
While there are additional corrections for guessing the flywheel HP & TQ figures, only the actual measured rear-wheel figures are typically used.
I'm not sure that a correction for tire-slippage can be entered. That's because you have no way to measure exactly how much slippage has occured.
Hmmm, it depends upon the version of the software and the model DynoJet you've used. The later ones don't require manual input of atmospheric conditions, which was a prime source of manually tampering with the results. These have built-in sealed and calibrated weather stations that provides automatic corrections to SAE figures.
While there are additional corrections for guessing the flywheel HP & TQ figures, only the actual measured rear-wheel figures are typically used.
I'm not sure that a correction for tire-slippage can be entered. That's because you have no way to measure exactly how much slippage has occured.
05-06-2003, 09:15 PM
#6
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A Dynojet is not measuring torque, it is calculating both torque and horsepower based on time.
For sake of discussion, read this article: German Motor Cars load vs inertia
For sake of discussion, read this article: German Motor Cars load vs inertia
05-06-2003, 09:19 PM
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05-07-2003, 08:08 AM
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Not all Dynos are the same. I had mine done on a Dyno Pac?? tires are removed, water supplied units are bolted to the hubs, transmission type entered, the printout included the gear ratio(there must have been a database internally as theyon ly entered the trans type and gear), air pressure/temp, etc. I assume all of that was relevant to the hp/torque curves.
05-15-2003, 02:41 AM
#9
Instructor
Not all Dynos are the same. I had mine done on a Dyno Pac?? tires are removed, water supplied units are bolted to the hubs, transmission type entered, the printout included the gear ratio(there must have been a database internally as theyon ly entered the trans type and gear), air pressure/temp, etc. I assume all of that was relevant to the hp/torque curves.
Dynapak? NZ-made, red things.
I figured it out - my car had an input for rpm (ie it plugged into the computer), it knows the hub turns, and (I think) you tell it the final drive. Geez, that doesn't make much sense at all. Sorry.
Dynapak? NZ-made, red things.
I figured it out - my car had an input for rpm (ie it plugged into the computer), it knows the hub turns, and (I think) you tell it the final drive. Geez, that doesn't make much sense at all. Sorry.
