torque for drive axle calculation

[RTAF]

I tested my engine by using chassis dynamo-meter.

Maximum torque is 55Nm. My adviser told me this is torque produced to the ground by the engine or wheel torque.

But when I calculated from gear ratio and engine torque from wikipedia (60Nm). wheel torque is more than 600 Nm (4th gear).

when I want to use the torque for drive axle size calculation. which one I choose use?

[Tilman]

Are you sure that this is the torque the dyno sees? There are standardized ways to calculate the torque at the crankshaft from the wheel torque. Our axles are designed for ~ 1000Nm.

[M. Nader]

Originally posted by Tilman:
Are you sure that this is the torque the dyno sees? There are standardized ways to calculate the torque at the crankshaft from the wheel torque. Our axles are designed for ~ 1000Nm.

I think you will get wheel spin before that. but i wouldn't go lower than that given that axles are a common failure source.

[Warpspeed]

Shock loading and fatigue are also worth thinking about.

What happens when you give it a good rev, and then dump the clutch in first gear ?

[Charles Kaneb]

Is the peak torque on the axles going to be engine-limited or tire-limited?

If it's engine-limited, I suggest carefully examining your gearbox and final drive ratio selections.

[Marvel]

Originally posted by RTAF:
I tested my engine by using chassis dynamo-meter.

Maximum torque is 55Nm. My adviser told me this is torque produced to the ground by the engine or wheel torque.

But when I calculated from gear ratio and engine torque from wikipedia (60Nm). wheel torque is more than 600 Nm (4th gear).

when I want to use the torque for drive axle size calculation. which one I choose use?


I am not a powertrain guy, but what I think is
(and I may be completely wrong also http://fsae.com/groupee_common/emoticons/icon_eek.gif )
(torque on dyano)x(primary drive ratio of gearbox)x(drive ratio at nth gear)x(outer sprocket dia/inner sprocket dia) must give wheel torque.
Also consider the type of differential used since it will split the torque at wheels...

Your case-
for instant, assuming-
primary drive ratio of your gearbox to be 2.11
Outer/inner sprocket to be 56/13
drive ratio at 4th gear to be 1

(55)x(2.11)x(1)x(56/13)=500Nm is the wheel torque
For final calculations, consider a proper sof and type of differential...

[M. Nader]

Originally posted by Charles Kaneb:
Is the peak torque on the axles going to be engine-limited or tire-limited?

If it's engine-limited, I suggest carefully examining your gearbox and final drive ratio selections.

Max torque is in first gear, but tire limited as slip is the problem (no load), although torque spikes do occur from shock loading and shifting.

[Matt Clark]

Originally posted by M. Nader:
Max torque is in first gear, but tire limited as slip is the problem (no load), although torque spikes do occur from shock loading and shifting.

yeah +1

@ RTAF we use max torque produced by the engine, then take primary gearing x 1st gear x sprocket gearing which gives max driveline torque, this is without a factor of safety

you also need to think about where this is reacted.

we use a spool, and thus when jacking could (pretty unlikely) see nearly all of this torque reacted though one tyre. This would obviously cause the tyre to slip, but as has been said already, tyres can take some high instantaneous loads. thus you must do some thinking about how much one half-shaft will see realistically in this case.

just remember to not just trust a number, think about where it is being reacted and what is really your worst case to be designing for.

for a guide we also design each half-shaft to take around 1000nm and this is on the safe side.

[Warpspeed]

You can get a much higher torque spike than just peak continuous engine torque, if the clutch is suddenly dumped at high engine revs.

[Dash]

Also, think about the fact that even though you don't have tire grip you are still diffusing the torque. T = I*Alpha

[Vatsal Agarwal]

Hi, We are a first year team and we are facing the following problems in relation to torque calculations.

How do i calculate the initial torque required to set the car into motion, and is this equal to the maximum torque of the car?
We are using 2 Agni motors - 95R coupled by a single reduction chain drive and R13 tires.
How do i proceed with these calculations.
Regards
-Vatsal

[Jay Lawrence]

Vatsal,

To set the car into motion is a bit vague. Technically you want to accelerate it and for this you need F=ma in the most basic and linear sense. This is the bulk of it, but you may also want to consider rolling resitance of the tyres at a given pressure and any other negatively acting forces that are present (air resistance, toe resistance, hills, etc).

The maximum torque of your motors is the maximum torque of your motors (data you should know before you buy your motors, assuming they're electric), and the maximum torque of the tyres can be found in the tyre data. You probably need to research which of these is more important in your design process.

[Gruntguru]

Originally posted by RTAF:
I tested my engine by using chassis dynamo-meter.

Maximum torque is 55Nm. My adviser told me this is torque produced to the ground by the engine or wheel torque.

But when I calculated from gear ratio and engine torque from wikipedia (60Nm). wheel torque is more than 600 Nm (4th gear).

when I want to use the torque for drive axle size calculation. which one I choose use?
Your post includes a common fallacy. Chassis dynamometers do not measure torque. The interface with the car is a linear motion and the applied force is linear (commonly called "tractive effort")

Warpspeed is right. You ignore all the above torques and calculate the effect of a dump-clutch launch.

[Warpspeed]

Try carefully placing a house brick onto your foot, not that heavy, huh ?

Now drop the same brick from three feet high onto your foot.
The brick doesn't weigh any more, but I bet it's really gonna hurt.

Hammer blows, and vicious clutch dumps can break things that constant applied torque will not break. It has to do with inertia and shock deceleration.

[Vatsal Agarwal]

Originally posted by Jay Lawrence:
Vatsal,

To set the car into motion is a bit vague. Technically you want to accelerate it and for this you need F=ma in the most basic and linear sense. This is the bulk of it, but you may also want to consider rolling resitance of the tyres at a given pressure and any other negatively acting forces that are present (air resistance, toe resistance, hills, etc).

The maximum torque of your motors is the maximum torque of your motors (data you should know before you buy your motors, assuming they're electric), and the maximum torque of the tyres can be found in the tyre data. You probably need to research which of these is more important in your design process.

I have the maximum torque produced by the motors and also maximum torque of tyres. can we calculate the force F=zmg(rest weight of the body), consider z is cofficient of friction. how do i calculate 'z' between tyres and tarmac.I think the equation F=ma would give the torque based on accleration not the intial torque required by vehicle to set in motion.
Please help me out with this confusion.

[thescreensavers]

Originally posted by Warpspeed:
Shock loading and fatigue are also worth thinking about.

What happens when you give it a good rev, and then dump the clutch in first gear ?

How does one look into figuring out the shock loading and fatigue? We have done the formulas to figure our safety factor(its 4.5+) with an input of 400 ft/lbs per drive shaft(800 from the engine) but Shock Loading is Worrying me.

[Warpspeed]

The whole thing is scary, it's not just shearing something clean off from a one time overload, but fatigue and eventual cracking that can sneak up on you.

I think the guys running super thin wall tubular driveshafts are just moving the problem somewhere else.
A pair of springy torsion bar shafts will absorb some of the shock, and if you paint a long stripe down the shaft you can pick up any tendency towards any permanent yield.
Fatigue cracks will always start small in what should (?) be an obvious place, and they will be trying to tell you something.

[Jay Lawrence]

What Tony said^

Design for whichever torque is higher (engine or tyres) and add a significant safety factor to it (3,4,5?), then run it and observe. If you want to get it more precise you could perhaps use an FEA program to do a fatigue analysis, although I doubt you would be able to capture the quite extreme nature of FSAE behaviour.

Vatsal, 'z' is your rolling resistance and you can get an estimate from google. It is not just required to set the vehicle in motion but is present all the time and is one of the many forces that needs to be overcome to provide you with the 'a' in your F=ma. Think about what you are doing when you set the vehicle in motion: you are making it go from rest to some velocity within a given time/space.

For a practical solution to your question, perhaps use an old car/someone else's car and pull it from rest using a fish scale or some other linear weight scale. This will give you the weight (F) required to move the vehicle from rest. I still do not know why you would care, however.

[Warpspeed]

All this is going to be pretty difficult to calculate or simulate, even if you could fudge up some numbers and relevant safety fiddle factors to work with.

But it's pretty easy to measure!
Assuming you have a pair of typical torsion bar type driveshafts, splined at least at one end.

What you could try is sliding a thin wall tube over the driveshaft, and rigidly securing it to the driveshaft only at one end.

At the free end, there will be relative motion between the inner loaded shaft, and the outer unloaded thin walled tube, as the shaft twists under load.

All you need then, is something to record the extreme peak of angular travel, such as a hose clamp or tie wrap attached to the driveshaft.

After a few savage clutch dumps, lock the inner end of the driveshaft solid, and try turning the wheel hub with a torque wrench or long lever and a known weight to duplicate the maximum peak torsional twist recorded.

It's a bit Mickey Mouse, but I bet the numbers are going to probably be as close to reality as anything else you can do.

[Marvel]

Originally posted by Vatsal Agarwal:
I have the maximum torque produced by the motors and also maximum torque of tyres. .......
Please help me out with this confusion.
I once had the similar confusion.
What I have learnt from this forum and big guys of this forum is you have two ways out-
1. Engine limited torque
2. Traction limited torque
(Also read Tune To Win )

Method 1-
(torque on carnk)x(primary drive ratio of gearbox)x(drive ratio at nth gear)x(outer sprocket dia/inner sprocket dia)=
torque from engine available at tire.

Method 2-
Just go in the reverse direction.
From the tire data-at a particular load condition get the value of coefficient of friction 'u'
With the tire radius and lateral force, find the torque reacted from road to tire.

Assuming the higher value and proper FOS, you have to proceed in designing your components.

P.S. the selection of type of differential also has a huge effect in torque biasing.