Jesus-Ninja
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| posted on 2/9/08 at 10:09 PM |
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Any mathematicians? Torque, mass and broken gearboxes
It's no secret that is torque that kills gearboxes, especially big launches and low gears due to the effect of overcoming inertia.
But it's not torque alone, as mentioned, inertia has an effect too. What's the relationship between the torque, the mass of the vehicle
and the breaking point?
Consider a car weighing a tonne will break it's gearbox with 300lbft of torque delivered from a standstill.
What amount of torque will be required to break the gearbox if the car weighs half a tonne?
Suspension geometry tool here >>> http://www.locostbuilders.co.uk/viewthread.php?tid=81376
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watsonpj
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| posted on 2/9/08 at 10:59 PM |
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If you talking about a catastrophic failure due to the shock load it will depend on if the tires spin up on the lighter car. If they don't then
the failure will also most likely occur at the same 300lbft of torque, this is because the failure point within the gearbox will suffer the same
stress in either case. If the lighter car doesn't bog down however you will probably not get this kind of failure but will head towards a
fatigue type failure. This failure type takes many repeatative cycles and as the stress goes down so does the number of repeatition in a non linear
way as in the graph below
Image deleted by owner
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Jesus-Ninja
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| posted on 2/9/08 at 11:02 PM |
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Now that's the kind of answer I was looking for! 
Suspension geometry tool here >>> http://www.locostbuilders.co.uk/viewthread.php?tid=81376
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smart51
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| posted on 3/9/08 at 06:59 AM |
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Or put simply, the 1 tonne car and the 0.5 tonne car both put the same load on the gearbox as the 300 lbft of torque does work accelerating the mass
of the vehicle, however, the 1 tonne car accelerates half as quickly off the line, so the load is applied to the box for twice as long. You might
therefore expect 1st gear to live half as long on the heavier car.
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dhutch
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| posted on 3/9/08 at 07:50 AM |
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quote:
Originally posted by watsonpj
If you talking about a catastrophic failure due to the shock load it will depend on if the tires spin up on the lighter car. If they don't then
the failure will also most likely occur at the same 300lbft of torque, this is because the failure point within the gearbox will suffer the same
stress in either case.
If the engine delivers 300lbft, the box will receave that. And if the box it in whatever gear, that will pass a certain torque onto the diff, etc,
through to the load (wheels).
As you say, if the wheels lgiht up, break traction, then the load drops a lot, and so does the torque.
If the wheels dont spin up, and you maintain 300lbft thoughout accelleration it doesnt, on paper, matter on the weight of the car as you say.
However with a lighter car, if you do still maintain 300lbft it will accelerate faster as said, so will do so for less time.
Alternativly you may actaully find that the torque is less, becuase the car is lighter.
Ie, If you put the same power down, the 1/2ton car will accerate in half the time. But if in practice the half ton car only accelerates 50% faster
than the one ton car, the acutall power put down will be less.
There is also the case of shock loading. Where by you rev the engine up and drop the clutch. Or have a bad gear change. Which is quite likely when you
would get a catastophic failure. In which case agian, the lighter car wins.
Daniel
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watsonpj
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| posted on 3/9/08 at 08:00 AM |
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Smart51 if you look at the graph you'll see its more likely to be less than 1/2 as long.
cheers
Pete
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82 Locost
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| posted on 3/9/08 at 09:12 AM |
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Also, from the graph I'm not sure that Torque relates directly to Stress and that Cycles relates to Duration.
Cycles relates to the number of applications of Stress, and would therefore depend where the Stress were measured. For example, a spinning gear would
experience Stress on the teeth everytime they were engaged/disengaged, i.e. up to 7000 times per minute (assuming 7000 RPM redline). However, the
shafts would experience a single long-duration Stress with much less damaging results as there is only 1 Cycle.
I'd estimate something like Life being a function of Power Squared, i.e doubling the torque would reduce the lifespan to a quarter, trebling it
would reduce the lifespan to a ninth. 10 times power = a hundredth of the life.
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alistairolsen
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| posted on 3/9/08 at 09:15 AM |
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also depends which bit of the box youre talking about.
Everything deforms, if elastically youre ok, if plastically your box is f**ked.
Imagine a driveshaft, it will twist slightly when you accelerate.
If the torque applied is the same in bth cases, the car weighing half as much will twist it less for a given rate of change of torque.
(If you bury your foot, one car will accelerate while the other twists up the shaft and snaps it in an extreme case)
apply this to your input shaft, mainshafts, prop, driveshafts...
[Edited on 6-9-08 by Fozzie]
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MikeRJ
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| posted on 3/9/08 at 09:52 AM |
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quote:
Originally posted by alistairolsen
Imagine a driveshaft, it will twist slightly when you accelerate.
If the torque applied is the same in bth cases, the car weighing half as much will twist it less for a given rate of change of torque.
For the same applied torque the driveshaft will twist exactly the same amount. The lighter car will accelerate faster though.
The problem with heavy cars is the peak (rather than sustained) loadings tend to be higher, since you can push more torque through the drivetrain
without breaking traction (which is like a safety valve in this case).
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richardlee237
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| posted on 3/9/08 at 10:04 AM |
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The torque that the engine can supply is not just that derived from the steadystate operation of the engine, but in addition the inertias of moving
parts of the engine, particularly the flywheel. These inertias will supplement the torque if you try to slow down the engine.
So the situation you have is dependant very much upon how fast you try to change things. These torque peaks are what break things.
By the time the rear tyres are slipping the damage has already been done.
For example Formula one engines have little or no flywheel and very low inertias so they can accelerate very quickly and the transmission can be built
for less "over" torque but it also means that it is very easy for the car to "bog down" at the race start as the stored energy
is small.
Quote Lord Kelvin
“Large increases in cost with questionable increases in performance can be tolerated only in race horses and women.”
Quote Richard Lee
"and cars"
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Ivan
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| posted on 3/9/08 at 10:06 AM |
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It also depends on what the weak part of the box is - ie in fith gear you can stress the casing as much as in first - and a light car won't
relieve stress by wheelspin in fith so is as likely to burst the box as a heavy car.
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alistairolsen
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| posted on 3/9/08 at 11:29 AM |
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quote:
Originally posted by MikeRJ
quote:
Originally posted by alistairolsen
Imagine a driveshaft, it will twist slightly when you accelerate.
If the torque applied is the same in bth cases, the car weighing half as much will twist it less for a given rate of change of torque.
For the same applied torque the driveshaft will twist exactly the same amount. The lighter car will accelerate faster though.
The problem with heavy cars is the peak (rather than sustained) loadings tend to be higher, since you can push more torque through the drivetrain
without breaking traction (which is like a safety valve in this case).
right! course it will!
imagine a straight shaft with an engine on one end and a flywheel on the other, let the clutch in and measure the twist.
now double the mass of the flywheel and you will have more twist!
I agree if you take both of them, side by side, and examine them while accelerating under constant torque that the twist will be the same. The inital
twist as the shaft tries to accelerate the flywheel from stationary will give evry different results however!
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Jesus-Ninja
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| posted on 3/9/08 at 11:47 AM |
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So, long story short: My gearbox will likely live longer for being in a lighter car....?
Suspension geometry tool here >>> http://www.locostbuilders.co.uk/viewthread.php?tid=81376
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MikeRJ
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| posted on 3/9/08 at 12:06 PM |
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quote:
Originally posted by alistairolsen
I agree if you take both of them, side by side, and examine them while accelerating under constant torque that the twist will be the same. The inital
twist as the shaft tries to accelerate the flywheel from stationary will give evry different results however!
Another good reason to get a lighter flywheel
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alistairolsen
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| posted on 3/9/08 at 12:11 PM |
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reduce inertia from rotating and linear accelerative loadings, reduce shock loadings by driving sympathetically and it should last considerably longer
then in a larger car of the same power/torque in my opinion.
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richardlee237
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| posted on 3/9/08 at 12:39 PM |
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Very simply
The more mass you have either rotating or linear, the more inertia you have and the more torque required to achieve the same acceleration.
The more acceleration you want the more torque you need for given masses.
Put the 2 together and you can see that torque required will rise rapidly if you want both a heavier car and more acceleration.
Inertia is nothing more than the force opposing change which is produced if you try to change the system and is generally proportional to the masses
in the system and the acceleration rquired.
The point to realise is that these forces do not exist until you impose change and then the size of the forces is dependent on the speed of change
(acceleration) and the mass of the system
Quote Lord Kelvin
“Large increases in cost with questionable increases in performance can be tolerated only in race horses and women.”
Quote Richard Lee
"and cars"
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iank
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| posted on 3/9/08 at 01:18 PM |
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Also be aware that a lighter car will lose some of its safety factor if fitted with super sticky slicks as they won't spin up as easily.
--
Never argue with an idiot. They drag you down to their level, then beat you with experience.
Anonymous
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Jesus-Ninja
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| posted on 3/9/08 at 02:38 PM |
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quote:
Originally posted by MikeRJ
Another good reason to get a lighter flywheel
I hadn't really thought about the flywheel as a factor, but of course it makes sense. Hmm, I wonder if fidanza do them for Saabs.....
Suspension geometry tool here >>> http://www.locostbuilders.co.uk/viewthread.php?tid=81376
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Jesus-Ninja
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| posted on 3/9/08 at 02:45 PM |
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quote:
Originally posted by Jesus-NinjaI hadn't really thought about the flywheel as a factor, but of course it makes sense. Hmm, I
wonder if fidanza do them for Saabs.....
...apparently not...
[Edited on 3/9/08 by Jesus-Ninja]
Suspension geometry tool here >>> http://www.locostbuilders.co.uk/viewthread.php?tid=81376
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