Weight and transaxle durability.

[Cliff Beer]

OK, here's the question: will a transaxle in a lighter weight car be more durable/last longer than in a heavier car, assuming the same engine?

Seems like the gut reaction is "yes" as the transaxle has less mass to propel down the road in the lighter car. However, the engine is putting out the same hp and torque, and loading the transaxle to the same extent in either car, so isn't the load the same? Obviously, the lighter car will accelerate faster so the duration of the power load during acceleration may be less than in the heavy car, which in itself potentially increases durability/longevity.

I would submit that the answer is not simply "yes, the lighter car is easier to propel down the road so it must be easier on the transaxle."

Thoughts are appreciated.

 

[Jac Mac]

As long as you dont go and fit some sticky tires or aero aids to load it up again You can make a 'light' car awfully 'heavy' with wings etc. Having tires with a lower traction rating can make life easier on the transaxle as well.

 

[Randy V]

What Jac said - and I'll add the benefit of an external cooler for the oil in the gearbox.

I'm also of the mind that an open differential would potentially limit the traction thereby reducing internal loading on the gearbox itself.

At least that's the route I'm going...

 

[Jim Rosenthal]

Very sticky tires also increase the loading on the transaxle and CV joints. Maybe the reason the rubber doughnuts worked back in the day was that the tires had less traction than modern ones....

 

[Randy V]

whoops editing error

 

[Randy V]

Very sticky tires also increase the loading on the transaxle and CV joints. Maybe the reason the rubber doughnuts worked back in the day was that the tires had less traction than modern ones....

And the shock loading of gear changing and road bumps/divots would be at those donuts rather than being carried all the way back into the transmission to then be absorbed by the clutch disk/hub... Good point!

It would be interesting if there were someone who could create a captive shock absorbing hub that could be bolted to the wheel side of the CV/Axle assembly.. Similar to what has been used in many different motorcycles for years;

edit: odd - I can't seem to get the images sorted.. Ahh there we go..

This looks to be a more modern variant;

 

[Wayne M.]

Cliff
I am in agreement with you. I feel the transaxle would last much longer in a lighter car. To overcome the inertia of a 2000lb car is much less that that of a 4000lb car. So why wouldnt the trans from a 4000lb car with 200hp have the same loading as a 2000lb car with 400hp??? Im not sure of all the physics but sounds good.

wm

 

[Jim Rosenthal]

Randy,
The last photo of the more modern unit is interesting. It looks like the drive arrangement in a marine transmission, like a Twin Disc, which is what you find behind big diesels in boats. There are a series of rubber blocks arranged radially, and the other side of the drive is a wheel with spaces which accommodate the blocks. This cushions the shock of the takeup when you put the boat into gear. (sometimes boats are shifted rapidly from forward to reverse hundreds of times in a week, and the torque of the diesels versus the inertia or opposite rotation of the shafts and props makes for some significant shock loads) A hub arrangement like this would be good on a GT40; it could also be made to serve as a joint if needed.

 

[Cliff Beer]

Thanks all. I definitely understand that different variables can affect durability - wings, stickiness of tires, shock loading related to clutch action, etc. What I'm suggesting is that if we assume all these variables are equal between the light and the heavier car, will there be a difference?

It's a bit of a brain teaser but when I think about it, it seems that the weight of the car isn't a factor between the two as it relates to durability because the engine is loading the transaxle with the same hp and torque in either car. Perhaps the lighter car gets to speed faster and therefore the transaxle doesn't have to bear the load for as long but that's the only difference I'm coming up with. Hhhmmmm...

 

[John Milmont]

I'm no pro; but it seems like if you have an engine with X amount of torque, the trans is going to see the same amount of torque no matter how much the car weighs, as long as the tires are not spinning. In a lighter car, you may have to be going faster for that to happen...
I think what does happen with weight is you have a greater chance to put a damaging shock load though the trans. Think about it, if you have a 5000lbs car and you pop the clutch at redline, its going to take more torque to break traction with that massive weight pressing down on the tires. These shock loads, which are just huge torque spikes, are going to be higher in a heavy car vs a light one. So, it would seem your margin of error would decrease as as weight increases.

 

[Jac Mac]

Hang a tennis ball on a piece if string and slap it with your hand- moves quite easily doesnt it!
Now hang a cricket ball/baseball from the same piece of string & slap it with the same force you slapped the tennis ball-- doesnt move as far & your hand hurts , OK your hand was the transaxle being 'turned' by the same amount of force 'Torque' & the cricket/baseball was your heavier car.

 

[Keith]

One of the things we all forget about when thinking of these issus is that:

Force = Mass x Accereration

Thus, relatively small changes in mass (weight in this case) makes relatively large changes to the force (torque in this case) to do the same work (acceleration in this case).
Keith

 

[Jack]

Well said Keith!

 

[John Milmont]

One of the things we all forget about when thinking of these issus is that:

Force = Mass x Accereration

Thus, relatively small changes in mass (weight in this case) makes relatively large changes to the force (torque in this case) to do the same work (acceleration in this case).
Keith

Yes, but acceleration is proportionally going to decrease as mass increases.

 

[Keith]

Anyone who drove an original Lotus Elite will well remember "rubber doughnut" wind-up in the rear IRS set up, but it was certainly cheaper to replace those items than shafts and CWP's. A typical Colin Chapman solution.

 

[Cliff Beer]

Well, I'm glad we got that straightened out.....

 

[glen]

i enjoyed that discussion, am i a bit sad??

 

[Thomas Rouw]

I've litterally abused the gearbox in my sylva striker for years and years on end, it was a simple stock toyota T50 box which was desinged for a 120hp and the 20V engine in front of it had 163. I had seen a lot of track, about 18.000 kms of road use, yokohama sticky's and an uprated exedy clutch, could not break it.

the thing was the striker weighed only 498kg's. (oh yes it was fun!)

if one normaly does this in a 900kg toyota AE86 corolla the box would have been long gone.

my mind says the load on the box would be the same, because acceleration f.i. 0-100km is build up of 4 things, engine power, vehicle weight, end speed, and most impotand Time. two things change, weight and time, weight get's less and time gets less, so in the end the amount of energy gone throught the box will have been the same, but in the end the lighter car will survive and the heavier one won't........

grtz Thomas