Broken Transmission

OWNED!!

 

Brian, et al,

I was considering 19" wheels up front, and 20" for the rears for my 01 360. had just looked at a set being sold by an owner who wants to replace them with the factory wheels before selling his 360. I don't have any other dimensions for the wheels at this time. Based upon what I read on this thread, is there a maximum size aftermarket wheels/tires, if any, that would still be considered "safe" for the car/transmission?

Thanks

 

well said.

 

Thanks for the advise Brian.

 

Very, very true. That´s why the only thing I´ll ever change on my 360 is to the rear challenge grill...

 

So Brian, are you saying you don't think rims like these would be a performance upgrade for a 348 or 360?
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hahahahaha, good one!

 

I have 20" tires and wheels on my CL55 (Installed by the former owner) but I am not tempted to change the wheels on the 360. Just a slight change in offset or weight can make a big difference. The CL is not driven aggresively other than in a straight line.

 

I believe that. Not the greatest knowledge at times! Dan

 

I'm just a fat ole billybob and I don't understand why big tires/wheels are an issue.
1- I posit the idea that guys on oem wheels and slicks ala challenge cars have a higher grip level to the road than any street tire. In fact a car with no aerodynamics can do 1.5 g's on slicks. With minimal aero the 360 has maybe it can do 1.5 gs easy and maybe more but not much. No street driver will ever see these numbers so the pressures on the gearbox will always be less than racing on a track. We don't see 360 gearboxes exploding on the track do we?

2- wheel size...I don't see how this can have any effect unless one subscribes to the idea that the larger radius wheel applys more torque back to the gearbox under loads. But is that any worse than the higher rpms a gearbox is exposed to with a smaller wheel such that the momentum is really the same effecting the gearbox? Someone please do the math for me I got a headache.

3- tire size/width? I'm not sure on that one either. A wider tire does not necessarily increase the contact patch and grip. Contact patch is a function of relative width but also weigth of car. So if you have a narrow tire your contact patch is longer front to back and less side to side. But if you have a wide tire your side to side is longer and your front to back is narrow. The result I think is the same "area" contact patch. So I think 18" or 20" wheel the contact patch is the same area=same relative torques can be applied back to the gearbox?

I vote for bad metal, something out of spec causing wear and then catistrophic failure. How about that 0.002" clearance Dave talked about where something was just out of spec and the gear did not go completely "home" on the main shaft during factory assembly. Then as it slopped back and forth on the shaft it destroys itself and the companion gear? To me I think the owner was pretty oblivious to the world. That box had to make some nice noise before it went and with that much destruction it is hard to find out what broke first. Now I know why Ferrari thinks it needs those dumb slow down lights and feedback between the left and right sides on its engine mangement...

 

Is this true? I've been scratching my head. Can someone do a footprint & measure then compare them? Say a 295 & 265, what's the difference in the contact patch?

 

I did the math and it shows that the torque in the driveshaft increases with ALL of the following:
a) larger overall tire diameter
b) higher coefficient of friction, i.e., stickier tires
c) heavier wheels
d) heavier car

Within practical variations of the above the biggest single contributor is stickier tires.

Note: I arrived at this by doing a free body diagram of a wheel showing all forces and using equilibrium equations at the point of impending slip.

....damn, now I have a headache

 

Very well done. The tire profile will also influence the forces. Not that the net will be much higher but the force/time will be much higher because the tires won't buffer the interface nearly as much.

 

I don't get it here.

Torque is a rotational force. Tires, wheels, and drivetrain components present resistance to the torque.

If the engine and gearbox are the same with the delta being wheels and tires, then can someone please explain how the " torque in the driveshaft increases"?

And I am asking this in a pure mechanical sense for clarification purposes.

How many gearboxes blow up like this?

I have to agree with fatbillybob that there was some notice going on in advance here that may have been ignored which was a result of "bad metal, something out of spec causing wear, and then catistrophic failure."

How do wheels and tires come into play whereupon in many cases the replacement wheels are lighter (less mass) than the OEM?

 

Simple. Torque is force x distance, hence a rotational force.

Assuming a larger radius (wheel) and the same mass (force), the torque on the shaft will be higher.

 

I am sorry guys for being dumb...I don't get it.
a) ?????? lets assume the corner weight at the rear of a 360 is 1000lbs. My 348 is 850lbs. Lets assume the largest moment you can put on the gearbox is taking the 360 at full throttle lift the back wheels and drop the car onto the rear wheels. I think that is 1000lbs at the corner acting through a 18 or 20" wheel trying to stop the motion of the spinning wheels and torque of the motor. All force dying in the gearbox. That's 1000lbs acting on 18" or 750 lbs/ft or on 20" 833 lbs/ft...which is only and 11% increase in torque for 2" of wheel. I don't see why a 360 gearbox can't handle only 11% potential torque or transient "resistive force". As someone stated above.

b) yes but not street tire is stickier than a slick race tire and 360C's are not blowing up. I cannot see how a sticky bigger street tire is the biggest issue. Once you have run on slicks even the best street tire seems like junk, marbles, grease, driving on ice!

c) well yes but that depends on relative angular momentum. Is the 360 made so badly that this small difference in angular momentum destriys the gearbox. Isn't the angular momentum of the oem wheel at top speed most for which the car was designed likely greater than the maybe heavy aftermarket wheel that sees slower street speeds?

d)not an issue in this case

Sorry, I can't remember my math so everything for me has degenerated into empirical land

 

FBB:
I only did the exercise to see for myself what characteristics would have an effect on halfshaft torque. Each of the ones I listed does, but I'm not suggesting any one of them would lead to a transmission failure. I would hope they aren't designed that close to the limit!

I based my sticky tire comment on a coefficient of friction increase of 0.3. A large spread perhaps. Probably like going from a good street tire to a Hoosier, but my point is that tires have a more pronounced effect than a typical increase in wheel/tire weight. Actually it's the moment of inertia of the wheel tire combination that matters...but that data is tougher to come by.

 

Great input and work from everyone, but I have to believe we had a weak component here, poorly assembled transmission, or both.

Plenty of folks have very big tires on their cars, and I haven't heard of anyone else complaining of a transmission failure.

Look at the tires Gig has on his hot rod. Huge is the word that comes to mind. How's your tranny, Cig?

There was an 04 spider in Fla that was lowered and had huge 20" rears. Don't hear him complaining yet.

Dean's 360 in Discovery Bay has some pretty good sized rears. Dean, how's is it holding up?

I am sure very large tires can impose an effect, but drastic failures I do not read mentioned here too much.

So, what's the true skinny on this?

 

Most textbook analysis of torque don’t take into account real world effects like gear lag and slack in components. But when doing failure analysis these transient effects are very important. The torque numbers generated by the engine and quoted by the manufacturer are steady state values. When torque is suddenly applied to components in a drivetrain and there is a load on the other end, certain components will see momemtary torque values much higher than steady state.

One way of visualizing this is to imagine for a moment there is “slack” in the gear train. Now, suddenly slam a shift like you are trying to chirp the tires (OUCH! Why do people do this!). When the “slack” is suddenly taken up, the gear teeth will “slam” together and the main shaft will feel a torque much higher than it does a tenth of a second later. All the while the torque produced by the engine has remained constant.

A linear analogy of this is to simply stand on a scale. My body produces a vertical force of 155 lbs. due to my mass and acceleration(gravity). I can measure this with a bathroom scale. Now if I jump up in the air and land hard on the scale, I can produce a vertical force of 200 lbs., even though my mass and acceleration have not changed.

So yes, torque in the driveshaft does momentarily increase. Ask any trucker about this. You can actually snap a driveshaft if you are in the wrong gear and pulling a heavy load and the rig starts bucking.

Well many years ago when I was a skinnybillybob I was pulling a boat with an old 1966 Ford F250 4x4 and became stuck in a long line of traffic in the Oregon Coast Range, due to a wreck ahead. I decided to bail and head home a different direction, but there was no room to properly turn around. Being a young male with a 4x4 I decided to simply put the truck in low and back the boat over the edge of the mountain so I could turn around. I did this and the boat was almost dangling from the trailer hitch. When I tried to yank it back up I stripped first gear (see photo). Thinking fast I went for second and amazingly pulled it back up.

This is why most young men shouldn't own a Ferrari - abuse them and they will break!
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I think we have beat this horse to death. The facts are that the instantaneous force is much higher than steady state and larger tires increase that force.

Having said all of that - You are right, the gearbox should hold up to this anyway.

 

Glasser1,

Great analogies all of which make great sense and which I understand. I get the feeling you almost went "over board" with that boat issue!!!

John,

You are right. You are the voice of reason.

And yes, I agree that we have beat this thread to death!

However, I think it has been a very informative and valuable thread. After all, how often does Rifledriver post pictures!

I do agree with FBB about noises possibly being ignored. In flying multi-engine airplanes one always listens for unusual vibrations other than props being out of synch. Prop synch is an easy fix with throttles, but odd vibrations indicate something may be wrong and can't be fixed while flying.

So the moral is to listen to your machine. It will talk in its own language.

 

Well said, I have been telling clients for years that the engineers spend millions of dollars and hours designing and setting up the cars, do you think the "dude" at "bobs wheel and tire" knows more than them. Its built that way for a reason. I see so many cars that have big wheels and are lowered and the owners complain "they handle funny". Je do ya think!!