Anyone have some good info on transaxle mounts. I road race and am looking for options before the clutch install. I've seen the recent write up, but am a little suprised at the price and configuration so there must be something more to the design that hasn't been explained.

I found a good looking one in Ecklers, it's polyurathane and costs $169 http://www.ecklers.com/product.asp?pf_id=39834&dept_id=1746 and is rated for extreme duty.

Also found one at mid american motorworks for $129 http://www.mamotorworks.com/corvette-1-327-3924.html

Given these configurations vs the newly popular Pfadt's, What's the difference? I see the out riggers on the Pfadts.

Thanks

 

ttt for more info

About to order from Mid American Motorworks. Jeff was very helpful and is building a mean C5 machine himself.

 

The one from PFADT just looks like more engineering went into it.

The two above, while heavier duty than stock, are the same basic design.

Of the two above, I'd go with the Poly one.

Of the three, I might try the PFADT. Much more expensive. Keep in mind that the install can be done/undone later without taking much apart, see thread a few down.

 

The Pfadt C5 Transmission mount takes a different stance on controlling the transmission and differential on your C5. On the factory mounts the transmission and differential are allowed to rock back and forth on the mount with little resistance. The engine mounts normally have to take all of the rotational load from the engine. This is unlike the C6 which has a transmission mount on either side of the the transmission to help contain the rotational load from the engine.



The Pfadt Transmission mount is designed to take some of this rotational force and distribute it across the rear sub-frame to two polyurethane pads. Eliminating the ability of the transmission and differential to simply rock back and forth on the factory mount.

One experience that many have in their Corvette is the feeling of wheel hop. This experience can be two different things. The first is the rear tires are actually hopping on the ground. The other possibility is the drivetrain is shaking. The Pfadt Powertrain mounts substantially reduce this tendency.

Please let me know if you have any other questions! I am more then happy to help!

 

You can get the Pfadt mount through Mid America as well, it just hasn't been added to the website yet....

Please feel free to call us with any questions you may have. Have a great weekend!

 

Josh,
I'm having a hard time picturing how the loads are distributued with your mount design compared to the stock mount. Some type of diagram would really help. Does your mount add damping?

Thanks,

Eric D

 

There has been some confusion about what and how this mount actually works. Well here is a quick diagram that shows what is actually going on.



You can see that the torque from the transmission needs to be controlled some how. In the stock mount this is done only by the rubber in the small center mount. In the Pfadt mount this is done by transferring this torsional load onto two polyurethane pads that are at the end of the orange "stabilizer bar", as I have affectionally dubbed it. The above diagram shows that the torsion load translated onto the sub-frame through one of these pads. If the torsion load was the other direction then the opposite pad would transfer the load.

I hope that this quick diagram helps in visualizing what our mount does. If this is not clear then please let me know and I will try and put together a more detailed diagram.

 

Awesome!

Not to answer for everyone, but thats very clear to me.

Even if the rotational forces are being stabililzed by solid motor mounts, I would imagine there are still forces that twist the trans axle back the other direction from putting the power down on the road.

very good, thanks!

 

Yes, and the DTE brace helps with that.

 

Josh,

Thanks for taking the time to post the diagram, and yes it helps. I know the C6 has mounts for the diff near the output shaft covers that take some of the torque. I'm still a little fussy on where the torque on the diff comes from. I thought that was the job of the prop shaft housing?

Thanks again,

Eric D

 

The torque is reacted out in its mounting, and in the case of the C5 its the motor mounts and the differential mount. What Pfadt has done is move the reaction points outward and provided better support for the differential and the drive train. While more money, its a much better design approach than the other two posted by the OP. If you look at the design and the cost of materials, I think the Pfadt unit is actually very reasonably priced.

 

Man, I'm really confused here. Vettenuts, you mention the torque is reacted out in the motor and differential mounts. Where does this torque come from to react? I'm really sorry, but I am having a hard time visualizing where this torque that is being reacted out comes from.

Thanks,

Eric D

 

The torque comes from the rotational forces produced by the power produced by the rotating mass of the engine and drive train as it makes the rear wheels push the car forward.

It's why you see the engine jump to the right when you blip the throttle. The right front pulls off the ground higher when a car jumps off the line (and you hear of people twisting the frame)

All the movement of the engine and drive train basically equates to power that doesn't get put down to the wheels. At a certian power level or level of demand (performance) the stock mounts are no longer effective in controlling the alignment of the drive train thus prematurely wearing bearings and can even break housings..... which is why DTE braces are attractive....

When your up at high rpms and you let off the throttle without shifting (manual) and you feel the car snap back the other way, that's because of the transfer of torque back through the drivetrain. You just created a 700 ft lb of torque swing going from producing 400 ftlbs of torque to propel the car to using 300 ftlb to slow the engine. But rember that torque is controlling the 3000+ lb car. And all that is relying on a little mount 4 inches wide to controll all the weight of the rear end and rotational forces.

Road racers really know this transfer of torque as they use the throttle to steer the car. A little lift in the middle of the turn can make all the difference in your line and you can go right back to putting the power down out of the turn. What you don't want is the mass of the drive train slopping around to unsettle the car, much less with the repitition prematurely wear drive train components.

Drag racers need it for the instant shock delivered at the drop and transfer or power. Pretty easy to see that when everything torques over.

 

The engine is generating torque and transferring it to the rear wheels through the drive train. So in effect, the engine is trying to twist, raising one side and pushing down on the other. If you lift your hood and blip the throttle you will see the motor twist. The only things stopping the motor/drive train from twisting right out of the car are the various mounts that hold it in place. Up front are the motor mounts, in the rear are is the differential mount. The torque tube holds the transmission and differential in place so that the shaft internal is what rotates.

At the differential , imagine you are trying to tighten something with a wrench. A short wrench makes it harder to tighten (apply a torque) while a longer wrench makes it easier to tighten since the additional length give you more leverage. In each case, you are applying a force through your arms, but the torque applied is a function of the leverage arm length. Think of this in reverse, if there is a torque applied you have an easier time holding ti with a longer wrench. The Pfadt brace has lengthened the arm length and in effect decreased the forces required to counteract the torque from the motor/drive train.

 

Ok, now I think we are getting somewhere. Thanks for write up. On a conventional rear wheel drive car that doesn't have the torque tube over the propshaft like in the corvette, I can see that the reacting torque has to find its way back to the engine from where it came. This means I have to put this torque somewhere like through the body of the car or through the tires to the front of the car to the mounts and then engine. In the case of the corvette I have a torque tube that allows this path right back to the source by-passing the body the ground or any other path. The torque inputs you speak of when you blip the throttle are different. This due to the acceleration of the engine inertia parts that are changing rate of rotation. This torque force would be near the same blipping the throttle out of gear as in gear. In other words, the defection you see the engine make blipping the throttle with the hood open setting in your driveway is about the amount you would see launching from a full stop. In fact the only other difference would be the added inertia of the clutch disk, propshaft and transmission input shaft.

This is why I was having a hard time seeing where the torque would be coming from. The Corvette has the propshaft torque tube as the return path, so why do I need anything more??

Thanks again for your input,

Eric D

 

Eric, you get it. It's the guys providing you with answers who don't.
You're just baitin' them, aren't you?

It looks a little like the Pfat item is designed to solve a problem which doesn't exist.

With the existing Corvette design, there should be no net side-to-side rotational loads on the rear end. It's "one piece" with the engine.

 

Engine, torque tube, tranny and differential case are one unit (essentially since all hard fastened together), where are loads reacted out to the structure? Seems to me the Pfadt bar has reduced the amount of displacement the drive train can experience as compared to the stock setup at the rear location where the other mount is located.

 

Warp Fator,

Seriously, no baiting. The great folks in this forum have taught me many things and I truly enjoy everything I've learned about Corvettes here. I don't mean to pester folks with my questions but if I have I apologize for that. If something doesn't quite add up in my eyes I tend to get obsessed with getting it right in my head. I have studied the C5 Corvette's drivetrain in detail and I believe this is one of its most unique features. I really could not understand where the rotational torque was coming from others were talking about.

Vettenuts,

The only deflection (displacement) that I'm aware of is the differential output shafts to the wheels and tires reacting on the diff case. The torsional forces here under a hole-shot can cause the torque tube to bend upward but not to the point of yield. On the C6 diff case there are added mounts forward of the output shafts to transfer these loads to the sub-frame. If a car's output power is increased from stock on the C5, I can understand the need for adding structure between the transmission and the differential case. I have a hard time understanding the need to change the rear differential mount.

Eric D

 

Eric,

Should have checked your bio, didn't realize you had a technical background or I wouldn't have provided that explanation on torque in the prior post. You and I are a lot alike, this is where I like to use a white board .

When I look at the design, it seems to me to limit both rotational deflection around the torque tube axis and any lateral displacement (up/down & side to side) at the differential mount. Correct me if I am wrong but it would seem to me that there are two axes of torque, one about the torque tube itself and one about the wheel axis. Since the entire drive train is essentially soft mounted, wouldn't higher power applications or stickier tires, etc. tend move the drive train around more. I would think with the clutch engaged and car accelerating, the torsional loads on the mounts would increase since they need to react out the shaft loads being applied. Are you saying the engine mounts would react out all of the engine torque and therefore the differential mount is only taking lateral loads?

Would like to continue this discussion as I too would like to understand your discussion point, but unfortunately I am on the road this week and carrying a laptop through airports has gotten to be too big a pain to make it worthwhile so I would need to pick this up at the end of the week

 

Sorry Eric, you had such a good grasp on things that I thought you might be messin' with people.

I also see only a torque load counter to wheel rotation....one which would tend to raise the torque tube.
This load can be severe enough to break the mounting flange where the diff and trans join.
That's what the DTE brace is for, as you already mentioned.

If the torque tube housing twists, I suppose the differential brace might do some good. But it's hard to imagine it twisting enough to take up compliance in the "stoppers" at the end of the brace, and actually load the brace.
If it does though, I suppose the brace might serve to reduce some of the "spring" in the system, and that might be helpful for wheel hop. With all the other spring in the system though (driveshaft, halfshafts, tires, suspension bushings), such a reduction would seem trivial.

At the moment, I can't see any other potential benefit. Assuming that the torque tube housing twists, I don't see how a few degrees (?) of differential rotation in the direction of crankshaft rotation would matter.

Like you, I also fail to see any torque loads on the engine mounts, other than inertia loads from engine rpm changes.

Edit:
That inertia torque load on the whole rearend/torque tube/engine assembly could be pretty high during a clutch dump though.