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Old 05-03-2007, 10:05 PM
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Effects Of Offset Driveshaft On Rear Suspension Geometry

I have a triangulated four-link rear suspension on my street rod. One powertrain setup I am considering would require me to use a radically offset (to the right) driveshaft. The four-link geometry I am using is 83 GM G-body, where the upper control arm pivots are directly over the center of the axle; approximately eight inches apart.

To do this I would have to have a rear end made with the diff offset to the right, which would move the driveshaft away from its near centered position between the control arms, to under the right upper mount or possibly outside it. Hope that all makes sense...

What effect do you think this would have on the rear suspension geometry, if any???

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Old 05-03-2007, 11:01 PM
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A lot of rear ends are not centered ...
Mine is offset 1 inch ...





My other 32 is the same way. A good many Fords came offset from the factory. How much offset are you talking about ??

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Old 05-04-2007, 01:07 AM
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I had to run out and take a quick measurement... The differential would be offset 8.5" to the right. That's why I am curious about what, if any, effect it may have on the rear suspension.

If I am thinking properly, it would seem to lesson the effect of torque on the chassis when launching the vehicle (drag race type standing start, with good traction). There should be less leverage on the right wheel, due to the shorter axle/tube on the right; and a longer leverage arm to overcome on the left.
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Old 05-04-2007, 01:38 AM
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The older ford rangers (2nd gens) were offset to one side about 4" from the factory. The driveshaft had to clear the fuel tank on the drivers side.

I don't know anything about racing or suspensions.
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Old 05-04-2007, 07:21 AM
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As others have stated, lots of cars have a side offset built into the driveshaft.
But, with that much offset you need to check your U joint operating angle. If you have a height offset as well as a side offset and your driveshaft is short the max operating angle for your rpm range may be exceeded. Here is a link to a page with a spiffy chart...
http://www.drivetrain.com/driveline_angle_problem.html

I'd mock it up and see what your combined angles were.

I would also think that it would have a tendancy to unload 1 wheel at a hard launch, but I'm guessing on that one.

later, mikey
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Old 05-04-2007, 09:24 AM
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Thanks for the replies guys. There would be no side offset in the driveshaft angle. The extreme offset differential on the rear end is to line the driveshaft with an offset tailshaft on the transmission.

This for a mid-engine setup I have been contemplating. I'm basically just trying to collect information to make a logical decision on it. Loading and unloading of the wheels under power is the type of effect I am interested in. Basically, what effects the torque will have on the suspension, as it would be introduced so far off center.

I knew there were factory vehicles with offsets, but didn't realize they could be up to four inches. Come to think of it, 4WD trucks with solid front axles have extreme lateral offsets up front. The transfer case hangs off the side of the transmission, and the front driveshaft has to clear the engine and transmission. I guess any effect would be more noticeable on the front, so maybe my idea would work.

I have also, seen dirt track rear ends with an extreme offset diff, but never got to talk to a owner/driver/builder about it. I was hoping there might be a few around here...
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Old 05-04-2007, 09:45 AM
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Possibly PM BillyShope with a link to this thread.
he has a very in depth knowledge of suspension dynamics, from an engineer's perspective.

http://www.hotrodders.com/forum/members/billyshope.html
Check out his home page, you will see things that will amaze you.

Now I'm curious...Are you using a front wheel drive motor/trans turned sideways? Is that where the offset comes from?
Later,
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Old 05-04-2007, 09:57 AM
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Todd,
As I understand the question, you are asking about offsetting the rear differential (eg. one axle longer than the other, pinion yoke not cetered in car) but keeping the drive shaft centered in plan view looking from above?

You are correct in that circle track cars run offset diffs- my DIRTcar quick change has one axle 8+" longer than the other axle.

My understanding of what you are doing is that the 4-bar attachment points will also be moved over to one side by 8"?
The G-body cars/ rears have instant center and roll center pretty much centered from the factory. Moving the 4-bar mounts will offset the instant center of the 4-bar to the right- also the rear roll center will be offset to the right. This could be weird in a street rod application. Your car's weight (centered in chassis for the most part) will be rolling about the offset roll center and it would react differently in right and left-hand turns on the road. Not an insurmountable problem, but could require alot of geometric figuring and moving (cuting re-welding) attachment points around.

Also if the springs are offset as well, your wheel rates would be different L &R. Would need to get different rear springs Left & Right- not a big deal.

Is there reason for using the G-body suspention?

If you go with ladder bars- even if they're offset and a long panhard bar (or wishbone) centered in the chassis- at least your roll center would be centered. Or maybe an offset torque arm (centered in chassis)- locating links- panhard arrangement.

Last edited by novajohnb; 05-04-2007 at 02:14 PM.
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Old 05-04-2007, 10:01 AM
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As long as the rear end and trans tail shaft are parallel then you should be OK. They don't need to be in line but be "parallel" in the horizontal and vertical axis. Parallel, meaning they are pointing in the same direction, but not necessarily in line.

Vertical:
If the engine is tilted down at 2 degrees then the rear end should be tilted up 2 degrees.

Horizontal:
In any case, the engine/trans axis should be running straight down the car (not "angled" toward one side or the other). The engine can be offset to one side as long as it sits square with the frame. This is because the rear end sits square with the frame. So don't try to reduce the angle by angling the engine/trans toward the rearend. This only makes the u-joints spin at different speeds which cause vibrations and u-joint wear.

Then the drive-shaft angle should not exceed 5 degrees. Inverse Tangent (rise/run), remember trig class?

The parallel tolerance is about 2 degrees.
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Old 05-04-2007, 10:10 AM
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Quote:
Originally Posted by powerrodsmike
...Now I'm curious...Are you using a front wheel drive motor/trans turned sideways? Is that where the offset comes from?
Later,
Mikey
Close. I thought about that, but I would have to spend money eliminating the differential inside it; not to mention what it takes to build one that can handle ANY abuse. The solution I came up with is the Eldo/Toronado transaxle. It's basically a TH400 inside, and would already be "pointed" at the rear axle. The diff unbolts and I would just have to fabricate a tailshaft housing. It will also allow me to run a longer driveshaft.

The driveshaft would have no lateral offset or misalignment. It would run straight from the tailshaft to the offset differential (overhead view).

The upper control arm mounts would NOT be moved with the differential, so the instant center and roll center would not be changed laterally.

Last edited by toddshotrods; 05-04-2007 at 10:17 AM.
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Old 05-04-2007, 10:28 AM
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Quote:
Originally Posted by powerrodsmike
...Check out his home page, you will see things that will amaze you...
I extended the invitation; hope he comes... It's gonna take a while to read and absorb the info on his site, but it's gonna be fun
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Old 05-04-2007, 10:53 AM
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Thanks for the compliments. Just added a page on the torque arm suspension. Page 26 of:

http://home.earthlink.net/~whshope

As for the subject of this thread: The driveshaft U-joint provides a couple to the rear axle assembly and, as you look back at the rear axle assembly, you can move that couple anywhere...up and down or left and right...and the effect remains the same.

And a bit of trivia: Some of the Mopars not only have the engine offset, but at an angle! I would assume this can be found in other makes, too.
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Old 05-04-2007, 10:54 AM
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[QUOTE=toddshotrods]The solution I came up with is the Eldo/Toronado transaxle. It's basically a TH400 inside, and would already be "pointed" at the rear axle. The diff unbolts and I would just have to fabricate a tailshaft housing. It will also allow me to run a longer driveshaft.

QUOTE]

Will it all turn the right way? Or will you need to get a set of reverse gears cut?


Mikey
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Old 05-04-2007, 11:56 AM
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Quote:
Originally Posted by powerrodsmike
...Will it all turn the right way? Or will you need to get a set of reverse gears cut?
You're very perceptive! I would be turning the engine and trans 180* in the chassis, which would cause the tranmission output shaft to be reversed. My solution for that would be in my custom tailshaft housing. I would have a single gearset which would reverse the rotation again, and allow me to raise the output shaft for a lower engine/trans installed height = lower cg.


Quote:
Originally Posted by BillyShope
As for the subject of this thread: The driveshaft U-joint provides a couple to the rear axle assembly and, as you look back at the rear axle assembly, you can move that couple anywhere...up and down or left and right...and the effect remains the same.
Thank you a million times over! That's exactly what I was looking for. I was just concerned about any major effects it could have on the suspension dynamics. With that solved it's just a question of feasibility (project budget), and overall design. I figured I should find out if it would even work, before spending hours on design and budgeting.

The reasons I am even considering this, as opposed to just running a traditional mid-engine transaxle and independent rear suspension, are strength and design. It takes a LOT of money to come up with a mid-engine transaxle that is even close to the strength of a moderately built performance automatic. With the older Eldo/Toronado transaxle being based on TH400 components, I can build a relatively bulletproof transmission for a reasonable cost. I am leaning heavily towards a Poncho motor, as well, that would bolt right up to this transaxle - no adapter plates/kits needed. Also, in the strength department, I can (reasonably) build a 9-inch based rear end that will stand up to any abuse I can throw at it.

That rear end also reaches over into the design aspect, as I love the look of a stick axle rear end in a street rod. You'll be able to see all the goods from behind (open bodywork) and I just love the idea of something like a quick change, or Mark Williams modular rear end between massive tires. Also in the design considerations are the dynamic advantages of having a true mid-engine layout. No tricky suspension needed for adequate traction, just the right tires and setup. That should equate to better handling, and more enjoyable cruising. Finally, the aesthetic design possibilies are endless when I don't have to work around a big motor in front of the "cockpit". I have incredible freedom in wrapping the bodywork over the chassis.

Last edited by toddshotrods; 05-04-2007 at 12:03 PM. Reason: typos
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Old 04-19-2009, 11:21 PM
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Greener grass on other side of fence

Not really surprised to find non-engineers speculating about design possibilities, because it is entertaining, and it does fuel the imagination and it seems the grass is always greener on the other side of the fence. But actually when we begin to study chassis design and geometry there are real reasons why cars are designed and built the way they are. Cars are designed by highly educated engineers. I am not an engineer at all but from what I have read, all of the offsets and angles are carefully engineered for many reasons, and to change them more often than not will undoubtedly produce some undesired effect. By turning the Eldo/Toronado front wheel drive system sideways, and the driveshaft placed too far to one side, the torque of the engine may apply a force to flip the car over on it's side! The harmonics/vibration oscillations of a faulty engineered mismatch of components could make a person car sick in a short amount of time. The placement of engine mounts and components affects the safety of a car in a collision; how the heavy engine component will break away and if it does where it will go etc. There are rotational forces like inertia and momentum at work that demand to have specific angles and turning motions for the chassis and drivetrain to work right and feel right. I have no degree in physics, I'm no automotive chassis engineer, but I have enough education to respect their designs which may seem to be simple, but they are actually the result of a combination of very complex principals of design. I'd say stick with the tried and true methods, and while the grass may look greener on the other side, it probably isn't, and worse, maybe that grass is only like weeds unfit to eat... maybe a car unfit to drive. -to "do it right" will take a real bit of educated/engineered design, beyond a group consensus of street smart enthusiasts.
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