Driveshaft Length Question - FFCars.com : Factory Five Racing Discussion Forum

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post #1 of 8 (permalink) Old 05-16-2017, 05:36 PM Thread Starter
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Driveshaft Length Question

When I first installed my driveshaft it was way too short, it seemed like there was 3" of the yoke hanging out past the tailshaft seal. Too much for me considering we have 510 horses to harness. The machinist that lengthened it asked for the measurement from each side of the rear flange, that the u-joint bolts to, to the output shaft of the trans. I took the measurement at 4" ride height. When I re installed the lengthened driveshaft I noticed the yoke slid right against the tailshaft seal. I guess he forgot to allow for the rear articulation. I've upped the ride height to 5" and now have 3/8" between the tailshaft seal and the ears for the u-joint on the driveshaft. I want the ride height back to 4", so my question is how big should the space between the seal and the ears for the u-joint be to allow for anticipated rear axle movement? My driveshaft is already 3/8 shorter than the length suggested in the build manual. And thinking about it, setting the pinion angle may have changed the length needed too. Still doesn't change the question.

Thanks, Glen


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post #2 of 8 (permalink) Old 05-16-2017, 08:04 PM
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When you're making these height changes, most likely you are changing the pinion angle. Better check that at the desired ride height, before taking any drive shaft length measurement.

If you have a custom driveshaft made at a place like dyno tech, the main things they want are a measurement between the u-joint saddle face at the pinion shaft and the the end of the transmission, and the amount the tail shaft protrudes beyond the tail shaft. They also need some width info on the u-joints. They take care of making the driveshaft the right length. If it doesn't work, it usually means an incorrect measurement.

Custom Built Aluminum & Composite Drive Shafts & Parts Shop | Dynotech Engineering Services

http://www.dynotecheng.com/downloads...%20to%20CL.pdf


This article says that there should be 1 inch of the yoke shaft showing, at ride height.

How to Measure for the Correct-Length Driveshaft - Hot Rod Network

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post #3 of 8 (permalink) Old 05-18-2017, 08:08 PM Thread Starter
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So, since my driveshaft is about 10 inches long instead of the 40 incher in the article how will that affect that 1 inch measurement?

Thanks, Glen

Coupe 0652-408W/510 hp-TKO600 .64OD-3.73 TractionLok-3 Link-26x10 front 26x12 15" Mickey Thompson-Heat and A/C- Complete Kit-First Start 7/10/15 First Go Kart 8/15/15 Painted 11/12/16 Titled and Registered 3/24/17 Legally on the road 4/20/17
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post #4 of 8 (permalink) Old 05-19-2017, 01:20 PM
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The shorter length will increase the amount of movement, but not a lot. This would be roughly calculated by making the assumption that the driveshaft can move up or down by 2 inches in each direction. If the shaft is 10 inches long, then the length needed to with the pinion at the travel limit is the the hypotenuse of a right triangle with one side of 10 inches and the other side 2 inches. The answer is the square root of 104, which is 10.2 inches. A 40 inch driveshaft would move about 1/4th that amount.

Also a factor is how much movement you get from a typical 4-bar suspension. The amount of length change should be less, since the 4-bar arms should be longer.

If the splined yoke is 4 inches long it could move 1 inch inward before bottoming out and 1 inch outward would leave 2 inches of engagement with the transmission shaft.

Here's a picture of my car, with a driveshaft length around 40 inches. It's got a triangulated 4-bar suspension. It looks like about an inch of splined shaft showing. I know that the car is sitting at ride height because I used mock-up bars to replace the coil-overs, until that car was almost ready to drive. Even the front wheel alignment was done with mock-up bars in place.

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Last edited by daveS53; 05-19-2017 at 03:29 PM.
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post #5 of 8 (permalink) Old 05-19-2017, 03:40 PM Thread Starter
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Dave, Thanks for sticking with me on this. I'm pretty good with math, but it has been a long time.

This is the first time I wish my car had IRS, it sure would simplify this issue.

Set ride height and pinion angle where I want it.

Even at full compression, my driveshaft will not be straight. So is my take on this correct? Extend the centerline of the driveshaft. Measure distance to full compression measurement down from straight line mark. Install driveshaft yoke all the way in to measure hypotenuse. (add small fudge factor) Actually center of u-joint cap to center of u-joint cap would be the hypotenuse and longest possible length. At full extension, measure distance down from straight line mark. Using hypotenuse from last calculation and down measurement, calculate the yoke position on the output shaft. Then I should know how much of the yoke will be left in the trans at full extension. At this point, ride height is somewhere in the middle and it is what it is, and the yoke is somewhere in the middle as well. The yoke shouldn't crash the output shaft on a bump and it shouldn't pull out going over a jump.

If you can follow this (good luck) and it seems right, let me know.
Thanks a lot, Glen

Coupe 0652-408W/510 hp-TKO600 .64OD-3.73 TractionLok-3 Link-26x10 front 26x12 15" Mickey Thompson-Heat and A/C- Complete Kit-First Start 7/10/15 First Go Kart 8/15/15 Painted 11/12/16 Titled and Registered 3/24/17 Legally on the road 4/20/17
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post #6 of 8 (permalink) Old 05-19-2017, 07:01 PM
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I may have got you confused. The math just gives you an idea of how much movement you can expect when the suspension moves up and down. If you have the pinion angle set where you want it (usually parallel to the engine), all you need to do is measure the length from the face of the u-joint yoke at the pinion to the end of the transmission and the amount of the tail shaft protrudes from the transmission. It's a simple tape measure job that roughly follows the angle between the pinion yoke face and the end of the transmission housing.

If the tail shaft protrudes a lot, the person building the driveshaft would know to expose more of the shaft on the transmission yoke, so it does not bottom out with forward movement. I left that calculation to the folks at Dyno Tech, since I was buying a new driveshaft.

The only other difference between your car and most others is the driveshaft may move an additional .15 inch due to it's short length. Add another 3/16" of yoke shaft exposure, to be safe.

https://www.iedls.com/#Yokes

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post #7 of 8 (permalink) Old 05-19-2017, 07:55 PM Thread Starter
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I left those calculations up to my machinist too, that is how I got here. You did exactly what I needed, a place to start calculating. If I was looking for an easy way out, I would have bought a finished car. I haven't seen the Pythagorean Theorem in at least two decades. Thanks Again

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post #8 of 8 (permalink) Old 05-19-2017, 08:51 PM
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Quote:
Originally Posted by gsides9 View Post
I left those calculations up to my machinist too, that is how I got here. You did exactly what I needed, a place to start calculating. If I was looking for an easy way out, I would have bought a finished car. I haven't seen the Pythagorean Theorem in at least two decades. Thanks Again
You're on the right track. Maintain as much engagement of the yoke with the transmission shaft at the greatest outward travel and make sure it can't bottom out with inward travel.
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