(A forum of almost all Factory Five owners)

See Greg Dunn's note about driveshaft angularity

Author Topic:   driveshaft length
Steve Dunkel posted 02-26-99 07:29 PM PT (US
Has anyone tried a shorter driveshaft length?
A friend of mine who is a ford mechanic and fellow factory five builder went with 9 3/4".
Do you think that the length of the shaft might be the reason for binding that a few owners have been experiencing?
BIG RON posted 02-26-99 07:52 PM PT (US)

That's about the length I went with since the machine shop said that's what my driveshaft would be if they cut and welded it correctly. It was from a Turbo Coupe so it might be a little different from a Mustang .I don't think a little play could hurt anything. Any other ideas ????? BIG RON
Greg Dunn posted 02-26-99 08:32 PM PT (US)
I've got to agree, mine's cut to spec and seems a bit too long...first time I hear it "thump" it's coming out and getting shortened again. My advice is to go at least a 1/2 inch more than spec.
Jason Knepp posted 02-26-99 08:36 PM PT (US)
Tremec and I went 7.25 from center to center. It looks great. Have not driven hard yet though. I will let ya know.
Steve Dunkel posted 02-26-99 08:43 PM PT (US)
If you are talking about the tremec trans is it the same length as the t-5? I was going to go one inch shorter, but that sounds really short!
Irv Lennert posted 02-26-99 09:43 PM PT (US)  
I disagree shorter is not better. If you shorten the shaft, its relative joint angles will increase with the same wheel (suspension) travel. These will both increase the possibility of binding and increase U-joint ware. Unless you’re really pushing into the tail shaft I would not shorten it.
-Irv FFR 1470
Greg Dunn posted 02-26-99 11:17 PM PT (US)  
Steve, the tremec is a little longer so you will need a slightly shorter shaft. And I agree with Irv, the shorter it goes the greater the angle. Supposedly the angle shouldn't be more than 6 degrees. (ERA NOTE!  6 degrees translates to little more than 1" travel in each direction. ERA's independant suspension allows 3"+) I did notice that when my car's suspended by a jack and the safety wire fully extended that the joints are binding..not healthy (have to try from getting airborne) - the other side of the coin is who's had a problem - I know of only one and it was caused by having wheels with wrong offset, tires too big. He adjusted the body higher to avoid rubbing and may have had a loose pinion to boot - needless to say the 3/4" tubing was ripped in half, two holes in the tunnel and a small crease in the rr wheelwell when the shaft spit out.
John Sawyer posted 02-27-99 03:34 AM PT (US)  
I'm Steve's freind, and I'll explain why I went with 9.75". When I talked to the shop about shortening the shaft, they said it would be better to measure the length on the actual car as opposed to taking the measurement from the manual. The procedure they recommended was to insert the slip yoke all the way into the trans, then slide it out about an inch, then measure the required length. I checked out several Mustangs at our shop and noted that the slip yoke was 1 1/8" out on average while sitting at ride height. I used 1 1/8" on the Cobra and made my measurement. It came out to exactly 9.75". Several posts were made about binding under hard acceleration. Under hard accleration, the rear of the car squats putting the suspension into compression (not rebound). If your joints are binding due to excessive angle when your rear is at maximum rebound travel (rear axle hanging against limiter straps), that is a different problem. My concern was with how far into the trans the slip yoke ends up when the shaft is 10.75". I was concerned that the "binding" on hard acceleration that people were having might actually be due to the yoke bottoming against the trans. Maybe I'm wrong, but I guess I'll find out when I get FFR1475K on the road!
Rick Framil posted 02-27-99 05:44 AM PT (US)
I am going to soon build my second cobra replica the first was a CMC ( yikes that sucked ) the second will be a FFR. I have also done other cars that required driveshaft shortening.

i have always diregarded the manuals on done an independent measurement. each time the measurement has never been what the manual said. i would recommend doing this.

the car should be at ride height with all wheels carying load. insert the tranny yoke all the way in then pull it back out 1 1/4 measure from the center of the u-joint opening on the yoke to the center of the u-joint opening on the rear end flange.

i have never had any problems with the driveshaft yoke coming back into the tranny.

hope this helps


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