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Author Topic:   Critical Speed Driveshaft
Dirt Forum Champ
Total posts: 1875
posted September 22, 2005 08:37 PM  
I have been destroying U joint bolts(the u bolt), I mean strectch oblong them, whatever almost every weekend east a set. Then The crush sleeve on my new gear is about half destroyed after one race.
We are going to beef up things like the solid crush, but have any of you guys had this problem.
I called a few places and they thought maybe it was a driveshaft problem. I have a 28 inch driveshaft, .120 wall, steel, 2 inch dia. driveshaft. I am going to go talk to one of my proffesors about critical speed for the driveshaft, and looked on one webpage, but i needed some other info i could not remember.
I think for 06 I will go to a 2.5 inch dia driveshaft to be safe.
Have any of you guys had similar problems and how did you fix it?

Dirt Forum Champ
Total posts: 1172
posted September 22, 2005 09:23 PM  
Make sure your u j oints arent in a bind ie too much angle in it at a static setting like setting there race ready. maybe tranny angle too low at tailshaft or too much pinion angle in rearend . I know its possible for some new yokes to not have as much clearence for the joints to have max clearence and require grinding. Yes for serious racing use a solid crush collar on your pinion shaft and not a reg crush sleeve.

Dirt Forum Champ
Total posts: 1875
posted September 22, 2005 09:47 PM  
We had not had any problems before. With my 355 turning it abotu 7400 max.
Now I have a 350 turning 8900+ And problems are showing up all over. So first thought was the critical speed of the driveshat might be approacing, so afriad it was wobbling and tearing it up. I will check all the angles this weekend, we have a big race to take it too.

Dirt Maniac

Total posts: 168
posted September 22, 2005 10:23 PM  
Zero you get circle track mag. ? There is a article in there november 05 issue about drive trains and it talks of critical speed of drive shaft. Sounds like it would give you a lot of food for thought. Where you going to a big race memphis missouri is got a cople good shows coming up in october.

Dirt Forum Champ
Total posts: 796
posted September 23, 2005 07:02 AM  
Get rid of the U-bolts and go to the Moroso girdle deals. We had alot of trouble out of the strap type bolts. Until we replaced them with the girdles, they make 3 different ones, red blue and gold, all 3 different sizes.

Are you running a 1310 series in the rear? the small one, or a bigger 1350 series?


Dirt Freak

Total posts: 402
posted September 23, 2005 08:26 AM  
Shaft speed doesn't change if yor running a shorter gear with more rpm. Have you verified a drive line inbalance? Pinion angle and phasing? How about the u joint saddles are they the right size for the joint cap?, Try changing yokes.

Dirt Forum Racer

Total posts: 89
posted September 23, 2005 12:17 PM  
I had more problems with a small shaft, I know another mod who had tried the 2" shaft and destroyed them. Oh yeah, not to be jerk to leapinlizar but engine RPM =driveshaft RPM, reqardless of rear gear. (UNLESS YOU GEAR THROUGH THE TRANS) If he is now turning 8900 then a 2" driveshaft is PROBABLY past its crital RPM.

Dirt Full Roller

Total posts: 56
posted September 23, 2005 04:52 PM  
Make sure the driveshaft is balanced, use solid spacer on pinion, make sure yoke is in good shape, and check u-joint angles. I would check the u-joint angles at static but I would also raise the car and let the rear hang and rotate the driveshaft and check angles and clearance, if the pinion is down to far and the tranny up to high at full suspension it could install bind, if the pinion angle is where you want it and the tranny is to high and you are using a motor plate and mounts without a tranny mount, just shimming the motor up with a couple washers under the motor mount makes a big difference at the tailshaft.

Dirt Freak

Total posts: 206
posted September 23, 2005 05:37 PM  
It sounds to me like the pinion yoke is loosing it's preload due to the crushsleeve failing.This in turn causes the yoke to wobble which destroys the u-joint and the ubolts.The small diameter driveshafts will flex and distort at a lower RPM than the larger ones.For torsional strength smaller is not better.Why do you need to turn a motor that tight anyway?Those kind of revs are hard on everything.Been there done that.

Dirt Maniac

Total posts: 102
posted September 25, 2005 06:56 AM  
That is a lot of rev's for a wet pan. Zero, what kind of pan do you use? Also, is this a 4 bar car? My guess is severe pinion angles combined with the rear wanting to drive up under the car.

Dirt Roller

Total posts: 24
posted September 25, 2005 01:39 PM  
Make sure you use quality u-bolts such as Dorman We used u-bolts from NAPA once twisted the u-bolts, off they are Junk.

Dirt Forum Champ
Total posts: 1875
posted September 27, 2005 01:21 PM  
The tranny is 1:1 falcon so locked up, 8900 is 8900 at the driveshaft also. The we took alot apart over the weekend, and found the crush sleeve problem. it was messing up. I am shooting for 8400-8600 but missed the gear the other night and was running 8900. We went back over everything and no wear or other problems motor wise.
I am on a 4 bar car and will start to shim the motor up under the front mounts.
thanks guys.

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