Inadvertently exceeded driveshaft "Critical Speed" at Willow Springs.

[jbc426]

While exercising my '68 convertible Barracuda during the Spring Fling Track Day a few weeks ago, I experienced a deep, high-resonnant frequency vibration when the car was in excess of about 105 mph. The vibration became more intense as I increased my speed increased past 130 mph, and went away completely under 105'ish mph. I immediately suspected a driveline issue, but know I had gone to great lengths to get everything lined up after installing a 200R4 mated to a stroked 5.9 Magnum. My current driveshaft is a Strange engineering built 3" chromemoly unit with forged ends, 1350 U-joints and a Mark William's forged transmission yoke. The transmission has a fresh tailshaft bushing, and there is 1" of clearance once the drive shaft is bolted in.

I measured and verified my driveline angle as "0" degrees at the transmission and "2" degrees down at the pinion. I am running slapper bars to control spring wrap. Dr Diff helped me get that dialed in. I'm learning now that i should get the pinion angle closer to just under 1 degree for very high speed use.

While researching this issue on-line the other day, I began to learn about the "critical speed" of driveshafts, and how that is dependent upon a variety of factors including length, construction materials and other items. Depending on where you look, it seems my current driveshaft has a critical speed of around 5500 to 6500 rpm. That correlates to the incidence of vibration I experienced at the top end of 3rd gears 1 to 1 ratio and beyond into 4th gear's .67 to 1 overdrive ratio. Theoretically, my current set-up would have the driveshaft approaching 10000 rpm wound out to the rev limiter in 4th gear.

I had no idea a high quality chromemoly driveshaft with 1350 U-joints and forged ends was essencially a fuse waiting to blow at high speed. This is obviously a very hazardous situation given that sudden catostrophic failure of a steel driveshaft can occur when critical speed is exceeded. That could have ruined my whole day or life. I thought I should pass this information on, given the amount of people adding performance upgrades to our old Mopars these days.

It seems the only solution is to buy a higher quality drive shaft with a much higher critical speed rating than a steel or aluminum shaft. That leaves an expensive carbon fiber unit from QA1 or another manufacturer to solve the problem.

The whole situation reminds me of the old saying, "Speed cost money, son. How fast do you want to go?"

 

[clifftt]

Do you run a driveshaft loop? Might be cheap insurance.

 

[jbc426]

Do you run a driveshaft loop? Might be cheap insurance.

The driveshaft is currently just captured by my exhaust. Not ideal, and soon to be changed.

 

[replicaracer43]

Drag cars often turn RPMs into the 8k range and run steel driveshafts, i would probally be more interested in the 0 degree at the tailshaft of the trans. Are you saying that the driveshaft and yoke are perfectly inline? as in 0 degree?

 

[jbc426]

Drag cars often turn RPMs into the 8k range and run steel driveshafts, i would probally be more interested in the 0 degree at the tailshaft of the trans. Are you saying that the driveshaft and yoke are perfectly inline? as in 0 degree?

No, the trans centerline is at "0" and the pinion is pointing nose down "2" degrees. The rear end is below the transmission centerline, so not sure exactly how much angle the driveshaft leaves the transmission at.

 

[Marcohotrod]

increasing shaft diameter increases critical speed

 

[Killer6]

January 2012 issue of Car Craft, good article w/chart,up
in dia. raises cr. spd., as does switching to alum. in the same dia., or do both for a big jump. Depending on the actual length, a 3.5" alum. shaft will increase safe rpm
1.25-2.25K

 

[Killer6]

This is why I'll repeat what I voiced in the rear gear poll,
I'm not fond of overdrives,I'd rather have an extra deeper
1st gear instead. Run a strong rear w/3.23's max, and
these vibration problems are minimized.The prop shaft
will have to handle more launch torque,but that's the least
of your worries on a road course.

 

[66fs]

I used to take my 64 Barracuda to 140 MPH and felt no vibration. 273, 4 speed, stock HD rear leaf springs, factory driveshaft, factory trans and pinion angles. Does not seem right to me that it would be driveshaft.

 

[jos51700]

A sufficiently sized aluminum shaft should be more than stable. Cars have run faster than yours (with overdrive) long before carbon fiber shafts were available. Crown Vic cop cars have aluminum shafts that ought to work well in an A body. It ultimately depends on the mass of the shaft.

Have you taken the shaft to a different balance shop? Checked trans mount, u joints, tail bushings (new doesn't mean "right"), etc?

 

[jbc426]

A sufficiently sized aluminum shaft should be more than stable. Cars have run faster than yours (with overdrive) long before carbon fiber shafts were available. Crown Vic cop cars have aluminum shafts that ought to work well in an A body. It ultimately depends on the mass of the shaft.

Have you taken the shaft to a different balance shop? Checked trans mount, u joints, tail bushings (new doesn't mean "right"), etc?

An aluminum 4" might take care of it, but critical speed is a calculable factor, and the math still doesn't add up for predictable rpm safety. I will run my numbers past the techs at QA1 to verify my set-up.

The 52" long center to center driveshaft was made at Strange Engineering, so it is a quality piece. Regardless, I took it to my local shop to have it checked, and he was amazed at how well balanced it was. He also inspected both u-joints and all checked out good. I replaced my transmission output bushing and it was nice to begin with.

I'm going to adjust my pinion angle to just under 1 degree down and my transmission angle to 1 degree up. That's 2 degree down pinion angle in relation to the transmission centerline with equal angles at each end of the shaft to equalize and cancel out joint phasing.

I'm also going to install a driveshaft that's critical speed is rated higher than I could potentially spin it.

 

[jos51700]

Are you able to redline in top gear?

 

[jbc426]

Are you able to redline in top gear?

I don't know if it will. With the current cam (214 @ .050 w/ 110 lobe separation) should be 410+hp. I would definitely need a longer run than I had at Willow Springs to do it. But, it was pulling hard at 130+ mph, so it seemed that it would eventually redline in 4th.

I'm upsizing the cam (234 @ .050 w/ 108 lobe separation)to better match my intended use. It will bump the power up to 510+hp, and make it easier to potentially approach redline in 4th.

It only has to rev to 4690 rpm in 4th to make the drive shaft hit critical speed now.

 

[jos51700]

I'm not an expert on this by any means, but if you can hit redline in top gear, I thought that meant you go to a steeper rear gear, which would slow down drive shaft speed?
On the japbikes, we'd always redline on the gear under the top gear.

 

[mguner]

I used to take my 64 Barracuda to 140 MPH and felt no vibration. 273, 4 speed, stock HD rear leaf springs, factory driveshaft, factory trans and pinion angles. Does not seem right to me that it would be driveshaft.

Were you running an overdrive with lower rear gears? I was looking for the ratio the OP is using....

 

[jbc426]

Were you running an overdrive with lower rear gears? I was looking for the ratio the OP is using....

Yes, overdrive at .67 to 1, and I'm running 3.91's with 28" tall tires. I can easily wind out 3rd, and it is still pulling hard in 4th. I have yet to wring it out in 4th yet, but I wound it it up pretty good at Willow Springs. The vibration was really starting to concern me though.

Opps, answering for the wrong guy. Sorry.

 

[66fs]

Were you running an overdrive with lower rear gears? I was looking for the ratio the OP is using....

If I remember correctly It was a regular A833. Not sure what gears were in the rear. I see where you are going.

 

[jos51700]

I was thinking about this on the way to work this morning. I've had my Dakota to 120, and it's wore the hell out, with overdrive and 3.55's. No shake.
How are you certain that isn't a tire issue?
NOT doubting, just wondering if you've somehow eliminated these as the source of the problem. Remember, is almost impossible to get something perfectly balanced, we just balance tires and wheels to be in balance below certain thresholds. It's easy to spit a weight at that speed.
Additionally, are you sure your tires are speed rated? Tires do funny funny stuff when pushed up too those speeds and the drive shaft sure sounds straight and balanced. (It is straight, right? It can be bent and still be balanced at certain speeds)
Forgive me if you covered that and I missed it.

 

[mguner]

I drove my 71 Demon on the street for a while with 4.56 gears and had a new shaft made and balanced to cure some vibration issues like you are having. Phasing and balance are critical on higher speed shafts for sure.

 

[jbc426]

I was thinking about this on the way to work this morning. I've had my Dakota to 120, and it's wore the hell out, with overdrive and 3.55's. No shake.
How are you certain that isn't a tire issue?
NOT doubting, just wondering if you've somehow eliminated these as the source of the problem. Remember, is almost impossible to get something perfectly balanced, we just balance tires and wheels to be in balance below certain thresholds. It's easy to spit a weight at that speed.
Additionally, are you sure your tires are speed rated? Tires do funny funny stuff when pushed up too those speeds and the drive shaft sure sounds straight and balanced. (It is straight, right? It can be bent and still be balanced at certain speeds)
Forgive me if you covered that and I missed it.

I appreciate the input. No apologies necessary. Tires can always get knocked out of balance or throw a weight. My rear tires are rated at 186 mph, the fronts are rated to 146 mph; so they should take it. I had them rebalanced just before the trip down to LA and they were smooth as silk the whole trip.

A few months earlier, I had a rear wheel bearing go bad on the passenger side. That vibration seemed like a drive line vibration at normal freeway speeds. That's when I had the driveshaft double checked. Strange built the driveshaft and stated this morning that it has a critical speed of 7000 rpm, but take away 20% as a safety margin. That's only 5600 rpm.

What is the overdrive ratio on your overdrive, and what rpm were you spinning at 120 mph?

The sensation of the vibration is

 

[jos51700]

I appreciate the input. No apologies necessary. Tires can always get knocked out of balance or throw a weight. My rear tires are rated at 186 mph, the fronts are rated to 146 mph; so they should take it. I had them rebalanced just before the trip down to LA and they were smooth as silk the whole trip.

A few months earlier, I had a rear wheel bearing go bad on the passenger side. That vibration seemed like a drive line vibration at normal freeway speeds. That's when I had the driveshaft double checked. Strange built the driveshaft and stated this morning that it has a critical speed of 7000 rpm, but take away 20% as a safety margin. That's only 5600 rpm.

What is the overdrive ratio on your overdrive, and what rpm were you spinning at 120 mph?

The sensation of the vibration is

If strange rated your drive shaft, they should already included the factor of safety. If they took twenty percent, and then you do on top of that....You could really be selling it short. I don't know that, but might be worth calling and asking.
That jaunt was in my 5.2 Dakota. 3.55 rear with whatever overdrive ratio a 727 has.

I'm really curious to see how this turns out for you. Drive shaft failure scares the hell out of me! At least you know it's not tires!

 

[GMachineDartGT]

We have had countless driveline issues in the shop with shafts made by "experts". Once we check runout in three places on the shaft, that usually tells a different story. We ONLY have Dynotech make our driveshafts period. I don't trust anyone else. My Dart has mild gears (2.94) and 1:1, but 150mph on the straight at NJMP, no issue.

 

[C300]

Had my Monti SS with stock 200R4 up to 130 ish lots. Cruise 110 all day long. Smooth as glass. Will have to put my drive line protractor on it to see the stock angles. One of the most important things in building a car is drive line angle. Jbc426, you said in another thread that US car tool was going to make a trans mount that was lower. Would like to know what you get now with a protractor. If it's not optimal, I'll build my own. Not wanting to cut up a finished product. That pan looks REALLY low too. What's the ground clearance for it now ?

 

[jbc426]

If strange rated your drive shaft, they should already included the factor of safety. If they took twenty percent, and then you do on top of that....You could really be selling it short. I don't know that, but might be worth calling and asking.
That jaunt was in my 5.2 Dakota. 3.55 rear with whatever overdrive ratio a 727 has.

I'm really curious to see how this turns out for you. Drive shaft failure scares the hell out of me! At least you know it's not tires!

No, Strange states the critical speed is 7000 rpm for this specific shaft. They further state that to factor in safety, it should only spin to 80% of 7000 rpm.

With a .69 to 1 overdrive in a 518 transmission and a 28 inch tall rear tire ( likely shorter than stock), you are not quite spinning the drive shaft to 3200 rpm.

 

[jbc426]

Had my Monti SS with stock 200R4 up to 130 ish lots. Cruise 110 all day long. Smooth as glass. Will have to put my drive line protractor on it to see the stock angles. One of the most important things in building a car is drive line angle. Jbc426, you said in another thread that US car tool was going to make a trans mount that was lower. Would like to know what you get now with a protractor. If it's not optimal, I'll build my own. Not wanting to cut up a finished product. That pan looks REALLY low too. What's the ground clearance for it now ?

With the thicker Schumacher motor mounts, the transmission centerline was at 0 degrees. My pinion angle was 2 degrees nose down. Not sure how much ground clearance it has, but its been more than enough thus far.

I am swapping factory rubber mounts back in and using the Schumacher torque strap to hold the motor down to avoid breaking the front mounts and get rid of the engine vibration that is fed into the chassis with their harder mounts. This will slightly change my transmission centerline from 0. Once I find out what that is, I am going to shim the rear end so that it is just under 1 degree nose down in relation to the transmission centerline. This way both ends of the driveshaft will have nearly equal angles, but the pinion angle will be slightly nose down in relation to it < 1 degree to allow for upward rotation under throttle. It was at 2 degrees nose down before.