Inadvertently exceeded driveshaft "Critical Speed" at Willow Springs.

[jbc426]

With the thicker Schumacher motor mounts, the transmission centerline was at 0 degrees. My pinion angle was 2 degrees nose down.

That must have been someone else that stated US Cartool is going to make a lower mount. Ground clearance is fine on my transmission. My exhaust use to scrape on very tall speed bumps before I switched to TTI step headers and their 3 inch H-pipe exhaust.

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.

Must have hit post twice.

 

[Mopar to ya]

Man, if you can wind it out in 4th gear, you'll be screaming. And it isn't going to happen in a quarter mile track. I run a 408 and a 200-4R, 4.10 gears and 27.75" tires and I never even get to overdrive at the track. I had a street race against a Mercedes SLK55 AMG on the highway that took us up to 145 mph and I had more to go before I maxed out my rpms. I actually had my driveshaft built at a local shop that used what they call "race tubing". I assumed it was just a feel good term, but that shaft with the 1350 yokes has had the crap beaten out of it for four years now. Same u-joints and everything. Like you, I had / have a harmonic vibration. Mine comes in around 80-90 and goes away after 115 or so. I tried pinion angle, balancing the driveshaft, different wheels and different tires. I still have it, and I am at a loss. I'm getting a new torque converter in a few weeks and I'll see if that makes a difference. While the transmission is out I am going to get a new bushing in the tailshaft because it feels a little sloppy. I wish you luck, and if you find the problem, please let me know. I will do the same.

 

[jbc426]

Man, if you can wind it out in 4th gear, you'll be screaming. And it isn't going to happen in a quarter mile track. I run a 408 and a 200-4R, 4.10 gears and 27.75" tires and I never even get to overdrive at the track. I had a street race against a Mercedes SLK55 AMG on the highway that took us up to 145 mph and I had more to go before I maxed out my rpms. I actually had my driveshaft built at a local shop that used what they call "race tubing". I assumed it was just a feel good term, but that shaft with the 1350 yokes has had the crap beaten out of it for four years now. Same u-joints and everything. Like you, I had / have a harmonic vibration. Mine comes in around 80-90 and goes away after 115 or so. I tried pinion angle, balancing the driveshaft, different wheels and different tires. I still have it, and I am at a loss. I'm getting a new torque converter in a few weeks and I'll see if that makes a difference. While the transmission is out I am going to get a new bushing in the tailshaft because it feels a little sloppy. I wish you luck, and if you find the problem, please let me know. I will do the same.

I bought the tool and changed my tailshaft bushing. Turns out it was fine to begin with. Mine stays in 3rd through the 1/4 too. I was able to hit 4th at Willow Springs though, and that's when the vibration started.

I spoke with both Strange Engineering and QA1 regarding my driveshaft speeds. Even a 4"aluminum shaft would exceed critical speed on the top end. It's not worth the risk for me. I'm pretty sure I am going to have to pry open the wallet and opt for a carbon fiber driveshaft. I'd hate to be wring out the old 408"r passing through 140+mph and have the driveshaft explode.

 

[jos51700]

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.

I bought the tool and changed my tailshaft bushing. Turns out it was fine to begin with. Mine stays in 3rd through the 1/4 too. I was able to hit 4th at Willow Springs though, and that's when the vibration started.

I spoke with both Strange Engineering and QA1 regarding my driveshaft speeds. Even a 4"aluminum shaft would exceed critical speed on the top end. It's not worth the risk for me. I'm pretty sure I am going to have to pry open the wallet and opt for a carbon fiber driveshaft. I'd hate to be wring out the old 408"r passing through 140+mph and have the driveshaft explode.

I'm surprised to hear that about strange, but it's good to know. I'm curious how much a cf shaft costs now. ..

 

[Mopar to ya]

4.10 gears ... the driveshaft turns 4.1 revolutions for each revolution of the wheel? Or ring gear? The rpm of a 15" wheel at 100 mph is around 1100 and at 130 is around 1400. Times that by your rear end gear, mine being 4.10 and the driveshaft rpm shouldn't get close to the max of 7000 rpm. This, of course, is assuming my mind is working correctly and the wheel rpm is correct.

 

[jbc426]

4.10 gears ... the driveshaft turns 4.1 revolutions for each revolution of the wheel? Or ring gear? The rpm of a 15" wheel at 100 mph is around 1100 and at 130 is around 1400. Times that by your rear end gear, mine being 4.10 and the driveshaft rpm shouldn't get close to the max of 7000 rpm. This, of course, is assuming my mind is working correctly and the wheel rpm is correct.

Try to forget the rear end ratio for a minute unless you are also calculating speed. If you are winding your motor to 5000 rpm in 4th with a 200R4 overdrive and lock-up converter, you are spinning the driveshaft at nearly 7463 rpm. My rev limiter is set at 6700 rpm, but the motor wont pull it that high in 4th, that would be 10000 rpm at the driveshaft. That's with the .67 overdrive ratio.

The speed can them be calculated with rearend ratio and tire size. With my 275/60-15's & 3.91's and the motor rev'ing to 5000 rpm, the car will be at about 160mph. That's fast enough for me, and has the driveshaft spinning at 7463 rpm. That's too fast for my current driveshaft. A 4" aluminum drive shaft can deal with about 8900 rpm for about half the cost of a carbon fiber shaft, but with nearly a 17% safety margin.

 

[Mopar to ya]

How do you get 6700 rpm out of a 408? Mine hates anything more than 6000. I set my rev limiter at 6200, but I can hear the engine hating me at 6000.

 

[Killer6]

4.10 gears ... the driveshaft turns 4.1 revolutions for each revolution of the wheel? Or ring gear? The rpm of a 15" wheel at 100 mph is around 1100 and at 130 is around 1400. Times that by your rear end gear, mine being 4.10 and the driveshaft rpm shouldn't get close to the max of 7000 rpm. This, of course, is assuming my mind is working correctly and the wheel rpm is correct.

The prop shaft turns 4.1X both, ring gear&tire revs are the same(hopefully),or you broke somethin'. Tire diameter and rear axle ratio are the only determinants of driveshaft rpm's at a given mph, terminal velocity is varied by power level,proper trans gearing, and aerodynamics. For instance,the laser XE turbo I had did 114mph in 4th, and not 1mph more in 5th. As a matter of fact, it would start to lose speed on a grade & I would have to grab 4th to maintain speed. The later A-555 had 3.87 final drive instead of the A-525's 3.56's,and might of allowed a few more mph w/the same OD 5th. BUT, there is no driveshaft involved in that gearing. In a RWD, a less steep OD would be better. If the OP does get over 510 ponies kickin', he will most certainly be dangerously close to pushing up high into that OD gear before topping out. Aero stability would be the next concern on the agenda......

 

[jbc426]

The prop shaft turns 4.1X both, ring gear&tire revs are the same(hopefully),or you broke somethin'. Tire diameter and rear axle ratio are the only determinants of driveshaft rpm's at a given mph, terminal velocity is varied by power level,proper trans gearing, and aerodynamics. For instance,the laser XE turbo I had did 114mph in 4th, and not 1mph more in 5th. As a matter of fact, it would start to lose speed on a grade & I would have to grab 4th to maintain speed. The later A-555 had 3.87 final drive instead of the A-525's 3.56's,and might of allowed a few more mph w/the same OD 5th. BUT, there is no driveshaft involved in that gearing. In a RWD, a less steep OD would be better. If the OP does get over 510 ponies kickin', he will most certainly be dangerously close to pushing up high into that OD gear before topping out. Aero stability would be the next concern on the agenda......

I suspect the "pucker factor" will be my limiting thing as far as terminal speed goes. The car was dead stable and felt great at 135mph with the top down, but things start to change beyond that speed in terms of how the car feels etc.

 

[Mopar to ya]

I've had mine to 147 once just cruising and another time at 145 racing a Mercedes. Both times I had more to go, but the 1973 steering, suspension and brakes had me just about crapping myself. It was right after the Mercedes race that I ditched my T rated BFG Radial TA for some V rated Nitto tires. I'm with jcb about the pucker factor. The only thing limiting a 408 / 200-4R is ball size.

 

[Mopar to ya]

By the way, I went to my local transmission guy who impressed me with his 200-4R knowledge. Also, he was building one for his GTO, so I trusted him more than I normally might. He adjusted my TV cable for higher line pressure and firmer, more timely shifts. I was taught that at wide open throttle the TV should be all the way open. He has a kit that pulls it even tighter for more positive shifts. He believes at least part of my vibration is the worn tailshaft bushing. My vibration is very small compared to what it once was, so I'm getting a new bushing and hoping that helps even more. You said you bought a tool to do your bushing. Where did you get the tool, and can it be done on vehicle, or do I have to pull the transmission?

 

[jbc426]

By the way, I went to my local transmission guy who impressed me with his 200-4R knowledge. Also, he was building one for his GTO, so I trusted him more than I normally might. He adjusted my TV cable for higher line pressure and firmer, more timely shifts. I was taught that at wide open throttle the TV should be all the way open. He has a kit that pulls it even tighter for more positive shifts. He believes at least part of my vibration is the worn tailshaft bushing. My vibration is very small compared to what it once was, so I'm getting a new bushing and hoping that helps even more. You said you bought a tool to do your bushing. Where did you get the tool, and can it be done on vehicle, or do I have to pull the transmission?

I have the link for the tool at home and will post it later. It's a nice simple tool for R&R'ing the bushing with the transmission in the car.

 

[Mopar to ya]

Sweet. I have very little money to play with. If I have to spend, I will, but if I can do it, that's so much better. There is a WIT Transmission shop a block away from my shop. They stock everything. I'll bet they have the bushing.

 

[Killer6]

I've had mine to 147 once just cruising and another time at 145 racing a Mercedes. Both times I had more to go, but the 1973 steering, suspension and brakes had me just about crapping myself. It was right after the Mercedes race that I ditched my T rated BFG Radial TA for some V rated Nitto tires. I'm with jcb about the pucker factor. The only thing limiting a 408 / 200-4R is ball size.

I have driven a "brick" rite around 145, but that was all she had at the time. The aero concerns are that frt. end lift can cause the nose to hunt around like the steering was loose or sloppy, I could feel this in most of my cars around the 120 mark, so tires and steering are not always to blame for that.I added an air dam to that car for that and increased cooling air to the rad. worked OK 'till I killed a dog that ran out in front of me one nite w/it.

 

[FredS]

jbc, what's the update on your harmonic?

Just talked to my local driveshaft shop, and asked questions, and he stated they spin their shafts to 11K RPM when balancing, and he said my current 3inch tube shaft with 1350 ujoints would be just fine. He also says, you have up to 6 degrees before it becomes a problem with harmonics, or risk in breaking anything. These guys do a lot of race cars, and lifted trucks, and I've used them many times.

dshaft length and tube diameter relate, and being under 50 inch, 3 inch tube would be fine, he says.

 

[72Duster440]

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.

I will second Dynotech, I have gone through a lot of driveshaft issues on my Jeep when I went to 5.13 gears, all of the places listed in the back of car mags couldn't figure it out. Dynotech took care of the problems, and they are local to me, excellent place to deal with. I've used them many times.