Driveshaft vibration
Discussion
I have a 3" chromoly steel shaft almost 53" long, suplied from the US complete with front trans slip yoke, and rear pinion yoke, U-bolt style fixing.
Due to a cock up, I had to have this shaft shortened by approx 1" locally, re-welded and balanced.
I fitted it, and it drove fine, until approx 6-6500rpm shaft speed was reached, at which point a massive vibration enters the car. This is at about 130mph or so.
I then had the shaft re-balanced by same firm, they did add some small weights. They balance at max 3000rpm, which is claimed to be accurate to 14000rpm. I re-fitted and placed car on axle stands, and drove up to same speed in the air ( stands under axle ). Same vibration enters car.
The shaft runs virtually straight, with gearbox end at very very slight angle.
Rear end is 4-linked rose jointed Ford axle.
There was info with the shaft suggesting it could vibrate at its "critical speed" which would be about 7000rpm for my shaft length, and you should stay below this speed.
Im using 3.27 gears, if virtually any other car had this shaft it would vibrate a lot lot sooner on the road, and be totally useless, as many use much shorter gearing..
Axle and old 2 locally made piece shaft had been fitted and working fine with old engine/trans combo. Only change to the axle is the fitment of the U-bolt pinion flange to suit the new shaft.
WTF is going on ?? Is the shaft still at fault ? Is this critical speed thing nonsense ? ( I do understand materials have natural frequencies where they tend to vibrate etc )
What do I need to fix it ? Can anyone actually test spin the shaft on a rig to 6500rpm ?
Due to a cock up, I had to have this shaft shortened by approx 1" locally, re-welded and balanced.
I fitted it, and it drove fine, until approx 6-6500rpm shaft speed was reached, at which point a massive vibration enters the car. This is at about 130mph or so.
I then had the shaft re-balanced by same firm, they did add some small weights. They balance at max 3000rpm, which is claimed to be accurate to 14000rpm. I re-fitted and placed car on axle stands, and drove up to same speed in the air ( stands under axle ). Same vibration enters car.
The shaft runs virtually straight, with gearbox end at very very slight angle.
Rear end is 4-linked rose jointed Ford axle.
There was info with the shaft suggesting it could vibrate at its "critical speed" which would be about 7000rpm for my shaft length, and you should stay below this speed.
Im using 3.27 gears, if virtually any other car had this shaft it would vibrate a lot lot sooner on the road, and be totally useless, as many use much shorter gearing..
Axle and old 2 locally made piece shaft had been fitted and working fine with old engine/trans combo. Only change to the axle is the fitment of the U-bolt pinion flange to suit the new shaft.
WTF is going on ?? Is the shaft still at fault ? Is this critical speed thing nonsense ? ( I do understand materials have natural frequencies where they tend to vibrate etc )
What do I need to fix it ? Can anyone actually test spin the shaft on a rig to 6500rpm ?
Its nearly 5 feet long then.
No centre support bearing i presume? The old arrangement being 2 shorter parts with a bearing would have vibrated at way over the speeds youre running and hence the reason they were ok.
This new one would deffoo seem to be the cause of your aggravations as itll reach its critical vibration frequency much lower down...it does seem a rather long prop to me to be run unsupported.
No centre support bearing i presume? The old arrangement being 2 shorter parts with a bearing would have vibrated at way over the speeds youre running and hence the reason they were ok.
This new one would deffoo seem to be the cause of your aggravations as itll reach its critical vibration frequency much lower down...it does seem a rather long prop to me to be run unsupported.
Actual shaft is 53" ( measured between UJ centres )
a 3" shaft is a fairly common thing as far as I can see, although unsure about length.
If I was using say 4.1 gears, which are quite common in the US, it would hit the vibration at only 100mph or so. That would be totally useless.
Surely my driveshaft isnt that much longer than what other cars are using ???
a 3" shaft is a fairly common thing as far as I can see, although unsure about length.
If I was using say 4.1 gears, which are quite common in the US, it would hit the vibration at only 100mph or so. That would be totally useless.
Surely my driveshaft isnt that much longer than what other cars are using ???
IIRC 58" is the max recommended length for a one piece drive shaft. As the shaft gets longer toward the 58" limit RPM would need to go down as the longer shafts have a tendency to bow when they get close to their critical speed.
I would guess that the reason that balancing hasn't helped is that you are getting bow. Chrome-moly is a stronger steel so it is usually thinner to save weight and keep the same strength. It may have helped to have had it made in a 4" diameter as the greater diameter would help to decrease the "bow" effect.
Drive shafts can be made out of Aluminum or Carbon fiber, but when these materials are used the diameter is always increased to resist "bow". You may need to have the shaft cut in two and run a center bearing to cure the problem.
I would guess that the reason that balancing hasn't helped is that you are getting bow. Chrome-moly is a stronger steel so it is usually thinner to save weight and keep the same strength. It may have helped to have had it made in a 4" diameter as the greater diameter would help to decrease the "bow" effect.
Drive shafts can be made out of Aluminum or Carbon fiber, but when these materials are used the diameter is always increased to resist "bow". You may need to have the shaft cut in two and run a center bearing to cure the problem.
I do not have room for anything larger diameter than 3". Tunnel in the car is very tight, and Im not cutting it. The 4130 tubing is actually quite thickwall. I'll measure it later, as I have one I cut up.
A 2 piece would be ideal, but finding someone to make one with a suitable centre bearing and strong enough for my needs is proving very very difficult.
Ive enquired with a few places. Some did get back to me, but still waiting on a price ( 3 months ago )
I did place an order with place.....2 months ago, and they still havent bothered, and its not specced as strong as I would like.
I want a shaft I can fit, and forget about, and never having to worry about breaking it, even if I have a go with slicks. The current 1 piece fits that bill....it just doesnt work normally.
A 2 piece would be ideal, but finding someone to make one with a suitable centre bearing and strong enough for my needs is proving very very difficult.
Ive enquired with a few places. Some did get back to me, but still waiting on a price ( 3 months ago )
I did place an order with place.....2 months ago, and they still havent bothered, and its not specced as strong as I would like.
I want a shaft I can fit, and forget about, and never having to worry about breaking it, even if I have a go with slicks. The current 1 piece fits that bill....it just doesnt work normally.
Stevieturbo, just thought of this, what are your angles?
They should be no less than 1 degree and no more than 3 degrees. You probably have checked, but I remembered that the angles can influence vibration as well. Are the u-joints phased properly ?
Are you using a double cardon u-joint at the differential, I believe that they help cancel vibration with long drive shafts.
I know some (or all) of these suggestions are basic, I'm just trying to cover all the bases.
Not trying to be a pain, just trying to help.
>> Edited by Trooper2 on Saturday 21st May 19:10
They should be no less than 1 degree and no more than 3 degrees. You probably have checked, but I remembered that the angles can influence vibration as well. Are the u-joints phased properly ?
Are you using a double cardon u-joint at the differential, I believe that they help cancel vibration with long drive shafts.
I know some (or all) of these suggestions are basic, I'm just trying to cover all the bases.
Not trying to be a pain, just trying to help.
>> Edited by Trooper2 on Saturday 21st May 19:10
I have no way of measuring the angles, but would be confident to say axle end is almost, if not straight. Trans end has a bit of an angle, but is to be expected as axle moves up and down. But I mean that is a very very small angle. The vibration doesnt feel its coming from the trans end.
Standard 1350 UJ's front and rear, U-bolt fixings at rear.
When you say phased, it is fitted as the "balancer" marked them. But I did also try swapping it 180deg too, but to no avail.
Looks like I may keep searching for someone to build a 2 piece....Ive been searching the web, and it seems the critical speed thing is a problem for all.
Standard 1350 UJ's front and rear, U-bolt fixings at rear.
When you say phased, it is fitted as the "balancer" marked them. But I did also try swapping it 180deg too, but to no avail.
Looks like I may keep searching for someone to build a 2 piece....Ive been searching the web, and it seems the critical speed thing is a problem for all.
what you need to remember is that dynamically the axle nose moves when you are applying the higher torque at higher speeds. So the aim should be that when you have the car mid laden, doing around 80 mph, that you can project a line down the centreline of the crankshaft, and have it go straight through the prop and axle pinion.
I worked on driveline issues for a number of years, but I had the benefit of being able to use accelerometers to generate frequency plots and work out what is going on.
You don't have that luxury, so you are limited in what you can do.
What I can say is that when you balance the prop you should have 17 gcm imbalance as an upper limit, and 9 g cm is a good target for a luxury car with a vibration problem.
I'd get the prop imbalance measured, get them to tell you what the measurements are, then have them put some effort into it.
I worked on driveline issues for a number of years, but I had the benefit of being able to use accelerometers to generate frequency plots and work out what is going on.
You don't have that luxury, so you are limited in what you can do.
What I can say is that when you balance the prop you should have 17 gcm imbalance as an upper limit, and 9 g cm is a good target for a luxury car with a vibration problem.
I'd get the prop imbalance measured, get them to tell you what the measurements are, then have them put some effort into it.
I wouldnt think axle nose would move much under load. As I said, it is fully rose jointed with 3/4" joints
Problem is, the local firm who has balanced say it is near on perfect using their machine, at 3000rpm.
I believe the problem is beyond their ability to measure.
Ive now sent emails to several driveshaft suppliers Ive found online. Hopefully some can help me out pretty quick.
Problem is, the local firm who has balanced say it is near on perfect using their machine, at 3000rpm.
I believe the problem is beyond their ability to measure.
Ive now sent emails to several driveshaft suppliers Ive found online. Hopefully some can help me out pretty quick.
Another idea, Stevieturbo, put one or two stainless steel hose clamps on the drive shaft (depending on how bad the vibration is) and mark the starting location. Try moving the heavy side of the clamps from starting position to 90 deg from were you start,if no improvement then 180 and 270 deg Etc.
place the clamps toward the end of the drive shaft that you feel the vibration is coming from.
This is a trick thats been used in the U.S. trucking industry for eons. It's easy and it just might work.
place the clamps toward the end of the drive shaft that you feel the vibration is coming from.
This is a trick thats been used in the U.S. trucking industry for eons. It's easy and it just might work.
Surely all this is missing the point? According to the manufacturer, shaft flex is going to cause vibration at around 7000 rpm. So it should come as no great surprise that you're getting vibration at around 7000 rpm. Surely you're stuck with this unless you either drop the rpms or stiffen the shaft, for example by shortening it.
Impossible to drop rpm's. Not being able to go over 130 is totally useless, as will already be very close to it at the end of the 1/4.
There is also a chance I will try shorter gearing at some point in the future, which will increase rpm's.
It seems a 2 piece is the most likely solution. I just hope someone can make a good strong 2 piece shaft.
Of course, even if they can, from past experience waiting on Americans to do anything, I'll be lucky to have it for Christmas.
>> Edited by stevieturbo on Saturday 21st May 20:54
There is also a chance I will try shorter gearing at some point in the future, which will increase rpm's.
It seems a 2 piece is the most likely solution. I just hope someone can make a good strong 2 piece shaft.
Of course, even if they can, from past experience waiting on Americans to do anything, I'll be lucky to have it for Christmas.
>> Edited by stevieturbo on Saturday 21st May 20:54
The issue is the first order bending frequency. The longer the shaft, the softer it is, so the lower the frequency it will have.
If you are running the shaft at 6000 rpm that is 100 Hz. If it's 7200 rpm that's 120 Hz.
A lot of sports cars are top speed limited to stop the prop getting near 1st order bending freq. and then failing at the designed in weakness to give good collapsibility in crash ("crash feature"
. The only ways to increase the frequency are to make it of stiffer material (i.e. CFRP) or make it in two parts.
If your prop is running near the FOBF you do need to try to improve balance, and ensure that run-out is well controlled.
For your application though I really think the best solution is a CV jointed two piece shaft - it won't be cheap but it will survive this arduous application.
If you are running the shaft at 6000 rpm that is 100 Hz. If it's 7200 rpm that's 120 Hz.
A lot of sports cars are top speed limited to stop the prop getting near 1st order bending freq. and then failing at the designed in weakness to give good collapsibility in crash ("crash feature"
. The only ways to increase the frequency are to make it of stiffer material (i.e. CFRP) or make it in two parts. If your prop is running near the FOBF you do need to try to improve balance, and ensure that run-out is well controlled.
For your application though I really think the best solution is a CV jointed two piece shaft - it won't be cheap but it will survive this arduous application.
You mention CV jointed ? as opposed to a Hardy Spicer universal joint ??
I am looking into a 2 piece further. I have actually had one on order now for over 2 months, but as I say...getting Americans to actually make anything seems a hard task.
I know they can do it, and they say they can do it, they just dont do it.
I never wanted a 1 piece in the first place, but few places offered a 2, so I went for the 1 anyway. I got fed up waiting on an alternative, I fitted the one piece to get the car driving.
Why I wasnt informed the 1 piece would have such major limitations when I ordered is beyond me. As far as I can see my current shaft is uselss.
>> Edited by stevieturbo on Wednesday 25th May 00:21
I am looking into a 2 piece further. I have actually had one on order now for over 2 months, but as I say...getting Americans to actually make anything seems a hard task.
I know they can do it, and they say they can do it, they just dont do it.
I never wanted a 1 piece in the first place, but few places offered a 2, so I went for the 1 anyway. I got fed up waiting on an alternative, I fitted the one piece to get the car driving.
Why I wasnt informed the 1 piece would have such major limitations when I ordered is beyond me. As far as I can see my current shaft is uselss.
>> Edited by stevieturbo on Wednesday 25th May 00:21
It sounds as if your plan for a two-piece prop would be the best bet.
I was wondering how normal big rwd saloons get away with this. My V8S is far too short to run into this sort of thing of course, but I thought surely the larger saloons will suffer the same problem? However, when I looked at a schematic for an SD1 I found that the diff had a huge nose on it and the actual prop was still quite short. Are more recent BMWs etc similar? If so, is there any prospect of swapping the diff for a longer nose one rather than engineering all new prop shaft mounts etc?
I was wondering how normal big rwd saloons get away with this. My V8S is far too short to run into this sort of thing of course, but I thought surely the larger saloons will suffer the same problem? However, when I looked at a schematic for an SD1 I found that the diff had a huge nose on it and the actual prop was still quite short. Are more recent BMWs etc similar? If so, is there any prospect of swapping the diff for a longer nose one rather than engineering all new prop shaft mounts etc?
My car is a 1980 Mk2 Granada. It originally had an IRS setup, with fixed mounted diff, which had a longish nose. Not that matters.
It had a 2 piece shaft as standard, as do most, if not all BMW's, Mercs etc etc.
I always knew a 1 piece shaft was a problem as far as fitment goes, as I now have a live axle. The tunnel was never built to have a prop moving up and down inside it, and also not woth a T56 gearbox.
With my old Rover V8/Supra gearbox and the live axle, I had a modified Supra 2 piece shaft, which worked perfectly. But with the new engine and power expectations the 1 piece seemed my cheapest and easiest option, guaranteed strength.
The old Ford/Rover shaft was minute when I was on the rover box, and it broke. When I went Supra box, the new supra shaft seemed massive compared to the ford one. Now, the shaft I have with 1350 joints for teh T56/8.8" rear makes the Supra shft look like a matchstick. That I like.
I fitted bumpstops to the rear end, lifted the car up a bit, and fitted the 1 piece shaft. It has touched the floor on occasion, but no biggy.
I had always wanted a 2 piece, but as I say, trying to get someone to make one is a nightmare. The centre bearing mount on my car is also tight for space, but at about 4" OD is workable.
So basically I want a 2 piece shaft with centre bearing approx 4" OD that will handle about 800bhp and some sticky road legal tyres.
Is it so much to ask ?
It had a 2 piece shaft as standard, as do most, if not all BMW's, Mercs etc etc.
I always knew a 1 piece shaft was a problem as far as fitment goes, as I now have a live axle. The tunnel was never built to have a prop moving up and down inside it, and also not woth a T56 gearbox.
With my old Rover V8/Supra gearbox and the live axle, I had a modified Supra 2 piece shaft, which worked perfectly. But with the new engine and power expectations the 1 piece seemed my cheapest and easiest option, guaranteed strength.
The old Ford/Rover shaft was minute when I was on the rover box, and it broke. When I went Supra box, the new supra shaft seemed massive compared to the ford one. Now, the shaft I have with 1350 joints for teh T56/8.8" rear makes the Supra shft look like a matchstick. That I like.
I fitted bumpstops to the rear end, lifted the car up a bit, and fitted the 1 piece shaft. It has touched the floor on occasion, but no biggy.
I had always wanted a 2 piece, but as I say, trying to get someone to make one is a nightmare. The centre bearing mount on my car is also tight for space, but at about 4" OD is workable.
So basically I want a 2 piece shaft with centre bearing approx 4" OD that will handle about 800bhp and some sticky road legal tyres.
Is it so much to ask ?
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