GT6 vibration from rear end

[Keith Scotch]

My GT6 Mk 3 ( non-rotoflex) is running fine until I get to around 50mph when I start to get a vibration from thew rear of the car which gets worse and noisier as speed increases. The tyres and rims are fine and balanced. I can't see anything obvious wrong with the wheel bearings or UJs. There doesn't seem to be any excess play or clunks. I'm left thinking that it could be prop shaft related (out of balance?) or maybe it is the bearings or UJs? Is there an easy way to track down the fault? Have I missed something obvious? Don't want to mess around removing the prop or wheel bearings if they're ok. Thanks

[JohnD]

Prop shaft possible, or else U/j .

Get it up on wheel stands, look under, shake the prop shaft for looseness.  Ditto all the suspension joints - a pry bar may be useful.

Then run the car on the stands, while observing it from a safe distance!   Can you see prop vibration at the  appropriate speed.    (This requires careful thought for a safe test!)

There is a method to 'rebalance' the prop, by putting two jubilee clips on it, with the screws on opposite sides, so they balance.     Then move ONE around.   If vibration worse, move it the opposite way, until the vibration settles.     THis isn't a quick procedure!

[dougbgt6]

Keith,

Before you start diving into the worst possible case scenario I would swap the tyres around. Just because they look OK doesn't mean they are.

Doug

[Clive]

But experience suggests the orop us orientation suspect.

My gt6 had an out of balance prop, made the car horrible 50-70mph. 

Current spit had a uj seize. Unbearable, but nonpayment in the uj, only obvious once dismantled. Changing the uj left the prop out of balance, so that was another job. 

If prop is the likely cause, find somewhere to fit quality ujs and rebalanced. 

Wheels are about the only other suspect, bearings and diffs don't vibrate. Bent driveshaft could.

[Pete Lewis]

Keith pop it round to sunny luton  and we can have a listen

firstly  props run at engine speeds wheels and drive shafts about 4 times slower so the frequency of the vibes is a good pointer to the culprit

also if someone has had the prop apart the yokes must be aligned to each other at each end of the prop one spline error on the telescope will give problems

so is it  a va va room vibes  or a round and roundy  vibe ??? 

ie  say  the prop at 50mph is at around 3000 rpm  the wheels are at 750    theres a big difference 

Pete

 

 

 

[JohnD]

Hang on, Pete!

The prop runs at a speed proportional to road speed, not engine speed, the proportion being the diff ratio.

It also runs at a speed proportional to engine speed but that depends on which gear you're in.

And the yokes that connect at each end have bolt holes spaced in pairs,  so each can only be connected in one of two ways, not four.      It is said that they were originally balanced in situ, and should be pop-marked before being taken off, so that they are reassembled in the same orientation, else vibration will occur.      With two ways at each end that's 2x2 =4 possibilities, and while one is perfect, two are bad and the last is AWFUL!      If you have had the prop off, then think about refitting it in different orientations.

But the balancing was done by welding a small steel plate to the shaft, about halfway down.     I don't think the plate was well protected against corrosion and can just rust away!   

Having said all that, my money is on a U/j fault.  A pry bar should reveal which one.

John

[NonMember]

My GT6 had similar vibration. On investigation it turned out the sliding spline on the propshaft was shot. That may be fitted at the front, so not readily accessible, but it's well worth a check.

[Keith Scotch]

Thanks for your advice chaps - much appreciated. Pete - I live in Flitwick so not far from Luton. I'll take you up on your offer of having a listen to it.

[Pete Lewis]

13 hours ago, JohnD said:

Hang on, Pete!

Im just trying to make the vibration frequency into a more understandable where it coming from   at 50mph Keith will be in 4th Ish 

 

and as Rob says  the telescope can be badly worn maybe because it never travels , my Vit6 was appalling when we got her .

Pete

[JohnD]

12 hours ago, Keith Scotch said:

Thanks for your advice chaps - much appreciated. Pete - I live in Flitwick so not far from Luton. I'll take you up on your offer of having a listen to it.

Best idea so far!  Pete will see you right!

[Pete Lewis]

we will attempt a test run at Mondays Pub meet at the Raven 

first problem is .....getting  me   into a GT6    then getting out again  

Pete

[Pete Lewis]

Just been out  with Keith and its pretty certainly propshaft 

its comming off so we can have a look , this could be phasing .or telescope wear, unlikely a UJ 

its a high frequency kicks in at 50 quite hard /noisy   but can be felt earlier at low speed 

all will be revealed sometime later 

think a trip to Bailey and Morris is on the cards 

Pete

 

 

[Pete Lewis]

just looked at normal triumph supplies and yikes props have certainly gone UP a lot 

luckily im just looking not buying  

will have a gander at the prop before Keith jumps into  deep wallet syndrome 

Pete

[Clive]

Dave Mac quoted me £195 recently, I doubt that included VAT.....

[Pete Lewis]

yes the 150ish has jumed to over 200 

maybe im just getting out of touch but after covid everywhere is playing price catch up 

Pete

[NonMember]

2 minutes ago, clive said:

Dave Mac quoted me £195 recently, I doubt that included VAT.....

That's still cheaper than I paid at Bailey Morris last month (for a GT6 prop).

[Clive]

Canleys are £218 incl. VAT, seems pretty good. HD version a bit extra.