Prop shaft Resonance?

[Dazzer]

Hi Guys

 

Tried a few searches on the forum to see if there was a similar problem to identify a mild resonance between 55 & 70 under light load, but not present when no power applied.

 

Vanishes completely under 50 and above 80+ or under hard acceleration (probably cant hear it due to grand prix noise levels!)

 

The resonance is a regular 'thrumming' but not bad enough to be disturbing, sounds very much like a muted Zeppelin airship!

 

Was just going to order a new propshaft but wondered if anyone had experienced a similar resonance.

 

I've checked diff oil level, greased the prop etc but no difference.

 

Best way to avoid it is to drive hard and fast. Ah well.

 

Cheers

 

Darren

[marvmul]

Trying to find the cause by replacing parts will be very expensive. Chances are minimal that it is the propshaft.

I believe TR4 or TR4A had a piece of metal bolt onto the gearbox to avoid resonance of the gearstick?

[TR 2100]

TR engines seem to have a natural resonance at the sort of speed you describe.

Often, this can make the exhaust, already critical for clearance especially though

the centre cruciform, vibrate on the chassis.

 

Could well be the propshaft UJs, though, as you say. Need to check out both.

 

AlanR

[vivdownunder]

Gee Dazzer, it's been a while since a muted Zeppelin went over !. (Presume you are talking film)

 

We'd probably relate the sound to 50 blowies on a cow pat.

 

I'm wondering if it's an exhaust noise.....perhaps something loose, or just touching on the chassis somewhere. Exhausts will resonate at specific engine revs. Or it could be something else loose that's vibrating under the car floor. I've had to find and fix all of these at times, including a rattling fuel pipe that had jumped out of a clip on the chassis. The fuel line can also rattle where it passes through the chassis cross member towards the front of the car, if the cushioning grommet isn't there.

 

With the tailshaft, if it's a uni joint on the way out, you can get a vibration noise, then as the needle bearings collapse, you get a distinct "clunk" every time you take off. That noise calls for urgent action.

 

It would be extremely rare for a tailshaft to just suddenly go out of balance, as a cause of the thrumming sound.

 

Do you have any opportunity to put the car up on a hoist and check out everything underneath ?. This would give you more of a chance to correctly ID the problem, and you might not have to buy anything at all.

 

Regards,

 

Viv.

[Dazzer]

Hi Guys

 

Thanks for the comments, and you'd be surprised what flies over North Yorkshire, plenty of cow pats too but not many blowflies! must be the damp!

 

Im going to investigate the exhaust further as it has the extractor manifold and larger bore pipe fitted, which on starting does touch the chassis. I'll also batten everything down and go out for another spin.

 

Thanks once again

 

Cheers

 

Darren

[Alec Pringle]

Hi Darren,

 

Coarse technique for identifying exhaust resonance . . .

 

Bolt a standard exhaust-fixing u-clamp midway along the suspect length of pipe (eg midway betweeen silencer boxes, or midway between pipe joins) and take a quick drive. If the thrumming note changes, you know you've found the resonator . . . if not, work your way along the pipe !

 

Cheers,

 

Alec

[Dazzer]

Hi Chaps

 

Yep it was... and still is the exhaust system, so thanks for the tips.

 

Probably live with it, as cant really get any adjustment on it. It might in fact be inherent in the system, as a little loud for my taste.

 

Extractor manifold, and one rear silencer, is this normal on a sports type system or should it have two silencers?

 

Cheers

 

Darren

[Alec Pringle]

Hi Darren,

 

if the exhaust touches the chassis on starting, that just might be a tired engine mounting allowing the lump to leap around more than intended - and can also be a factor in the periodic resonance of the exhaust.

 

Wrapping the manifold underbonnet, or even a section of exhaust pipe. is one way of reducing both noise and resonance. Another cure for a long resonating pipe is to cut and sleeve over - one size larger pipe, say 6"-8" long, a notch cut in both ends to allow tightening, and sleeve that over a cut in the centre of the offending pipe, u-clamp either end to seal.

 

Cheers,

 

Alec

[vivdownunder]

Dazzer, 3A's and 4's had a single pipe system with two mufflers originally. There was an 18" x 5" x 3" ahead of the diff and a 10" or 12" x 5" X 3" between the diff and the back of the car. 4A's had a twin system, except for where it came through the chassis cross.

 

My 3A has Triumphtune extractors and I just run one 20"L x 5"W x 3"H glasspak straight through muffler ahead of the diff, and in fact I have just fitted a new one, and it's a bit too quiet for my liking. It seems modern mufflers employ improved acoustic technology, so the rear one is optional. Of course everyone has their own idea on sound, and there's the MOT to consider, so that's just what I've fitted.

 

If you are renewing the muffler, beware the main suppliers, as they have been providing oversized mufflers that are a nightmare to fit, and some owners have had to cut front corners off and weld over a patch. Don't go any larger in diameter than the above, or you either foul the floor, or are so close you fry the floor and carpet. I prefer to buy mufflers from the local exhaust shop, as they are the experts with good advice.

 

The main problem with the engine pipe hitting the chassis is where it comes through the oval cutout in the chassis cross. If it's too close in the narrow vertical space available, I've squashed the pipe slightly oval to get better clearance. That avoids that terrible grinding noise right under your backside.

 

Regards,

 

Viv.