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Driveshaft Knocking Underneath


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Hi Guys got another problem with the car when under hard accelaration, especially if two up the driveshaft is lifting & hitting the underside at the end of the tunnel its the velocity joint that is hitting to be exact. i'm thinking I might have to raise the suspension to stop it happening its got avo adjustable dampers with seperate springs might need bigger springs open to any help

thanks Andy

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  • 2 weeks later...

How have you worked out what hits what? The differential should be firmly bolted to the chassis and should not move. The gear box is rubber mounted and can move an inch at most so the propshaft should not move enough to hit the tunnel.

Inboard ends of the drive shafts either side of the diff wont move much either so that leaves their outboard ends which will rise toward the inner wheel arch on acceleration and may hit, especially if two up. I seem to remember this could be an issue with the earlier monocoque cars and you could cut out some of the inner arch to fix. Probably a good idea to fit stronger or longer springs as well to reduce rear end squat.

Can you find witness marks on the arches or other bodywork or on the prop or drive shafts to confirm what hits what?

 

Nigel

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Hi ive been under the car to have a look there is quite a bit of rubbing underneath the plastic nut blocks on the handbrake cable are half worn away & there are bump marks on the chassis just above where the velocity joint joins the rear diff. its as if the front of the diff where it joins the driveshaft is twisting upwards on hard throttle. ive tried using spring assistors, but not cured it my only thing i think i can try next is to poly bush the rear end to stiffen it all up.

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Should have asked what car you have and what back axle. I have just assumed sierra donor, diff bolted to chassis with independent swing arm rear but you could have cortina or dolomite live axle which allows much greater movement of the rear of the prop.

 

Nigel

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Hi all I have a similar problem on my 2B When the o/s rear is loaded IE cornering hard there is a thumping from that side.I have checked everything but can't find it .Could it be a wheel bearing?.Also are the driveshafts supposed to be parallel mine sloping down to the hubs by 5 to 10 degrees.It is worse 2 up but it has been like this for about a year

HELP PLEASE Arthur

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Mines is a 7 with cortina rear end bolted to the monoque chassis there are 8 bushes altogether 4 bottom & 4 top going between the axle & the chassis. hope that helps Nigel

Andy

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I had this with my Cortina based S7.

 

In the end I replaced the springs and shocks with whatever was recommended on the Burton power website for a road going Cortina. The adjustable Gaz shocks current set about 1/2 way and also a pair of spring suppressors.

I've not had the problem in the last 1,500 miles or so.

 

Judging by the damage mine must have been like this since it was build back in the early nineties. The previous owner raised the suspension by about 1.5 inches which didn't make much difference with any weight in the car, and the bad roads round here.

 

Rich...

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The problem is torque reaction in the axle allowing it to twist when accelerating, pushing the nose of the diff upwards and thus the rear end of the propshaft upwards to a degree that allows it to contact the tunnel roof. To resolve this the axle needs to be better located by the 'trailing' arms. The cortina live axle has a lot of compliance built into the rubber bushes of the arms even when new. When old and soft, axle location is even worse allowing more torque twist so replacing with all new standard bushes will help axle location. A change to poly bushes would further improve axle location but possibly at the expense of some comfort. If you do go polybush use the softer road type not full race. This by itself may be enough to sort the problem of axle twist so would be the best first step IMO.

If you still got the problem then increasing the shock settings to slow suspension compression on acceleration may help. Fitting stronger springs of the same length would be the last step. You don't want to alter the ride height significantly by fitting longer springs.

 

Nigel

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I sorted this on my S7 but it took some serious engineering. Drop me your email and I'll send pictures and the like.

Basically you need to stop the axle rotating by fabricating and fixing new top arms parallel to and the same length as the bottom arms. When fixed in place they actually intrude in to the cockpit. The nose of the diff needs to be angled down slightly, and a panhard rod would be a good idea. Once in place you can rag it as hard as you like and it will just squat, and go!

Changing springs will have no effect on the axle rotation.

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Changing springs will have no effect on the axle rotation.
True but if it holds the diff one inch lower/further away from the body of the car than previously for the same forces it might not go bang bang bang on the underside.

 

Nigel

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thanks for the valuable info hopefully will have it sorted as soon as.I can't imagine what the previous owners must of put up with. just one of those things that i'm just gonna have to constantly tinker with so that i'm happy with it. tim I will send you my email address & thanks

 

Andy

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