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MG MGB GT V8 Factory Originals Technical - Body flex and vibration
|What are the symptoms of body flex in a V8 conversion?|
I have been tracking down the cause of some elusive body
shaking and / or driveline vibration. After switching rims
and tires I have adjusted pinion angle and switched driveshaft.
I am even wondering about my combination of spax shocks,
track bars and GT springs all round. The shaking/vibration
is not a constant occurrence. It seems worse some days than
others but always above 50 mph and worse at 80mph. It is most
noticable on a smooth higway at a sustained cruising speed.
It is not noticable around town or on rougher roads.
When I built the car I cut the top of the tranny tunnel as
shown in the Williams book and I dont know how well my body
guy fit the new tunnel top. Now I am wondering about body flex!
Car has SD1 Rover, 5spd Rover, Ford 9"posi, Rover3500S driveshaft,
new Yokohama tires on Minilite rims.
Anybody been down this road? Any input appreciated.
|Sounds like wheel balance problems if the problem occurs at around 60-70mph-if you are running big wheels -say larger than 185/70 on heavy wheels it is possible that you will get scuttle shake at certain road speeds on a roadster(you don't say which type that you have got).This problem occurs on 1800cc versions as well.|
The other problem might be the "track bars" that you say that you have-if they are any thing like the abortions that Moss sell then the rear suspension will bind which you may feel as a shudder thro' the shell.
Just a few thoughts......
|Thanks John, I have found a scallopped tire on the front of the car.|
I have Spax shocks (only 3 years on the car) on the front and back but
perhaps a bad shock is contributing. I have alloy wheels with 195/65-14
Yokohamas. I am also running track bars similar to the Moss
set-up. I'll disconnect them and see what happens.
What is scuttle shake exactly, what does it feel like? Do all roadsters suffer it the same?
I have a 79 Roadster.
Thanks for the ideas,
|Scutttle shake is a term derived from when older cars only had a ladder chassis which used to twist with road undulations and cause the 'dashboard' and the 'scuttle'(ie the area of body work in front of the screen) to shake sideways due to only being braced with ash framing.|
I have exprienced scuttle shake in a friends 1950cc '76
roadster fitted with 185/70 tyres on LE wheels(-the car was bodily sound-)when sitting in the passenger seat you could detect that the dash would shudder slightly at certain road speeds and undulations-I put this down to the heavier wheel and tyre combination flexing the front chassis legs slightly and this giving the shake.
I also think that the current fashion for putting the exhaust thro' the inner wing(fender) -ie cutting a large hole in the metal helping to brace the chassis leg will worsen the situation if the opening is not braced in some other way.A bigger anti-roll bar (sway bar) in poly blocks will only make matters worse as these forces are being fed into the c/legs forward of the bracing effect of the X/member
The Moss anti -tramp bars are crap becuase they cause the suspension to bind even when rose jointed-if you really get the power down they will also tear out of the floor-look at the bodyshell pictures in Roger Williams book of the RV8 front spring mounting to see how much bracing Rover had to put in to resist the loads.I'm currently using Rv8 single taper leaf springs with my own design of compliantly bushed top mounted radius arms.With a '79 it would be easy to use the existing rear anti-roll bar body mounting and void bushed knuckle(BTW don't scrap these any body as I hear that the good old Cortina/Taunus bush fits in a treat with the central bush taking a slightly larger bolt)and make a tubular arm with a rose jointed connection to the OE a/roll bar mounting on the axle-keep the OE A/r bar if the car is not lowered but use plastic bushed alloy blocks to transmit the tramp loads from the axle.
|Over some surfaces if I wedge the tip of a finger into the angle made by the 1/4-light and its sealing rubber on the windscreen I can feel slight scuttle shake on my 4-cyl roadster. It is this as much as anything, I believe, that causes the 'crack of doom' if the 1/4-light is pressed too hard against this rubber, instead of just meeting it.|
|My own conclusion to the problem has been sparked by the previous ideas...thanks|
guys. I beleive that I have at least one bad front lever shock. When I added the
Spax front shocks I used old lever shocks of which one was "stiff". I also made a
mistake of adding regular engine oil to them once I had removed the valves. I see it seeping out of the shafts.
I have been noticing a clonk coming from the front driver's wheel area and there is
some play in the steering column upon braking. I really suspect a bad lever to kingpin
connection. I think this explains the affect of different road surfaces on the feel
of the steering wheel.
I think it is time for a new pair!
Thanks again for your input.
This thread was discussed between 30/08/1999 and 01/09/1999
This thread is from the archive. The Live MG MGB GT V8 Factory Originals Technical BBS is active now.