[Staowners] SCUTTLE SHAKE REVISITED

[CotaLymepond at aol.com]

Ah  Heck ... here's my 5 cents worth
 Scuttle shake is when we see the front portion of our cars  vibrate. (if you 
really want to experience this phenomenon, go for a ride  in an old T-series 
MG)  The body on a roadster is somewhat flexible by  design, throw in 60 years 
of wear and tear, a little rust, and squashed out body  mounts and the cowl 
(American-ese for scuttle) will certainly shake around a  bit. This is to be 
expected on a rough road, However if this problem is  experienced on smooth flat 
pavement, and only at a certain speed then it has to  be a result of 
harmonics, a magnification of vibrations from a rotating mass,  most likely the wheels 
and tires, or an out of balance driveshaft. If running  the car up on blocks 
doesn't duplicate the problem then the imbalance is  probably in the front 
wheels. (although if the rear axle is sitting on  jackstands, then an imbalance 
in the rear might not gain resonance as the  flexibility of the tires would 
allow the axle to vibrate, whereas the  axle sitting firmly on stands would 
prevent the harmonics from getting  started and building on itself.)
The reason sedans don't experience this is because the roof structure ties  
the cowl into the rest of the body and the whole body would have to shake as 
one  unit, which would not seem as evident from  the drivers seat.  If  the 
problem was related to the engine shaking through the motor mounts, then the  
problem would be evident at certain engine RPM's, regardless of the vehicle's  
speed.
It seems that the most likely cause is an out of balance tire, or a bent  
wheel, or just an out of round tire, (very common on belted tires, even when  
they're new) I would be willing to bet that if one of these cars with severe  
scuttle shake was fitted with a set of new balanced radial tires, the problem  
would go away?
Just my thoughts <> Steve Cota <>



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