I don’t know if this is the right forum, but I cannot see anything more appropriate. The car has skuttle shake, on an absolutely smooth road its fine, but any sort of surface imperfection sets off a slight but very noticable shake. Its not wheel balance because the steering wheel shows no oscillations/rotational motion at all, it shakes laterally from side to side along with the dash and windscreen. Its not bad enough that my wife in the passenger seat notices it, but my brother did and he’s a mechanical dunce. Bigger bumps produce more shake. I’ve had the wheels balanced anyway, they were not that far out (5g) now corrected, and I’ve checked the wheels for run-out. Two are perfect in both planes, the other two are out radially by 1.5mm and laterally by 1.5 and 2.5mm. The latter one is on the back. The back tyres are wearing absolutely evenly. The front tyres (Toyo T1R’s) are wearing on the inner and outer edges, worse on the inners, as though they have been run under inflated, but that’s not the case as they were usually run at 35psi. The tracking and camber are spot on, not sure about castor as I have no easy means of measuring that, but the steering does not pull in either direction and self centres equally from each side, so its not far out.
Has anybody else had the same problem and found a solution. At present all I can think of is to strip the car and seam weld it and then put it back together, but I do not want to go to such extremes unless there is no easier solution. The rear sills had gone and I have repaired those properly, ie not just patched them. Also I have rebuilt an MG Midget which required extensive chassis welding and that has no skuttle shake what so ever, even over the worse road surfaces, it quite puts the MX-5 to shame. What I am trying to say is that I know how to weld up sills. But maybe there is an internal problem further forward which was not in evidence when the rear sills were repaired.
I am not quite sure what you mean by ‘scuttle shake’ . To me the scuttle is the part between the bonnet and the windscreen, do you mean it vibrates? or have I missed something? A thorough check of the suspension for wear in the balljoints and wishbone bushes, then wheelbearings, followed by a similar check on the steering rack and its joints. Engine mountings, I could go on. Without the opportunity to check your car it is difficult to say more, just suggestions on where to start.
Scuttle shake (now correctly spelt) is a feature very well know to drivers of older open cars especially those with wooden frames. As you surmise the scuttle is that area aft of the bonnet. When it shakes the dashboard, windscreen, steering wheel, rear view mirror oscillate from side to side. I should have said that suspension and steering joints and wheel bearings have already been checked and found to be serviceable, but I had not thought of engine mounts so will check those out. I have also checked the propshaft joints for slackness, but not removed the shaft yet to check for roughness in the action of the UJ’s. On a billiard table surface there is no problem at all, but on our roads today it makes driving quite unpleasant and I’m starting to leave the MX-5 at home and take the 45 year old MG out in its place. Unfortunately my wife cannot get into the MG as easily as she can the MX-5 so I am under pressure to sort the MX.
New droplinks made a massive difference to my skuttle shake. But there could be a number of reasons for it, all of them suspension, unless your chassis is rotted away, in which case it’s probably that.
I reckon you might be running the Toyos at 35psi to combat the soft sidewalls? The first thing I would do is set them to 28 from cold preferably because 35 is pretty excessive in my view and 9 above book. Tyres are given recommended pressures to harmonise with springs, shocks, bushes and all else and my feeling is…35 psi from cold (?) will rise easily to nearer 38+ when run. Recipe to kick the 7 kinds of c**p out of any 5 I’d have thought. Failing that, I’d start to look for worn suspension components. All of them. I’m putting chassis rot at the bottom of my list until proven wrong. You will see I’m the owner of the now well exposed Mk2.5 whose rails were uttery porous. And yet, this being the Sport with factory uprated stuff + 16" Toyos, you would not have known there was an issue in terms of body flex or “scuttle shake”. The Mk2 shell is over 25% stiffer than a Mk1…just as well. My best guesses are grossly excessive PSI (my opinion), or worn rubber somewhere in the suspension…or perhaps…a duff/siezing shocker or two.
I understand what you mean now, I have never heard that name before! body shimmy, yes, still you learn something every day. If all the suspension, steering components are OK and the subframe etc. is not rotten I would look at the engine mountings, could be perished or gone soft. Another cause of this on older cars, not the MX5 was if the carbs were out of adustment particularly if they were twin or triple. So to update anything which causes the engine to run roughly and vibrate more than it should coupled with mountings which are past their best will exacerbate the problem. Gearbox mountings can also contribute but as the MX5 has a powerplant frame perhaps that would merit checking.
I’ve not said much, but I am reading all the replies for which I thank everybody, much food for thought. Its good that the general concensus seems to be that it is not structural rust in the forward chassis sills. I’m going through the suggestions one at a time and then trying the car, because although I want it cured, I also want to know what the problem was. Changing several things at once I’ll not know which one cured it.
I’ve moved the front tyres to the rear and vice versa, so the T1R’s are now on the back and all tyres are at 26psi now. The vibration or shake has changed slightly in character, but the intensity is about the same I think. I have some new tyres to replace the Toyos and will fit them soon.
Again, thank you all for the advice, keep it coming if any new thoughts occur, and I will advise if and when I find out what it was.
Not neccessarily, there is another thing that may be worth considering, how are your front dampers?
My Mk1 Merlot started to develop “scuttle shake” on some road surfaces and I had a sneaking suspicion that all was not well in the damping department. I tried the usual pushing down on the wing and trying to gauge the return was smooth and controlled and to me, it appeared alright. However, I took the car for the MoT and got an advisory on the l/h front damper which reinforced my theory. Mine is off to Dr Eunos soon to get new dampers fitted!
Are we not perhaps simply talking about the natural flexibility of a convertible chassis, especially when not fitted with all the aftermarket cross-bracing available?
I’ve had my Mk2 for almost a year now, and one of the things that astonished me, was the difference in rigidity, between having the hard-top fitted, ie from the time I got it, till about March when I took it off…
Having removed the hard-top the car was instantly seen to be more flexible, ie the windscreen/mirror seemed to “jiggle” side-to-side over anything but smooth surfaces. Got used to it fairly quickly, but the difference was VERY noticeable.
It’s entirely possible that at least “some” of the guys noticing this phenomenon, may be fairly new to drop-top motoring (as I am), and that there’s actually nothing wrong with their cars!
