Car is currently in for new clutch, I’ve asked them to look over the full drivetrain while doing the clutch and a few other jobs, first time I’ve used this garage but they appear knowledgeable with mx5s and enthusiastic, I’ll contact them and specifically mention the ‘shimmy’, they may have come across it before. Thanks for replies.
Power Plant Frame.
It’s the girder next to the prop-shaft holding everything true and in line, guaranteeing the UJs at each end of the prop-shaft have exactly the same working angle.
See the service manual’s instructions for installing the Diff for how to align everything correctly.
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Just sounds like a worn UJ if you ask me.
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Could be worth measuring the runout on the wheels. Last time I fitted new front discs I actually got out the dial gauge and measured (instead of just cleaning the hub faces and saying that’ll do). They were out of spec. Took a couple of goes at really scrubbing decades of crud off the hub faces before the discs were running true within spec.
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Just got car back from SPA in Ilkeston, it went in for new clutch and investigation of noise suspected to be gearbox. Luckily they had a donor car and we agreed price to swap out box, propshaft and diff, so noise gone and to my delight the shimmy at motorway speed. So although I can’t conclusively state where the problem was wrt shimmy, you can say it’s probably prop out of balance or u joints gone or worn out diff. Thanks for all the replies. Ps I can highly recommend SPA , they helped me with this and so much more.
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I thought I’d share on continuing lessons learnt wrt to motorway speed shimmy/vibration. Although I’d thought I’d seen this resolved as mentioned in earlier post, through out this summer I’ve still detected a reduced level of motorway speed vibration. So latest developments as follows; I’ve been using WR aftermarket 15” Alloys over the last 5 years, I’ve had these balanced to death and come to the conclusion the shimmy wasn’t wheels out of balance. Today I fitted a nice newish set of 15” motorsport Dynamics with newish tyres. I’d become convinced the aftermarkets were a very heavy wheel (in Mx5 terms) and that may have contributed to the issue. I weighed both for comparison, the motorsports with tyres coming in at 13.9kg and the aftermarkets 15.8kg. I was surprised as I’d expected a bigger difference. Despite this, the shimmy has definitely gone, I took the car through multiple test at speeds 50/55/60/65/70/75/80 both cruising and pulling hard acceleration - no shimmy. Add to this the car seems to smooth out the minor road imperfections, I’m so glade I reduced the weight, I know there are lighter wheels than the motorsports, but I think I’ve got the wheel and tyre combined weight back to approximately the original daisy wheels and tyres. Maybe Mazda always knew the car was sensitive to wheel weight. So next mod will be coil overs - more to learn about🤔
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I also have had the shimmy since I got the car. New tyres on 15" rims balanced countless times, new bearings & brakes all around together with new ball joints etc & the shimmy was still there. A couple of months ago for no other reason than I liked the look of them & they were a decent price I put on a set of 16" OE Mazda ones from the sport model & hey ho…no more shimmy. I have no explanation for this but motorway journeys are now a pleasure in comparison.
That’s incredibly similar to my experience. Out of interest were your previous wheels OEM or aftermarket? There’s a list of alloys typically used down the years of the Mx5’s which quotes weights (wheel only) it would be interesting to know the weight of your 16” new wheels. I’ve heard so many people’s experience with the issue and multiple theories and fixes, this suggest to me the problem could be a combination of issues and not necessarily always the same ones ie there may be 10 Potential contributing Facts and you may have any 5 that results in the problem. My personal theory is that the car isn’t that stiff as we know, over the years it becomes less so and starts to flex between front and back, I’ve stiffened the front and back with most of the known solutions, however I haven’t stiffened in between - akin to chassis rails (it was on my list) hence front to back flexing. Now imagine what happens at speed over what would appear to be a smooth surface in any other car, all the wheels/suspension are independently absorbing minor surface deviations, this will produce cross corner loads, the weak mid section can’t resist so the car flexes (shimmy) so heavier wheels going up and down have more inertia and hence travel further - more flexing ensues. I know this is simplistic and people will say “if that’s true then your suspension isn’t doing a good job” I’d go along with that, but probably it’s one of the multiple factors and not the only one. https://www.miata.net/faq/wheel_weights.html
Previous wheels were OE multi spoke arctic ones. Unfortunately I can’t now weigh them as a comparison as I have now sold them on.
In truth they’d be reasonable in terms of weight being spec’d with the car, it could be that the tyres were heavy, I suspected this with my aftermarkets due to not having heard of the brand before. It’s difficult to find tyre weight, but from now on I’ll weigh my old tyre with wheel and same with new tyres fitted, maybe a little late to do anything at that point, but if the shimmy returns it will be a good pointer. If you haven’t already - I can highly recommend changing the door rubber receivers, I put the eBay cheap ones on £12 - made a big difference to the doors rattling- although you do have to re tighten after a week or so
This may be a red herring but have those of us with persistent wobble/shaking/shimmy had the wheels’ runout checked as well as their balance?
Last time I fitted new brake discs I checked their runout with a dial gauge before I put the wheels back on. Turned out they were way out of spec. Never bothered checking before but remembering I had the gauge at the back of the cupboard I was curious to see how they measured up to the spec in Rod Grainger’s book.
It took a lot of repeated scrubbing of the hub surfaces with a cleaning pad to get them clean enough that the brake discs ran true and met the runout spec. Strikes me that this might apply to peoples’ wheels too.
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Excellent point, I haven’t done that, but I’d like to think that when the wheels are balanced (had mine done 4 times in 2k miles) excessive runout would be spotted 
, i suppose this could be added to my list of contributing factors and not the sole reason. I do check the brake disc mounting face is clean and same with wheel mounting face, I’ll check runouts on new wheel fitted and record, if the shimmy returns I’ll check again and compare. I would also say that several years ago I bought an old run around fiesta that had rear end wobble at speed, that turned out to be a buckled steel wheel the previous owner had bough as a spare that found its way onto the car after a puncture, that said it was miles out, my brother was following me and spotted it straight away. I can’t imagine an alloy being that buckled without cracking.