Chassis vibration/shudder

george.ene · Post by **george.ene** » Sun Mar 22, 2015 11:42 am

Howdy folks,

I've just purchased a new MX5 (98, NB, 1600, Manual).

This ride looks to have come with a few hidden covenants, turns out the engine came with 10,10,2,10 bar compression; which has been since resolved (turns out to be a inlet valve was .5 ml too small).

The car also came with another issue as well; the age old chassis shake when going over wavy roads (under 80 kmh), and 80-120km shudder. This is pretty bad, to the point of the whole dash shaking, yet there is no funny shake in the steering. The shake appears to be chassis resonant to the said conditions and when driven on a smooth road between the 80-120 kmh, the symptoms go away.

The car is not so standard, it came with adjustable coilovers all around, and aftermarket 14' Avanti alloys.

So far, I've had the rear wheels balanced, the alignment appears true, have swapped the front wheels with the rear with no fruiting results.

The next step will be to adjust the ride height, thus reducing the current preload but need to order the C spanners as the only place in Chch I've found charge $98 for a tool that will be seldom used.

What are you opinions? Have you seen this before? Any other possible workaround?

Many thanks,
George

george.ene · Post by **george.ene** » Sun Mar 22, 2015 11:48 am

To add to it, the whole front window visibly shakes right to left.

Have previously owned an NB (after the quakes), am aware of the occasional shake when you hid a pot hole but for this one it seems to occur on your standard christchurch roads, halswell direction where the streets are ok.

The car also appears to be certed and lowered, currently sitting at about 10 cm in height.

MrGrey · Post by **MrGrey** » Sun Mar 22, 2015 1:32 pm

Just wondering but has the vehicle been in a crash at any point?
Some of the other guys will have a much better idea than me but your problems sound severe.

marcellarius · Post by **marcellarius** » Sun Mar 22, 2015 2:52 pm

Hm, the springs you have will be much stiffer than standard, or than typical lowering springs.

My Tein coilovers make the car shake around a lot more on rough or bumpy roads. It can be pretty unpleasant. I've tried playing with the damper settings but it doesn't seem to make much of a difference.

george.ene · Post by **george.ene** » Sun Mar 22, 2015 4:30 pm

Mr. Grey - It looks to be fairly untouched as far as the chassis or previous crash history.

marcellarius - That would certainly make sense, and validating stiffness lead, I shall get my hands on some of those adjuster tools and give lifting it a go.. of course, while noting down how much on each corner before and after.

Given the current coilovers have only adjustable height, one would assume the preload would also be affected. Looks to be very low as is and one would notice a 5 cent coin on the road..

marcellarius · Post by **marcellarius** » Sun Mar 22, 2015 6:19 pm

Changing the preload doesn't affect the spring rate or ride quality -- it changes the height and where the car sits in the damper's stroke.

If your suspension is set too low then maybe it could have insufficient bump travel which could cause excessive harshness as the car hits the bump stops.

I guess it's possible that the dampers (shock absorbers) in the coilovers are worn out, or inadequate for the spring rate.

siren676 · Post by **siren676** » Sun Mar 22, 2015 6:35 pm

I take it the car doesnt have a hardtop? These cars are pretty floppy without one although i thought they'd greatly improved it on the nb's

george.ene · Post by **george.ene** » Sun Mar 22, 2015 9:56 pm

Nope, no hardtop on this little beastie Siren, I did read about the improvements of having one, along with underbody bracing and also that some of the NBs coming out stock with strut braces, however, will leave the expensive goodies as a last resort and start with the basics first. They do look pretty good with the hard tops though..

marcellarius, I did do some reading this afternoon, but cannot get my head around whereby if the springs are almost under full tension they wouldnt ride smoother by lowering the spring's cup a little, also resulting in higher ride height.

On the other hand, she is very low at this point, so the bump stops could certainly get in the way.

As far as the shocks go, the car seems to return to its ride hight without any additional bounces, no bleeding either and one would hope that not all would fail at once, which would certainly add to the diagnosis.

MrGrey · Post by **MrGrey** » Sun Mar 22, 2015 10:06 pm

I am purely speculating, but would a cabin brace possibly help stiffen things up a little?

george.ene · Post by **george.ene** » Sun Mar 22, 2015 10:11 pm

It would certainly help Mr Grey. I've already gotten in touch with Mazline to keep an eye out. Along with any additional bracing for that matter.

Before getting it heavier though, it would be wise to lift it a bit, especially with the Chch roads.

The C spanners are going to arrive later this week, so will keep updating..

Angreal · Post by **Angreal** » Mon Mar 23, 2015 7:31 am

I'd have to guess that reducing the preload would allow further suspension travel which should at the very least improve the ride. Once you've done that make sure you get another wheel alignment done though.

marcellarius · Post by **marcellarius** » Mon Mar 23, 2015 9:00 am

"Preload" is how much the spring is being compressed between the perch and upper mount when the suspension is at full extension. In essence, preload determines how much weight it takes before the suspension starts to compress, allowing you to adjust for the weight of the car to retain useful suspension travel.

This is important: preload does not change the total spring compression, or spring rate.

If you have strut with a 5kg/mm spring and put 250kg onto it, the strut/spring will compress by 250 / 5 = 50mm. If you wind up your spring perch to 20mm of preload, the spring will be putting 5 * 20mm = 100kg force onto the perch.

If you then put your 250kg weight back onto it, the first 100kg gets 'cancelled out' by the preload and the strut will compress by 150 / 5 = 30mm.

The total spring compression is still 50mm.

The OEM springs are very soft but long springs. They have lots of preload which accounts for most of the weight of the car, so that it has a useful range of suspension travel when loaded up.

They don't make good pogo sticks though, because it probably takes ~150kg before they will move at all.

Donovan · Post by **Donovan** » Mon Mar 23, 2015 2:48 pm

Is the shaking with the softop up, or happens whether it is up or down.

When I first got my NA, if I took the roof off it, it would shake something cronic, put the hardtop on, and it was as good as gold. I just ended up leaving the hardtop in place, as I bought the car for it's handling, rather than being a convertible.

I have don't a lot of extra bracing with the engine conversion, and it still shakes a bit with the roof off, no where near as bad, but still a little bit.

4A-GE · Post by **4A-GE** » Mon Mar 23, 2015 9:39 pm

I made my shake go away by getting a real alignment done and running factory tyre size (185/60/14) with factory pressure (28psi hot) all round.

Anything that isn't the above will cause the car to shake bang on 60 or in the usual shake bracket up to about 120.
Currently, I'm on the most aggressive road going alignment the guys would give me along with 205/50/15 at 36psi hot. Shake is less evident after new shocks and springs (dobis) and a whole bunch of bracing incl. the belt tower one, but is by no means gone for good.

To make it go away, do what the people who designed it said to do. No amount of bracing or hardtops will make it go away if it isn't factory spec.

SKREEE · Post by **SKREEE** » Mon Mar 23, 2015 10:34 pm

Hey George IF you want I could come by some time and you can bolt my brace in see if that helps anything prior to buying one. PM me if you want to.

george.ene · Post by **george.ene** » Tue Mar 24, 2015 8:14 pm

Thanks marcellarius, I understand what you mean. Aside from the 'resting weight' what I was trying to drive through is that the springs themselves are already compressed therefore the resting weight becomes 250 kgs, instead, so undoing the already compressed spring may drop it down to a reasonable .. say 200 kg loaded pressure (given its a coilover spring and they are a bit thicker by design).. worth a try..

Donovan, the car seems to be shaking both with the tent on and without, no real correlation , but then again I dont have a hard top base line..

Angreal, I hear you loud and clear, the camber is very likely to change .. aside that the wheel doesnt shake, no excuse for ruling it out and getting it done proper.

Skree - thank you very much, I will get in touch shortly..certainly sounds worthwhile.

Timmo · Post by **Timmo** » Wed Mar 25, 2015 11:34 am

What about your driveshaft? A previous car of mine had a bit of a wobble which had been caused by a dent in the driveshaft...might be something you can visually check?

marcellarius · Post by **marcellarius** » Wed Mar 25, 2015 1:13 pm

george.ene wrote:Thanks marcellarius, I understand what you mean. Aside from the 'resting weight' what I was trying to drive through is that the springs themselves are already compressed therefore the resting weight becomes 250 kgs, instead, so undoing the already compressed spring may drop it down to a reasonable .. say 200 kg loaded pressure (given its a coilover spring and they are a bit thicker by design).. worth a try..

Lowering the spring perch will lower the ride height and change where the car sits in its suspension stroke -- it will sit closer to the bump stop, therefore having less bump travel and more droop/extension.

Preload doesn't add to the load carried by the spring, it sets a minimum. At full extension, the strut and piston rod is holding the spring compressed but as weight gets applied this takes some of the load off the strut until it's the car's weight doing all of the compression, at this point the spring starts to compress further. The exception would be if you had more preload than weight, in which case the suspension would remain at full extension. If this were the case, the struts would be set up very wrong, I doubt the car would have got a cert like that.

I think this is a diversion anyway; it's unlikely to solve your problem.

Another thought, is it possible a previous owner replaced bushings with polyurethane ones? The OEM rubber bushes play an important role in isolating the body from sharp jolts, noise and vibration.

SLYDIT · Post by **SLYDIT** » Fri Mar 27, 2015 10:30 pm

before you get carried away...

what brand of tires do you have on it??

shitty tires (or unevenly/flatspotted tires) will cause the shimmy.

my first option would be to put a full set of new tires on it

george.ene · Post by **george.ene** » Sat Mar 28, 2015 9:09 pm

SLYDIT wrote:before you get carried away...

what brand of tires do you have on it??

shitty tires (or unevenly/flatspotted tires) will cause the shimmy.

my first option would be to put a full set of new tires on it

*Update*
Spent some time yesterday with my mechanic, discovered all tires to be "out of round". All by about a cm which is huge by decent tire standard. Went around and got a whole new set today. balanced fitted.
The issue still remains, probably worse now.

As about the ride height, I have not played with that yet, as the new tools dont seem to be able to crack the nuts on the coilovers.

The suspension is pretty much the last thing remaining but expensive so will be wanting to get a second opinion from a specialist. That said, this probably now be referred to a suspension place and or my mechanic..

Starting to panic that this thing has seen the wrong end of a few shrubs.. nothing obvious as far as the chassis having been straightened.

As about the drive shaft, it looks to be all good, not much play in it either - could be worth putting the back end in the air and throwing it in gear.

Last bit to touch on, does anyone happen to have a factory coilover set around the place they would like to sell/loan?

WideOpenThrottle · Post by **WideOpenThrottle** » Sun Mar 29, 2015 8:37 pm

Tried new RIMS and tyres?

Stevo · Post by **Stevo** » Mon Mar 30, 2015 6:06 am

Sounds like a bent or buckled rim to me. Possibly hit a curb or from those huge pot holes you have down there. Can you borrow a set of rims/tyres from someone?

george.ene · Post by **george.ene** » Mon Mar 30, 2015 8:55 am

I've lifted the car and they dont appear out of round, or is there any sideways jitter. The people that put on the new tires on Sunday also had a look at the rims on the balanced and were pretty contempt that they were OK.

Likely to go for a new set of campers + springs, however am getting a second opinion before splashing out from my mechanic..

NicF · Post by **NicF** » Wed Apr 08, 2015 1:20 pm

Put the rear up on axle stands and run it in gear. You'll be able to see any wheel wobble easily. If need be I have a set of wheels and tyres you can try out too, if you want to drive down to Fairlie.

Angreal · Post by **Angreal** » Fri Apr 10, 2015 2:12 pm

For one reason or another I had my butterfly brace removed... Didn't notice any change in the way the car handled

Chassis vibration/shudder

Chassis vibration/shudder

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