Questions for the mechanically minded

[Wivvix]

Since buying my MX-5 i've been trying to track down the source of vibration in my NA.

I've recently exhausted one potential cause, cost me only $1500 ( ), and am now trying to ascertain where to look from here.

I guess I have three questions.

1) Can the size and weight of a wheel (15-17", 15-25lb) impact or influence the rotation of wheel-driving components (i.e. the driveshaft)?

2) Can additional weight (~70kg) in a vehicle impact or influence the rotation of wheel-driving components?

3) Could poor aerodynamics of a raised soft-top exacerbate or cause vibration in the body of a vehicle?

Cheers

[StanTheMan]

all pretty unlikley if the wheel is straight & balanced & connected to the car centrally.
weight has nothing to do with vibrations provided those things are checked or...correct.....of course the lighter the wheel the better it will handle but that a separate issue.
If the wheel is buckled, unbalanced, uncentrally fitted.....for sure vibrations are very possible
Is your car an NA6? Na8, NB8A/c NC?

are you getting the 100 Km/H shimmy? in the NA?

soft top?.....I just can't see how.


maybe you could let us know what exactly the problem is?

[stevesports]

where exactly does the vibration come from? What does it sound like? or there is no sound at all. At what speeds does this occur?

or under what conditions does this occur?

i think these points could help us diagnose what is the problem =]

[pcmx5]

If you are sure that the wheels are not buckled and the balance is right it may be worth checking that one of the uni's in the tailshaft is not bindinging.Remove the shaft and test that the yoke will move freely in all directions.

This happened to me and the passenger seat used to vibrate,so did the softop, and the whole car shook at certain revs.

I am still to totally cure the issue but will do soon by getting a Gibson's(of Newcastle I think) conversion to replacable uni's

Peter.

[Okibi]

Wivvix please update your profile so we know where you live and can offer more specific solutions.

If you have your wheels fitted and balanced at a generic tyre workshop find out who the specialist wheel guru is in your local city.

Have them balanced with far less tolerance, they can also check if your wheels or tyres are out of round.

[mx52nv]

From this thread

As mentioned above, the two shops that have had the most experience setting up MX5s are:
1) Wheels World - John & Bob
2) Bridgestone Bentley - Kerry & Graeme Wade
Club members have had success with both companies for many years now

Without any details about your $1500 spent without finding the cause, I can only suggest going to see one of the two businesses for an opinion.

Of course, if this was already your first port of call, then may I suggest coming along to a club event so either myself, Ian or someone experienced to take a spin in your car and give you our opinion?

[Wivvix]

Thanks for all the replies everyone. It's a 95 NA8 in question.

The wheels are brand new Enkei RPF1's 15x7, tyres are Pirelli Dragons 195/50/R15 and were put on @ Wheels World.

I spoke to John about the vibration and we're going to go for a drive next week and see if we can pin-point a source.

From the sounds of things it's not wheels or tyres. Any roundness issues would be more prevalent at higher speeds, but the vibration i'm experiencing is through the whole body of the car and presents between 51 to 60kph, as well as 100 to 120kph.

The vibration isn't as bad with the roof down, but it's still there. If John is lost for ideas, i'll probably take it down to Veem engineering and get them to look at the driveshaft.

It's weird though because I went for a drive with Apophis with his 16" NB wheels on about a month or two ago, and could feel no vibration at all.

[fundies]

Could be anything from the clutch, driveshaft, wheels, to the shocks. Good luck.

[Mokesta]

You need to establish if the vibration is car speed related or engine speed related. Is it 50-60 amd 100-120 in 4th? What about in other gears? If another gear changes the speed at which the vibrations happen, then you have a drive-line issue (up to the box). If the vib is at a constant speed it is a suspension issue.

Assuming it is suspension I have personally encountered the following:

<1> Hubs out of balance so that the car would only be vibration-free if the wheels were balanced on-car. (1988 BMW)

<2> Wheels with all 5 holes in the centre drilled in the correct pattern but not in the centre of the wheel. The wheels/tyres would balance perfectly but you can balance an egg & she still won't roll nicely (1997 Australian-made new wheels for my AE86 Spriter)

<3> Tyres that weren't round. Same issue as above. Balancing machine says they're perfect but rolling on ovals. (Bob Jane All-Rounders "out-of-rounders" on Subaru circa 1992)

<4> Tyres with a tread pattern that doesn't vary around the wheel. That is, the tread block size is perfectly uniform, unlike just about all road tyres where the block "length" changes. Have a shocking harmonic at 30, 60, 120 km/hr. High frequency vibration that makes noise and shakes steering column, fillings and man-boobs.(Yoko AO32R on my Moke and to some extent the Toyo 888s)

<5> Axles bent at the flange to axle area (from the diff being dropped) making the wheels orbit and wobble. (Expensive jap-import diff for my Sprinter from Adelaide Jap who refused to give me my money back)

<6> Tyres that have a flat or hard spot in the carcass due to manufacturing fault or shipping damage. (not me, a mate)

All of these issues will have speeds at which they are worse as they are all a spring-mass system and therefore have harmonics.

The out-of-round and bent flange issues can be checked with a simple pointer or dial guage pointing at the tyre tread or (not for Bob-Jane's) the outer wheel rim with the wheel on-car and off the ground. Rotate the wheel slowly and observe the change in position of wheel versus pointer. If it is greater than 1.5 mm you'll feel it.

Amazingly, I had to actually demonstrate this technique to the tyre shop that sold me the wheels with holes drilled off centre! The fools thought that just because the balance machine said they were ok, meant they were perfect in every respect.

M.

[Adam_NAclubman]

Soft-top up is more aerodynamic than down, so unless you're comparing against a hardtop, thats not it

[Wivvix]

Went to Veem engineering yesterday. They think it could be a front uni, but would have to pull out the driveshaft, balance it, and spin it up to see if it is. Basically they'd want $250 to "have a look", and another $150-$250 if there are uni's to replace etc.

I'm going to wait and see what John at wheels world thinks after we've gone for a drive. I'm not convinced it's driveshaft related yet.

The vibration is definitely worse with the top up. Perhaps it's not the aerodynamics so much, as much it is creating a soft medium through which vibration can reverberate. The soft top is far less rigid than the body of the car, no?
Do you guys find that when you are driving on a freeway at 100kph, if it's a particularly blowy day, does your mx5 get blown around a bit? Mine feels/gets a bit skittish when there's strong winds, it's quite unsettling.

I don't think it's the shocks. The reason I say this, is because even on very flat roads without any noticeable bumps, it'll still shudder/vibrate at 50-60kph. If the shocks aren't having to do any work and there's vibration, then it can't be shocks, right?

Of the suggestions Mokesta provided, is it possible to test if <1> and <5> are the cause without committing hundreds of dollars? Who would have that kind of equipment? I'm pretty sure it's not tyre related, and the wheels are high quality Enkei's, i'd be very surprised if there was any faulty workmanship there.

Cheers guys.

[pcmx5]

[quote="Wivvix"]Went to Veem engineering yesterday. They think it could be a front uni, but would have to pull out the driveshaft, balance it, and spin it up to see if it is. Basically they'd want $250 to "have a look", and another $150-$250 if there are uni's to replace.

The mazda tailshaft will have to be modified if it is too accept other uni's as it is a throw away item.

I know Gibson's in Newcastle do a holden uni conversion for around $450.They said(I'm pretty sure) that it is the rear uni's that give the trouble.

My Uni vibration was much worse with the top up, more flexible is one cause but also asmall vibration at the roof base is a bigger one at the top!

Peter

[Wivvix]

Went to see John at Wheels World today. We put the centre-bore locator rings on the rims which as I suspected made zero difference, as was affirmed by our test drive, but no biggy.

Back at wheels world John did a little test to see if there was lateral movement in the wheels/tyres.

I didn't volunteer any information about which side I thought to be problematic, but he (correctly) diagnosed the rear right (driver-side) corner as having an issue. This corner smacked a curb.

So essentially there's something wrong with the rear right corner. My knowledge of mechanics isn't good enough to know which parts could be the problem here, and he was not inclined to speculate.

From what I could gather, it could be any of the parts between the wheel and driveshaft. Any ideas?

[Mokesta]

My guess is problem <5> from my earlier post. Jack up that corner of the car. Place a solid object close to the edge of the rim and rotate the wheel/tyre slowly by hand. Observe the distance between the rim and the fixed object. If the gap size changes, you have a bent flange/axle making the wheel wobble. If you notice a fluctuating drag on the wheel as you rotate it, unbolt the brake caliper and pull it away, then turn the wheel. If there is still a noticeable fluctuating drag on the wheel, you've got a stuffed bearing. Reach in to the drive shaft between the diff and the hub. Try to move it in-out (across the car) it should move a few mm, which is OK. Now try to move it up/down and front/back if it can move up/down or front/back you've got stuffed joints.

M

[franjae]

Some years ago, I had new wheels and tyres installed by John at Wheels World on a Merc 190E 2.6. The car vibrated at freeway speed and took John a long time to work it out. In the end he dismissed the wheels and the Pirellis were the culprit, for not being totally round! It was extremely time consuming as each tyre had to be moved around on the wheel to the best position. It would have been cheaper to ditch the Pirellis for something else.

Not sure if this is the same fault as yours. If you can try out another brand of tyres, do so and see if it makes any difference.