996 Wheel wobble under hard braking, wheel balance?

[tezza77]

After some hard and fast driving yesterday I noted a nasty wobble from the steering wheel under continuous hard braking (from 100+ mph). I cannot say I felt this through the brake pedal although in the excitement of hammering on the brakes approaching a 90 degree corner at 100 odd mph I may just have missed it but I doubt so.

Initial thoughts are obviously uneven disks or pads however, after purchasing the car knowing the previous owner had GT3 wheels on her and had only just put the original turbo alloys back on I decided to get the wheel balance checked, after all its the cheapest place to start. The garage were confident that although the balance was slightly out, it was unlikely to be the cause of the problem. This disks look spotless, I know its impossible to tell without proper equipment but they look almost new.

I had read briefly somewhere about cheap locking nuts causing an out of balance situation and I know that the previous owner had the garage who refitted the wheels pop on a set of locking nuts, (bound to be el-cheapos) so when the tyre fitter put the wheels back on I had him put the locking nut opposite the valve, prior to this they were next to the valve.

Now, its a wet miserable kinda day here so not really the weather for any hardcore testing but I would definitely say it's better going on my drive home from the garage, whether that's down to the re-balance or moving the locking nuts or perhaps even the several medium braking stints I did on the way into the garage to try and rebed the brakes I'm not sure but I wondered if anyone else had experienced this issue or if anyone had any definitive evidence that random locking nuts can affect the braking in this way?

I'm in two minds whether I'll bother with locking nuts at all now, surely my turbo wheels aren't all that nickable?

[anonymous-user]

If it was wheel balancing, it would vibrate at a constant speed, usually 75-85 mph due to resonance. Not under heavy braking where the frequency is changing rapidly. This sounds more like play in the suspension bushes, but impossible to say without investigating.

Edit: presuming it isn't the brake discs

[mohitos]

I had this on a BMW X3. After the main dealer replaced all the brakes and majority of suspension parts (under warranty) it was discovered that Kwik Fit had not balanced the front wheels after fitting new tyres. Fortunately the dealer were very good about it and did not charge me for the parts or labour.

Definitely worth starting with a wheel balance so perhaps you have already fixed the problem. Mine did not vibrate constantly - only when braking from high / motorway speeds.

Braking from over 100mph hey ? I guess you must have been on track or overseas ?

[tezza77]

uktrailmonster said:

If it was wheel balancing, it would vibrate at a constant speed, usually 75-85 mph due to resonance. Not under heavy braking where the frequency is changing rapidly. This sounds more like play in the suspension bushes, but impossible to say without investigating.

Edit: presuming it isn't the brake discs

There was a very minor vibration noticeable, nothing like the wobble under hard breaking though. The car only has 40k on the clock and was in for a porsche inspection just over a week ago and serviced and MOT'd in May so presumably bushes would be unlikely at that mileage or would have been picked up somewhere in that lot?

There was an advisory for slight play in a ball joint IIRC but the OPC said it was nothing to worry about just yet. I wonder if that is likely to be related?

Normal breaking is fine, no problems whatsoever from street speeds

mohitos said:

Braking from over 100mph hey ? I guess you must have been on track or overseas ?

err.. private road

[1point7bar]

If it happens when you use the brakes, it could be a sticking caliper.

How often do you stand it on it's nose?

[tezza77]

1point7bar said:

If it happens when you use the brakes, it could be a sticking caliper.

How often do you stand it on it's nose?

Heh, I only got it last week so not much (yet)

Not sure about the previous owner.

[anonymous-user]

Just to reiterate, out of balance wheels ONLY vibrate at resonant frequencies ie constant speed. Any transient vibration will be caused by something else - especially if there is no obvious vibration at any constant speed up to 3 figures. Warped brake discs are the obvious suspect in this case. Basically you can't have out of balance wheels that only cause a vibration under heavy braking.

[mohitos]

uktrailmonster said:

Just to reiterate, out of balance wheels ONLY vibrate at resonant frequencies ie constant speed. Any transient vibration will be caused by something else - especially if there is no obvious vibration at any constant speed up to 3 figures. Warped brake discs are the obvious suspect in this case. Basically you can't have out of balance wheels that only cause a vibration under heavy braking.

This has not been my experience, as above.

[Moosh]

Would check your discs have not become warped, if you were braking hard from 100mph, this may be the cause.

Keep us posted when you get the car checked over

[911p]

We had this on the previous C2 - from about 60mph, braking would see a juddering or vibration which got worse and worse the further into 3 figure speeds you went. In the end we went to JZ Machtech and they found the problem (disc not mounted dead straight, something on the hub when fitting), had them skimmmed and it was perfect after that.

[anonymous-user]

mohitos said:

This has not been my experience, as above.

Yeah I noticed that. But I can't see how it would be physically possible to have out of balance front wheels that did NOT vibrate at a particular constant speed on the motorway and yet manage to vibrate badly under braking, which would be an odd thing for a simple out of balance wheel to do anyway, given that this type of vibration is resonant.

In your case it sounds like most of the variables were changed and the true cause of the problem was lost somewhere along the way.

[S2Mike]

Yes, braking hard from those sorts of speeds could have warped the discs, to give you steering wheel wobble, to be wheel balance it would affect the steering when not braking as well, but maybe only at certain speeds, not all the time!
If you do brake hard from high speed then stop, dont use the hand brake or hold the car on the foot brake cos while cooling if the pads are against the disc it will cool at a different rate, this could cause warping.
I am not an expert but if you go on one of those track days they tell you not to use the hand brake or hold the car on foot brake after hooning round the track.

[1point7bar]

tezza77 said:

Heh, I only got it last week so not much (yet)

Not sure about the previous owner.

Something to try at the weekend maybe.

[tezza77]

Moosh said:

Would check your discs have not become warped, if you were braking hard from 100mph, this may be the cause.

Keep us posted when you get the car checked over

Would OPC do this? I dont know anywhere else locally I would be able to get this checked.

Unless... perhaps I would be able to tell with a set of vernier calipers, not sure how much warping I would need to see for it to cause that much of a wobble. I thought I had read somewhere that warped discs was a myth anyway?

[tezza77]

S2Mike said:

Yes, braking hard from those sorts of speeds could have warped the discs, to give you steering wheel wobble, to be wheel balance it would affect the steering when not braking as well, but maybe only at certain speeds, not all the time!
If you do brake hard from high speed then stop, dont use the hand brake or hold the car on the foot brake cos while cooling if the pads are against the disc it will cool at a different rate, this could cause warping.
I am not an expert but if you go on one of those track days they tell you not to use the hand brake or hold the car on foot brake after hooning round the track.

I wasn't braking hard to a standstill, only to be able to get around the approaching corners

After that the car had a thorough cool down before being parked up.

[1point7bar]

S2Mike said:

Yes, braking hard from those sorts of speeds could have warped the discs, to give you steering wheel wobble, to be wheel balance it would affect the steering when not braking as well, but maybe only at certain speeds, not all the time!
If you do brake hard from high speed then stop, dont use the hand brake or hold the car on the foot brake cos while cooling if the pads are against the disc it will cool at a different rate, this could cause warping.
I am not an expert but if you go on one of those track days they tell you not to use the hand brake or hold the car on foot brake after hooning round the track.

I see what you are saying, but would you not have to get the iron really, really hot first to change it's shape or even alter it's temper?

[S2Mike]

If you have watched rallying at night you will see the brake discs glowing redhot as they go past! That would be enough to do it!!

[mohitos]

uktrailmonster said:

Yeah I noticed that. But I can't see how it would be physically possible to have out of balance front wheels that did NOT vibrate at a particular constant speed on the motorway and yet manage to vibrate badly under braking, which would be an odd thing for a simple out of balance wheel to do anyway, given that this type of vibration is resonant.

In your case it sounds like most of the variables were changed and the true cause of the problem was lost somewhere along the way.

Nope, the garage road tested after each component was changed and the fault was not resolved until the wheels were balanced. It is possible that the vibration at constant speed is so minor that it goes unnoticed, and that hard braking accentuates the problem significantly.

Anyway, for the sake of balancing a wheel I would always start with the little stuff to eliminate the possibility. Many many years ago I had a problem with a Saab 99 which was pulling hesitantly under acceleration. The problem was only apparent after a specialist in older Saabs had serviced the car. At great expense the specialist replaced almost everything fuel related and rebuilt the carb. In the end the points he had fitted during a service were faulty. - a £2 part. Always start with the little stuff!

[anonymous-user]

mohitos said:

It is possible that the vibration at constant speed is so minor that it goes unnoticed, and that hard braking accentuates the problem significantly.

Anyway, for the sake of balancing a wheel I would always start with the little stuff to eliminate the possibility.

That would require some interesting physics indeed! I certainly couldn't begin to explain how that would be possible with a simple wheel balance problem. Do you not see the paradox here? The biggest vibration from an unbalanced wheel would most certainly be at the primary resonant frequency ie at a particular constant speed. If anything, braking would prevent vibration due to the rapidly changing rotational frequency.

I agree with starting with the simple stuff first though!

[MadMark911]

On my Boxster - I ended up changing quite a few things (rebalancing wheels, top mounts and track rod ends), before we discovered that it was a tiny amount of play in the "coffin" arms.

Every piece replaced in sequence, each one making an improvement .....