2003 MGTF Rear Suspension Problem

[ibosscat]

I Just Dont Believe It
Once more my rear suspension has collapsed, the lower shock absorber bolt sheared from the wish bone arm, due to bad road surface, I will admit I was not hanging about but??
Since having Spax Shockers fitted, first it was the O/Side now the N/Side.
Any ideas anyone??
My thoughts are either the shock bolt was done up with a air gun and over stressed or are the Spax shockers not long enough and when the wheel drops suddenly something must give???
Next thought is to get it back on the road and then get someone to check out the suspension properly.
Any ideas who locally in Essex??

[petevick]

is the inner boss that the bolt passes thru fitted into the recess in the arm ??, that takes any shear load out of the bolt.

[ibosscat]

Hi Pete,
Yes as far as I am aware the inner sleeve is / was located into the swing arm. Certainly the O/Side was when I repaired it. Should be stripping down the N/Side tonight.
Perhaps the bolts were over tightened so stressed.
Also one of my other worries is that the lower shock absorber bush is not at a straight angle to the swing arm so that the bolt is actually at an angle. (Not sure if this corrects itself when the weight is on the car)
Spax shockers fitted by local garage about 1 year ago. O/Side sheared after about 6 weeks. N/Side went this weekend.
Starting to wonder about letting the MG @ Swavesy have the car to supply and fit a comfort suspension kit and check out the suspension properly. No good having a car if its not reliable.

Thanks for any help

Steve

[petevick]

...Also one of my other worries is that the lower shock absorber bush is not at a straight angle to the swing arm so that the bolt is actually at an angle....

thats not right at all. When I fitted mine the bolts/bush were square to the arm.

[Debs]

The cause(s) of this failure are multiple:

Firstly the design of the damper/vertical link interface is such that it puts the bolt into 'single shear', which means it will be carrying more load than were it to be in 'double shear'.


Secondly the bolts themselves may be of a poor quality batch, or, more likely, were damaged by the mechanic when they were fitted. A small 'nick' on the bolt surface would be sufficient to act as a 'stress raiser' and initiate a High Cycle Fatigue failure (HCF).


Thirdly the bolts may well have been overtorqued when they were fitted. Most people don't realise that the torque settings given are for a lubricated fastener - ie they should be put in with oil or a copper based grease. They also don't realise that the bolts should not be fully torqued until the vehicle has been allowed to settle on its suspension.


Fourthly, there was an MGR notice put out that stated:

"Problem: Reports have been received that identify isolated instances of torque relaxation on the bolt used to attach the damper to the upper suspension arm. If undetected this condition could deteriorate and in extreme cases, the bolt could fracture."Solution: The torque of all four lower damper bolts needs to be checked. If found to be loose, the bolts need replacing."

If there has been a relaxation of torque on the bolt (ie it has loosened) then, owing to the fact it is in single shear there will have been a bending and hammering action applied (because of the bump-rebound cycles) that will have initiated HCF. Hence the need for replacement.


Fifthly, the incorrect bolts may have been fitted - the correct bolts are ATLAS 12 mm rated at 10.9 (the part number is: RYG000400) - BEWARE! I have seen similar bolts being supplied rated at 8.8 or 5.5 - these are not strong enough for use as a load bearing suspension component.


Finally, and I will bet this will be the over-riding cause, HCF occurs when there are alternating stress cycle reversals, such as in a con rod (for example). For such cycling to occur on these suspension bolts, then there has to be some movement between the bolt and the damper bush which allows the bolt to be shockloaded. To prevent this the fit between the bolt and the bush should be such that no movement can occur - Typically moderate shock loads can increase loads by a factor of 10, as well as causing local damage which can act as an initiation site for HCF if movement occurs.

ISO 286 -1 and -2: 1988 give a range of recommended design clearances for various fit requirements, ranging from press fits, to loose location fits. These fits are standard engineering practice across all mechanical engineering fields, and are standard practice in design offices. The fits are defined by a tolerance band on both components, based on their nominal diameter, and the degree of location required.

In this application a 'sliding fit' would be suitable. This would be defined as 'H7' on the bush, and 'g6' on the bolt shank.

For a nominal 12mm shaft and bush assembly: H7 would be a range of 12.000mm + 0.000mm to +0.018 mm on the bush, and g6 would give dimensions of 12.000mm – 0.006mm to -0.017mm on the shaft. This would give a required minimum clearance of 0.006mm, and a maximum clearance of 0.035mm on the diameter between the components.



I suggest you get a set of vernier calipers and measure the tolerances on the bolt shoulder and the diameter of the bush. I would suggest that there may well be wear in the damper bushes that lead to the problem.



My initial failures in 2006 were with bolts rated 10.9, springs softer than stock, and SPAX dampers. Bolts rated 10.9 should be perfectly adequate for this set up.The reason for the failure was that the muppet that fitted the dampers specified the wrong bushes with centres a whole mm greater than the nominal bolt diameter of 12mm - way outside the acceptable required minimum clearance of 0.006mm and maximum clearance of 0.035mm! Hence the bolts were undergoing a hammer action from the damper leading to HCF.

The failures in 2007 occurred as I was removing the bolts in order to swap out the soft springs for the X Power ones and were single overload stress failures - I believe the bolts had been overtorqued when fitted. Closer examination of the bolts indicated that the garage that did the rectification work after the initial failures had specified bolts rated 5.5 which, IMHO are not 'man enough' for the job when using stiffer dampers.

[ibosscat]

Hi Deb,
Oh wow, many thanks for all info.
This gives me a few more points to watch out for when renewing the shocker bolt and wishbone arm.
I know that when the garage fitted the new Spax shockers, spring last year they used the original bolts,
Obviously the O/Side I renewed all parts, and copper greased, inc a new shocker, when that sheared last June, All MG parts not repro. (Atlas Bolt 10.9)
Perhaps I should have renewed the N/side at the same time??
I will check the amount of clearance in the bush, make sure thats not excessive,
Really starting to wonder about the over torque though!!??

I still though have concerns over the angle of the shock absorber lower end / bush in relation to the wishbone arm,
just not straight

Have you any knowledge on this "Comfort Pack" that MG sell?? Are they any better then the Spax shockers??

Regards

Steve

[Debs]

I know that when the garage fitted the new Spax shockers, spring last year they used the original bolts

Muppets!

Obviously the O/Side I renewed all parts, and copper greased, inc a new shocker, when that sheared last June, All MG parts not repro. (Atlas Bolt 10.9)

The bolts should always be renewed.

You say you had a broken damper? Best practice is always to replace dampers as matched pairs.

Perhaps I should have renewed the N/side at the same time??

Yes.

Really starting to wonder about the over torque though!!??

The correct torques are:

Damper Lower Mounting - 100 NM
Damper Upper Mounting - 45 NM

I torque to 80% of the given values whilst the car is up on stands. I then lower the car off the stands and roll it forward and back to get the suspension to settle,. Then raise the car and torque to the final figures.

I still though have concerns over the angle of the shock absorber lower end / bush in relation to the wishbone arm,
just not straight

That is definitely not right, the bush should be perpendicular to the upper suspension arm. IIRC there is a recess in the arm that the end of the bush fits into. The trick is to get the spring damper unit into place and tighten the lower mounting before doing the upper mounting. I've not had any problems with my SPAX dampers in this regard.

Have you any knowledge on this "Comfort Pack" that MG sell?? Are they any better then the Spax shockers??

The 'Comfort Handling Pack' was developed for the MGOC, at the request of Roger Parker, by Andy Kitson of 'Vehicle Handling Solutions':

http://www.vehiclehandlingsolutions.com/

Andy was the lead chassis development engineer at MGR for the suspension of the MGF Tropht and the MGTF. It was developed to be the best compromise between handling and ride quality of the pre MY 2005 TFs.

The early TFs suffered from poor ride quality, often described as bone jarring. This was due to the fact that, for a number of reasons, the suspension set up was compromised from the point of initial production. Historically all modern MG chassis were developed in concert with Eibach (springs) and Bilstein (dampers). Unfortunately, owing to cost cutting, the only models that went into production using these components were the MGZT and Rover 75 models (hence their superlative ride quality).

When the TF went onto production MGR used cheap nasty dampers which had tiny pistons and reservoirs (35cc as opposed to after market dampers with 100cc reservoirs), and it is this that causes the 'choppy' ride.

Owing to public reaction, on MY 2005 cars MGR changed the spring rates for softer and countered this with an increase in size of the front anti-roll bar. However, while this gave better ride comfort it severely compromises the handling of the car when 'pressing on'. This is the set up that the LE500s are using.

Most 'experts' will tell you that the springing is at fault on the early cars and attempt to sell you softer springs (they don't tell you about uprating the front anti-roll bar). This is because they are complete muppets. There is nothing wrong with the stock springs, it is the OEM dampers that are at fault since they are under damped in bump and over damped in rebound - hence they can not control the springs properly. Indeed, I run X-Power springs which are far stiffer than stock with SPAX competition dampers and my car handles like a dream and perfectly acceptable ride quality for a sports car. I actually trialed a certain manufacturer's soft spring set up and it was appalling - the car handled like a boat with masses of corner entry understeer as well as little or no gust resistance to sidewinds when on the motorway.


MGR also further compromised the TF because, originally it was to have two suspension settings:

A 'sporty' firm setting to mimic the MGF Trophy and a softer setting to mimic the standard F. Unfortunately they developed the firmer setting first but ran out of development time to develop the softer setting. Hence, when the car was launched they merely jacked the ride height up by 25mm. This is why, if you go to a good tyre centre they will have two distinctly different wheel alignment settings for the TF - 'Standard' and 'Sports'.


With regards to the 'Comfort Handling Pack' this is basically a set of four replacement Bilstein dampers and I have heard good reports from several people. However, the dampers used are not user-adjustable in bump and rebound (unlike the SPAX) so are of no use to me because I like to change my set up for things like track days. The Bilsteins are adjustable in that they have a way of adjusting the spring seat height, thereby changing the ride height (four fixed settings IIRC) which you can't do with the standard SPAX (the competition dampers I use have fully adjustable spring sears so that I can set up the corner weights of the car).


Personally I think the Bilsteins, while good, are severly over-priced. I am currently looking at getting some dampers made up by 'Pro-Tech' since these are superb - they are what I use on my race car:

http://www.protechshocks.co.uk/

[Canalsman]

if you go to a good tyre centre they will have two distinctly different wheel alignment settings for the TF - 'Standard' and 'Sports'.

Which settings should I ask for on my 2004 unmodified TF160 when I next have the wheel alignment checked?

Chris

[Debs]

Standard Chris.

The Sports setting is for when you have lowered springs á la Service Pack 1, or X-Power springs.

[Canalsman]

Debs

Having read thoroughly what you have written about improving the ride quality without prejudicing the handling, can you offer some advice please?

I don't plan to change things immediately (mostly due to cash constraints), but my car will be going back to Steve McKie in Chesterfield in November for its annual service.

I might choose to get the dampers etc changed whilst it's with him.

Is this a good way to proceed? And if so, what parts etc would be required, and where might they be sourced?

Thanks for your evidently clearly knowledgeable advice ...

Chris

[Debs]

Personally I think the first step is to replace the execrable OEM dampers with some good mono-tubes.

If you are wanting to go down the route of retaining your stock springs (and maybe replace at a later date) then I would suggest the SPAX set up.

The standard SPAX have fixed spring pans and can be bought direct from SPAX.

Part numbers and prices are:

Front - G3253 £79.99 each

Rear - G3254 £79.99 each

ie. £319.96 for a set of 4 dampers. There is also a £15.00 delivery charge I believe.

http://www.spax-suspension.co.uk/tsearc ... 0TF%20Spax

I use X-Power springs which lower the ride height by 25mm, however these are uber stiff and not to everyone's taste.

Another option is to get Pro-Tech dampers. These are aluminium lightweight monotubes with adjustable spring pans. This way you can retain your stock springs but set your own ride height. Ihave no idea of part numbers or costs as yet. The dampers they did for my race Ginetta G15 came in at £70.00 each. I'd suggest you give them a call on 01225 705553:

http://www.protechshocks.co.uk/

Pro-Tech are run by the well known damper expert Rod Avo (ex SPAX, GAZ and Avo).


An alternative route is the GAZ replacement coilover set up. This is a kit of 4 dampers with adjustable spring seats as well as replacement springs (so you can set your own ride height). These are available from both the MGOC spares shop as well as the MGF Centre at around £550.00. The cheapest I have been able to discover s from Corby Motorsport at £434.47. The part number is: GHA356 for 4x dampers and 4x springs.

http://www.corbymotorsport.com/mg-tf-03 ... -p-71.html

I have no experience of this set up but, I believe both James Curgenven and Paula (MGF Register Committee Members) have both used it and were more than happy. One word of warning though, I personally don't like the way the GAZ springs are made since the spring ends are not chamfered and so will lead to stress raisers and possibly failure of the spring or spring seat. Whether these dampers can be sourced independantly (so you could use your stock springs) I don't know. It might be worth contacting GAZ on 01268 724585:

http://www.gaz-shocks.co.uk/contact.html


There is also a set up by Leda which is said to be good, a;albeit I can't find any pricings. Having said that, Leda are usually quire costly:

http://www.leda.com/pages/contact.html


Alternatively, if you want a 'fit and forget' set up, ie you don't need to adjust the suspension, then the VHS Bilstein kit is £527.85 from VHS or £528.46 from the MGOC.

[Canalsman]

Thanks Debs

So there is no fundamental difference between the SPAX and VHS Bilstein solutions other than cost - is my understanding correct?

If so, I might as well save a couple of hundred quid!

Regards

Chris

[Debs]

Fundamentally no.

The valving may be slightly different meaning the SPAX may be stiffer at the top end of adjustment.

The major difference is probably in the lifespan of the dampers - I would expect the Bilsteins to be of a higher quality finish and longer lifespan.

[FrAnC3SkO]

maybe

broken bolt depend

wrong spring plate position ( in my opinion to low )

i have a set of spax ( IMHO very poor quality shock ) and a ORAP MOTORSPORT (track oriented ) whit eibach spring

in ORAP SET i can change position of the spring plate ,whit low position i'havent correct piston shock ride

shock became a "solid element" and broke the bolts

it's possible in your opinion ??

what's your RIDE HEIGHT whit eibach/xpower spring ?

sorry ,is difficult to explain in english

[raymond.harper]

What setting did you have these Spax set too ? I know that in the MGF spax set up you can only have a maximum of 5 clicks as six clicks will cause cracking of the mountings. Is it possible in your TF that they have been set to their hardest setting which is causing the bolts to fail ?

[FrAnC3SkO]

What setting did you have these Spax set too ? I know that in the MGF spax set up you can only have a maximum of 5 clicks as six clicks will cause cracking of the mountings. Is it possible in your TF that they have been set to their hardest setting which is causing the bolts to fail ?

Hi Raymond

i don't think it's a problem about stiffer shock hydraulic

in my opinion when shocks WORK CORRECTLY you can use very very hard setting but piston must have correct riding lenght !!

if your shock (wrong spax too ) use too low spring mount ,you will have ( in compression situation) shocks "blocked" ...it's like hammering bolt !!!!!!!!!

NB my ORAP are 15 time hardest than spax (really)

[Debs]

What setting did you have these Spax set too ? I know that in the MGF spax set up you can only have a maximum of 5 clicks as six clicks will cause cracking of the mountings. Is it possible in your TF that they have been set to their hardest setting which is causing the bolts to fail ?

The TF has a completely different set up to the F - it uses coil over dampers with different mountings. I run around 16 clicks front and 20 clicks rear for Track Days and soften off to 12 and 16 for normal road use.

A stiff damper setting can not be the cause of bolt failure because no more load is being transferred - the same load is merely transferred at a different rate.

If, on the other hand you have springs that are going 'coil bound' then the shock loading at the damper mountings, especially on a bolt in single shear, will eventually cause the bolts to fail.

A certain MGF/TF 'specialist' insists that the X-Power springs go coil bound at very little suspension deflection. This is utter horlicks; I have tested the X-Power springs with a spring rate tester and they do not go coil bound. In fact you'd be on the bump stops way before this.



However, if you fit shorter springs such as the X-Power ones and you fit dampers with adjustable spring pans, as I have, then you need to be very careful! While it is acceptable to set the spring pans to adjust the corner weights of the car you should not use the spring pans to lower the car further because if you do so then you will rapidly run out of suspension travel. The upshot of this is that the car will regularly be hitting the bump stops. This will cause 2 problems:

1. As you hit the bump stop the suspension will momentarily go 'solid'. This will result in the car feeling bloody twitchy and skittering all over the place.

2. You will shock load the mountings and the bolts will eventually fail.

[FrAnC3SkO]

Debs i'm agree whit you

can you make measuremet of ride height in your MG TF ?

whell centre to body ?

lower bolt hole to spring pans ?

i'm waiting for 4 new Atlas 10.9 ....

[Debs]

Debs i'm agree whit you

can you make measuremet of ride height in your MG TF ?

whell centre to body ?

lower bolt hole to spring pans ?

I'm waiting for 4 new Atlas 10.9 ....

Sorry for the late reply, I kind of missed the continuation of this thread.

My TF is in the workshop just now because I am changing the suspension set up so I'll measure the ride height when it's all done.

For information purposes I had another rear lower damper mounting bolt failure. Closer inspection shows that the damper bush had worn and was allowing movement on the bolt. I have therefore come to the conclusion that SPAX dampers are not up to giving long term reliability when used with the Eibach X-Power springs.

I have at last been in touch with Pro Tech and will be dropping off a damper to them thos Friday such that they can use it as a 'mule' to make me up some proper racing dampers.

I will report back when I receive the new dampers.

[Debs]

I finally got my new rear dampers from Pro-Tech.

Gucci or what?





They come in at £75 each so they compare well on price with the Spax and are only half the weight of the Spax. As you can see the spring pan is adjustable so that the car's corner weights can be set up accurately. Additionally those on stock springs can use this to lower the ride height.

I'll report back further when I have fitted them to my TF.


For those that are interested I have asked Pro-Tech to retain the Technical Drawings of these dampers so that they will now be available to order.

The man to talk to at Pro-Tech is Kevin who can be reached on: +44 (0)1225 705553



In the early New Year I will be havig a set of front dampers made Watch this space...