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MG MGB Technical - New Rear Springs & Shock Absorbers

Hi All, I wonder if any of you suspension experts out there can help me please?

I have a June 1976 mgb roadster and have just replaced the rear springs and shock absorbers as the old springs were looking pretty sad and the interleafing rubber had stated to perish. I replaced the springs with BHH 1779's from the MG Hive (which are the correct ones for the car) and also while I was at it replaced the rear shock absorbers with reconditioned units from Rugby Classic Motor Garages. I have also fitted new Poly Bushes supplied by Autobush. I also replaced the Drop Links, U Bolts, Bump Stops, and Check Straps,

The problem I have is that the ride is now much firmer (which I guess you would expect), but it is also very bouncy over bumpy road surfaces (all to familiar in the UK) and is really now quite uncomfortable.

Has anybody else experienced this? I'm thinking maybe its the poly bushes are making the rear suspension too hard and inflexible, or is it just a question of new springs and bedding in?

Andy Robinson

I don't really see how it can be both firmer and bouncier. If by firmness you mean harshness and vibration, then that's probably down to the poly bushes. There are several grades of hardness, I've only ever tried the red ones which I found too harsh, and went back to rubber.

Springs have been a nightmare for some years now, the usual complaint is that rear springs are way to hard and too arched which the gives car a pronounced tail-up stance. They should be almost flat with normal loading. Fronts aren't much better having significant differences from the original specs in my experience, and even one from another. Part numbers mean nothing in terms of how closely they reflect the original specs. You need to measure your ride heights, from the centre of the axle to the bottom of the trim strip, and compare them here http://www.mgb-stuff.org.uk/ridetext.htm#height If nothing else it will give you an idea as to how much variation there is, not always intentional.

Rebuilt dampers can be equally iffy, they are only as good as the returned unit plus the rebuilder. I've had to reject some in the past as testing the arm on the counter showed a 'dead' spot with no damping.

Rebound straps ditto, most at the moment seem to be little more than rubber bands with no reinforcing as per the originals, and can stretch an inch or more, or even snap the first time they are asked to carry the load of the axle.
Paul Hunt

I don't know Autobush but unless they're sold as fast road or competition they shouldn't make the ride hard

provided everything was fitted correctly it could just be that things have to settle in

another possible alternative or contributor might be the tyre pressures, might be a bit high

or a possible contributor perhaps, are the tyres old and/or lack of use so there's loads of tread but the tyres have gone hard transmitting more harshness that was absorbed more by the previous worn suspension set up - tyres are an important part of the suspension handling and ride comfort

it could be any or a combination of above, check the tyre pressures and ages, drive the car a lot and see if things improve, if not report back with perhaps photos of the springs installed

tyre age and loads more info - http://www.carbibles.com/tyre_bible.html

hth
Nigel Atkins

Thanks Paul and Nigel for your replies.

The ride height of the car is 15.5 inches at the front and 16.75 at the rear (measured from the centre of the wheel to the bottom of the chrome strip, so the car does have a slightly "arse up" appearance. To cure this I'm thinking of using lowering blocks on the rear to bring it down by an inch.

The Autobush (http://www.autobush.com/) bushes were fitted as per their instructions with plenty of their white silicone grease used. They as far as I know they are for standard road use. I have fitted these bushes to the front suspension when I rebuilt it and have had no problems. I also fitted a 3/4 inch anti roll bar (I believe that 1976 models did not have anti roll bars fitted as standard as a factory cost saving measure)and this together with the new bushes significantly improved the handling.

The tyres are Firestones and have only covered 2k Miles and are a couple of years old. They are run at standard pressure - 21 psi front and 24 psi rear.

As I've only covered 100 miles with the new set up so it could be just early days, but I was concerned when I first drove the car after completing the rear suspension upgrade that the rear end had become a lot livelier.

Thanks for your advice.
Andy Robinson

Andy,
If you torqued everything up with the springs hanging you could be winding up the various bushes.
Jack under the dif and slacken the front eye bolt, the shackle bolts and the drop link bolts then re-torque. Were the check straps R/B ones, i.e.longer than C/B?
"Bouncy"? Could mean NO spring deflection but this would seem more like hopping, OR no damping, i.e., some more "up and down" after a bump. Did you check the dampers before fitting? You can get a subjective feel by the resistance of the levers, also you can get a good idea as to whether they are similar.
Allan Reeling

three quick thoughts

you've added an ARB (and an uprated one? at that) so that will make a difference to the feel, perhaps this has altered the balance of your car's suspension system overall

as Paul has put the quality and specification of the springs may be different from standard and as for recon dampers and rubber parts they can be very variable (or p*ss poor not to be polite) - (bear in mind this refers to USA)- http://www.youtube.com/watch?v=qYEx42iYXc8

1k-miles/year is what I'd call lack of use for the tyres (and car) but hopefully if the car has regular use and the tyres only 2 years from purchase (their age will be on the tyre) then they could be fine (if the car sits in one position on its tyres for 51 weeks of each year then probably less so)

ETA: I was typing whilst Allan posted
Nigel Atkins

You could try putting the bottom leaf on top.
John Bilham

G'day Andy,

In addition to the suggestions above I would offer that in my experience 21/24lb is way too low a pressure for modern tyres like Firestones, and is probably contributing to some of your problem. I would try them at a range of 28-32lb - perhaps 30 both front and rear to start and adjust progressively from there to your preferred rate (can of course be different front and rear) depending on how you like the car to drive.


Regards
Roger
R Taylor

As one of the 'features' mentioned was a firm ride, increasing the pressures that much is going to emphasise that even more.
Paul Hunt

One thought - are the rear shackles pointing backwards?

One suggestion - put some weight in the boot and see what the ride is like then.
Chris at Octarine Services

I have finally cracked a similar problem-too long a saga for here-but Chris' suggestion is where I started. Can you produce a ride that is suitable for YOU by adding weight to boot and /or a passenger.
Michael Beswick

G'day Roger and All, many thanks for all you inputs, it looks like with all your help I've cracked it! :)

Just for clarity, the new rebound straps that I fitted were for a 76 Rubber Bumper MGB.

Picking up on Alan's and Rogers suggestion the first thing I did today is increase the tyre pressure in the front to 28psi and the rear to 30 psi. The tyres are Firestone Multihawk's 175/70/R14. The other thing I did was (as Alan suggested), jack the car up under the Diff, supported it on axle stands, removed the rear wheels and then slackened everything off, front eye bolt, shackle bolts and the drop link bolts. I then disconnected the drop links to ensure that the dampers had a smooth action and were equal in resistance - which they were.

As per Chris's suggestion, I then put a good bit of weight in the boot (trunk) and gave the thing a bloody good bounce. I then retighten everything and took it for a test run. With the car on the ground the shackles are pointing backwards by about 30 deg - much better than before.

The result - well really a great improvement and the car handles like a dream and it goes round the lanes of this part of Essex much better than it has ever done before. A MGB is never going to give the ride of a luxury limo, but there was really a vast improvement. Firstly the steering was much lighter, obviously due to the increased pressure in the tyres. Secondly the rear end was much more stable and overall the ride very much more comfortable. I can only put this down to the retightening of all components and adding some weight in the boot when I did so (I removed the bags of sand afterwards). The rear ride height has also reduced to 16 inches, which is much more acceptable and looks much better.

Thanks all once again.

Regards

Andy

Andy Robinson

well done

commenting on the tyre pressures only -
the modern tyres seem to work well within a range of pressures

to me +6psi over book is about as far as you want to go and you might improve ride comfort further by reducing the tyre pressures

keep things as they are for a few more drives then try reducing the pressures by 2psi in each tyre (using the same (accurate, not forecourt) gauge on 'cold' (not still warm from use) and see if that's better by driving like it a few more times - repeat as required until you realise you've passed the optimum then go back up 2psi to get back to optimum

as even an accurate domestic gauge will have a tolerance for accuracy I think 2psi steps are best

my previous tyres were best at +2psi all round but my present hard feeling tyres are best at book settings for any sort of comfort at any pothole
Nigel Atkins

In general, the lower the pressure the softer the ride - and the slushoer the handling.
Back in the days before low profile tyres we needed to run 50-60psi in road tyres to optimise cornering traction on the track - anything less would see the tyres distort so much they would end up running on the top of the sidewall when under full cornering load.
But the ride was terrible!

Now with lower profiule tyres there's a better balance. But even then there is a trade-off between ride and handling. Higher pressures (up to an optimum) result in less tyre distortion and so more rubber on the road - but also a harsher ride.

If you prefer ride over handling, then follow Nigel's method to ge the optimum pressure for the eay you want to drive. If you prefer handling over ride, then basically do the reverse - increase the pressures until lateral grip is past its optimum then drop back a couple of PSI. The best way of picking this for cornering grip is to increase the prssure initially at one end only and assess the effect on over-/underster. If in doubt, reverse the pressure setting from end to end of the car and see if it makes any difference. If it doesn't, then optimum pressure for lateral traction has been reached.
Paul Walbran

whilst agreeing with Paul I'd add that everything is a compromise to get the best balance of grip, ride and handling to suit the car and driver

Andy may get a ride quality nearer to what he wants by reducing the tyre pressure, yes there might be small compromises in other areas but if the ride improves the enjoyment of the car will too

on a road going pretty standard suspension set up MGB I think you can go for a bit more comfort over a bit more handling of the tyres (that will be compromised by the car's standard suspension) but it's all a matter of overall balance and driver's preference and driving style - experimenting with tyre pressures within a sensible range is very easy to do and costs nothing to try, what suits one car and its suspension and driver won't be the same as another car and driver

you also need to take into account the differences in different make and models of tyres, some tyres seem to have stiffer and more upright sidewall, like the ones I have at the moment which transmit lots of road surface roughness to the car and occupants, and of course some make/models of tyre provide more grip than others by their design and manufacture

Nigel Atkins

Just be aware that departing significantly from the car manufacturer's recommended pressures MAY invalidate your insurance.

What significantly means depends on the insurance co.

I have never found it necessary to raise the front tyre pressure from 21 psi and only the back by 4 psi from 24 when driving fully laden.
Chris at Octarine Services

Andy,

Well done indeed. That was a good amount of fiddly, and cramped, work you did so it is pleasing that you are much happier with the driving outcome. Remember, if/as the back settles you may need to experiment with presures again.

Paul's comments and advice on adjusting fit with my experience.

On the insurance aspect Chris has raised, my understanding is English insurers may be more prescriptive than ours. At least to this point in time. Certainly there are insurance and registration issues re changes from manufacturer's specs, but with these older cars I think the view here is that contemporary tyre manufacturer's advisings would take precedence over those of the car maker, notwithstanding the less sophisticated suspension systems on our Bs.

I do wonder though when I see some of todays hot 4 cylinder hatches the young blades fancy with their 18 inch rims and 1/2 inch profile tyres!! They must be hell driving over a matchbox on the road?

21 psi at the front? Chris, you must bring the car out for a run on some of our NSW B roads sometime. Book your dentist for fillings before you leave (lol)

Must away, the Bathurst V8 race qualifying sessions are starting on TV. That means Sunday locked into watching the live coverage.

Regards
Roger
R Taylor

Roger,

They can't be worse than driving over the Belgian concrete slab roads and I certainly wouldn't want to reduce any of the tyres' suspension effect by increasing the pressure!

Interestingly, I can find no tyre manufacturer that gives any guidance on pressure - all the ones I have seen refer to the vehicle manufacturer.

The weight and suspension design are the key factors affecting the recommended pressures and they haven't changed unless you have modified the car.

The recommended pressures are intended to give a margin of safety - of course you can improve the roadholding under extremes but the trade off is loss of that margin, so when your high pressure tyre hits a pothole while cornering you may lose grip that you would not have lost at lower pressure because there was more compliance at the lower pressure.

If that loss of grip results in an accident then you could well face claims for contributory negligence or even prosecution for driving an unroadworthy vehicle.

I just don't think consumers second guessing the manufacturer is a clever game!
Chris at Octarine Services

"English insurers may be more prescriptive than ours"

If they can get out of it all too often they will, even though the 'transgression' they fall back on has had no bearing on what led to the claim. It's why we have Ombudspeople!
Paul Hunt

Gents, thanks all once again for all your inputs into my problem. which was originally about fitting new springs and shock absorbers and has now turned into a debate around Tyre pressures. Certainly from what I have read on the web, modern radial ply tyres have totally different characteristics to the type of tyres that were available when the car was designed. The issue also is that in general a 165SR - 14 tyre, with a rolling diameter of 620mm (24.39")are not readily available. To get a tyre that is near the original we have to fit:

• Radial 175/75 14 which gives a rolling diameter of 618mm (24.33") or a reduction in diameter of 0.32%
• Radial 185/70 14 which gives a rolling diameter of 615mm (24.2") or a reduction in diameter of 0.81%

So what we have with modern radials is a lower aspect ratio on the side wall on the tyre and therefore slightly different handling characteristics for a given axle load / tyre pressure / tread design.

I picked up these comments from the web:

1/ The proper pressure will depend on many factors including vehicle weight, tire construction, driving habits, etc. Also, it has been correctly noted above that the driver's handbook states pressures for bias-ply tires, not radials, which generally take much higher pressures.
Based on the weight of an MGB and the width of your tires, you should be running anywhere from 26-34 PSI. Typically MGBs on 165R14 tires run about 28F 30R for general driving. A pressure difference between front and rear of 2-3 PSI suits the MGBs handling characteristics well. As tires get wider, the pressure difference between front and rear typically gets smaller.
The bottom line is that you will need to choose a starting point and experiment a little. Set the pressure in the tires when they are cool, and when you make a change do it at the same temperature. After driving around for a day or two at one pressure, try making a change and see what it does to ride and handling. Go with what feels best for your driving habits and ride preference.

2/ I run 35 PSI on both front and rear tires (Michelin radials). I find that I have no problems with this and, for me, it makes the car handle better. Jacking up the tire pressures is an old racer's trick when running road tires in the race track.

The recommendation to use a lower pressure in the front tires was to give them slightly less grip than the rear tires, creating an understeer condition. It was felt, by the "safety experts", that the "average driver" was safer in a car that understeered rather than oversteered.

Tire pressures should be taken cold. When the tire warms up, driven more than a mile or so, the air inside the tire is also warmed and the tire pressure is greater than when read cold.

Les
Les Bengtson

3/ Do what suits you best. I use 21/24 plus 4 for 'normal' loads and speeds. For me this gives noticeably lighter steering at parking speeds, but without the harshness of going as far as 30, which is getting on for 50% over. By mistake one day I said '34' instead of 24 to a tyre fitter and he looked at me in horror.
PaulH Solihull


I think Paul sums it up well with "Do what suits you best". Lets be honest, the MGB is not the easiest car to park with 21psi in the front tyres. In fact you need the strength of Georg Hackenschmidt (Famous German Wrestler / Strong Man of the 1900's) to get into a tight parking space. So if a couple of additional PSI make it easier, then great!

Thanks all again for your help and let's take Paul's advice.........................

Regards

Andy

Andy Robinson

Andy,
I don't think the web has always served you well here but do experiment because the principal of experimenting and that the variances are on your car and you but I'm surprised you find the car steering so heavy at parking and wonder if it's just a difference of interpretation between us or if there could be wear or servicing requirements at the front of your car that make it so

as you've probably gone as far as you want to go on this subject now (if not please continue) I'll address the rest to others

For other readers here,
. tyres are a very complicated subject and modern tyres are very different to those of not that many years ago let alone 3,4,5 decades
. the relevant Driver's Handbooks I've seen give different pressures for cross-ply or radial or both
. understeer was(/is?) designed into cars but it's more than just with the tyres usually
. it's the rolling circumference that's of importance when comparing different wheel and tyre sizes - http://www.carbibles.com/tyre_bible_pg4.html
. most of the modern tyres of the size for Bs are designed for heavier saloon cars not light weight more sporting cars so the design, characteristics and load index are different to the Bs requirements

Paul,
I don't think I explained my 'method' very well, personally I'd start at book pressures then add at +2psi stages (to a maximum of 6psi) until I found what was best by having passed the optimum, I think comfort and handling wouldn't be that far apart or even my be together when you allow for compromise - Andy had started at 28/30 so I reversed the stages
Nigel Atkins

"Also, it has been correctly noted above that the driver's handbook states pressures for bias-ply tires, not radials, which generally take much higher pressures."

Handbooks for the very earliest models maybe, but by the time radials were being fitted to the MGB they specified radial pressures, and the Workshop Manual specifies both at 18psi front and rear for cross-plies, and 21 front and 24 rear for radials. It's these higher figures that people claim are too low for radials, which is obviously not correct, even if one prefers higher.
Paul Hunt

This thread was discussed between 08/10/2013 and 12/10/2013

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