MG-Cars.net

Welcome to our resource for MG Car Information.

Recommendations

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGB Technical - Lowering to chrome bumper height

Hi,

I'm going to lower a rubber bumper roadster soon down to chrome bumper level. I have a chrome bumper cross-member for the front.

Apart from the front cross-member which 'route' would you recommend for the rear? Going from simplest to best; or maybe simplest is best?

I don't mind welding so the relocation and welding of spring attachment points is not too daunting?

Also any particular pitfalls that might be encountered.

Ta in advance,

Iwan
Iwan Jones

Those of us who have experience of the chrome cars on their original 80 profile tyres may be smiling and perhaps wondering whether, given the state of the roads in UK, you are planning to limit the use of your car to the track. My recollection is two new exhaust systems a year - but that was on the normal road system. Since you have gone to the trouble of acquiring a crossmember, you might think about changing the suspension throughout - there is plenty of expertise here to help with that. If you want to lower the car further ( must of us are on 70 profile tyres.... you?), the most cost effective way will be to reduce the profile further; arguably 60 looks ok in the wheel arches while 50 looks a bit small. I have had 60s on my Summer wheels and still ground the centre box occasionally (the ferries are a b*gg&r)but 70s on the Winter set keeps both cars clear on tarmac. If roadholding is the main reason for change, you might consider 195x55x14 but the rear suspension needs to be in good order first.
HTH
Roger W

You may have fun aligning all the steering rack/column up at the front by simply swapping the X-member over without changing anything else. I think you'll at least need a C/B rack and the older type U/J for it all to work properly. The later C/B export cars were like this to mate the R/B style column with the C/B X-member/rack/etc

For the rear, the R/B cars have longer spring hangers at the front as well as different springs. You can simply re-drill the hangers using a C/B as a template and then get the correctly arched C/B springs to complete the lowering there. Don't forget to change your rebound straps as well as the lever arm damper actuation drop-links for C/B ones so as not to bottom out the dampers and destroy them.

If your car is a late R/B (77-end of production) you'll have the later style axle with rear ARB. The ARB should be fine, but the mounting point on the battery box for the rear flexi brake hose may need re-positioning so it doesn't bind under full upwards travel of the axle. If you have an early R/B (75/76) ignore the last paragraph!

Hope this helps, and as mentioned, don't go too low. It looks really cool till you rip your centre box off on a speed bump and it slams through your fuel tank then rear valance. Trust me ;)
Roadwarrior

Consensus is to remove the rear ARB.

You haven't said, but if you're converting to wires, your tyre choice will be limited.
John Bilham

As I recall the CB spring hangers go through the chassis rails, whereas the RB ones are below the chassis rails through extended side pieces. Both types have a strengthening tube going from one side to the other, if you drill through the chassis rails on an RB you will either need to add a strengthening tube or risk crushing the chassis rails. Given that there are lowered springs, with lowering blocks and longer U-bolts, I can't see the point in going to all that trouble. You will have enough issues to resolve with damper drop-links, bump rubbers, and rebound straps.

As Roger W implies, I've had to extend the rear shackles by over an inch on my CB roadster to limit the amount of bottoming and grounding when touring fully laden, and that's with new springs.
Paul Hunt

If you use a chrome bumper rack, isn't the pinion shaft shorter? This would mean that the upper column wouldn't be long enough to meet up with the lower column. Or am I wrong?
The "conventional" way to lower the front on a R/B car is to use shorter springs. Then you don't get into the problems with the racks.
Mike Howlett

Yes, which is why the change of U/J is also required as mentioned for the rack change. You may also need to source a US Style late CB column (very like the UK R/B type) but the UK one may be long enough to meet up with a bit of jiggery.

Neither front spring hanger at the rear go through the chassis rail. These are only the rear hangers, which are both close enough to not really have to change if you modify the fronts with redrilled holes.

Simply using shorter springs at the front alters maximum travel and can result in bottoming out issues on a hard driven car. He already has the C/B beam so why not go the whole hog?

I guess the simple route is to get lowered front springs and lowering blocks/ubolts at the rear. Then you have nothing to worry about elsewhere as such.
Roadwarrior

Iwan,

I own both a chrome bumper (68 GT) and a rubber bumper (79 roadster). I find the chrome bumper car hits speed bumps (sleeping policemen) and the drive way curb far more often that the rubber bumper car. On our local roads, which tend to be rough and rather poorly maintained, the RB car is far more pleasant to drive. I drove my original 79 B on the German Autobahn quite comfortably. My current 79 B has poly bushings on the front and rear and handles better than the original, purchased new, car did.

So, with this background, my question is: Why do you want to lower the ride height of your car? If it is for racing, or related, use, lowering might make sense. If it is for general driving, however, I find that I drive my rubber bumper more often because it is better suited to the condition of our roads. I can take a speed bump without coming to a complete stop and creeping over it (without hitting the exhaust system), something I cannot do with the chrome bumper car.

I would like to hear more about why you want to make this modification. I have a couple of formal "race driver's" schools and have done a minor bit of racing in the MGB and would appreciate your thoughts on why the lowering is needed since I have not seen such a need myself. Please take the time to enlighten an old man.

Les
Les Bengtson

"Why do you want to lower the ride height of your car?"

In most cases it seems to be mainly to make an RB car look like a CB, as the bumpers and grilles are often changed at the same time.

With the rapidly worsening condition of our roads, and the number of humps, 'tables' and 'pillows' (I can't go out or get home without going over at least three sets, and in some directions up to seven) lowering is becoming more of a liability. With the pillows if nothing is coming the other way I can reduce their effective height (in the MGBs) by driving between them. Otherwise I have to drive right against the curb or the exhaust catches, and that's on the RB V8 as well as the CB roadster.
Paul Hunt

Les,

I own two Chrome bumper Gt's one of which is 'stock' as you say in the states, the other has a stage 2 engine, handling and suspension mods and is lowered 1" for trackday/racing use. I have no big problems in rural North Wales with sleeping policemen or 'pillows' ... [sorry Paul], the roads are quite good too. No problems even with the track day Gt.

I now have a bare roadster shell that I can work on. It's a rubber bumper body. I'm afraid I don't like their ride height and I detest the rubber bumpers; also, if I fit chrome bumpers then the ride height will be raised (less weight). So it will have to be lowered.

Cheers,

Iwan





Iwan Jones

"I have no big problems in rural North Wales with sleeping policemen"

Just Richard Brunstrom, who put a speed camera in a horse box, I understand :o)

I see he has 'moved on', not long after making public pictures of an identifiable headless biker without informing the family.
Paul Hunt

Iwan-
The use of shortened coil springs in the front suspension should be avoided as they will leave the front suspension travel with less upward travel, result in an extreme Toe out front wheel misalignment that will result in accelerated tire tread wear, as well as create a bad tendency toward bump steer as a result of the change of angle of the tie rods. Braking distances will also increase as a result of the deformation of the contact patch of the tires. In addition, these shorter springs will need to have a much stiffer rate to partially compensate for the resultant decrease in upward suspension travel and to prevent the suspension from crashing into its bump stops, thus creating very harsh ride qualities. The most comprehensive approach to lowering the front end is to install a Chrome Bumper front crossmember, complete with steering rack and steering column, and then use an engine front plate (BMC Part# 12H 1387)and motor mounts (Metal Brackets BMC Part # AHH 5065 Right Side; BMC Part # AGG 5066, Left Side; Rubber Brackets BMC Part # AHH 5027, Right Side; MNC Part # AHH 7257, Left Side) from an engine intended for installation in a Chrome Bumper model car.

Be advised that the steering rack mounting brackets of the two different front crossmembers are set at different angles, and that the steering racks have different length pinion shafts and use different length steering columns, and thus cannot be interchanged except as a complete steering system, provided that the appropriate corresponding steering rack mounts are welded onto the front crossmember. For the rear suspension, you need only cut off the front spring hangers and weld in new Chrome Bumper leaf spring mount assemblies. These are available from the British Motor Heritage Trust (BMIHT Part #s BMH9002 for the right side, BMH9003 for the left side). These can be seen on the British Motor Heritage Trust webpage at: http://www.bmh-ltd.com/p61r.asp . You will also need to change the rear bump stops so that you will still have adequate suspension travel, and the limiting straps. If in the future you should decide to install a Rover V8 engine into a 1962 through 1974 Chrome Bumper model car, a change of the angle of the steering rack mounts, plus the later steering rack and its steering column from the Rubber Bumper model, will be necessary in order to clear the rear of the exhaust manifold on the engine.

MGBs are easily lowered by modifying the stub axles and replacing or reworking the springs. The rear leaf springs can be lowered by means of installing special lower-arch rear leaf springs, or by taking the less expensive approach of removing a leaf and restacking the short leaves and / or adding lowering blocks. It is best to rework the rear leaf springs, as this will decrease the rate of the rear spring, which in an mgb roadster improves rear grip. Unfortunately, this can result in the rear axle crashing into its rubber bump stops on bad roads, so a certain amount of discretion on the part of the driver becomes mandatory.

On an MGB that is being built for street use, several other factors come into play. The first is the roll center of the front suspension. As the front of the car is lowered, the Roll Center moves lower at least as fast as the Center of Gravity, thus reducing the benefits of the reduced vehicle roll rate. In addition, this further degrades the dynamic Camber gain of the suspension, which will result in a condition referred to as Bump Steer.

If you look at the lower A-arm on a lowered MGB, you will notice that its outer end typically will be higher in relation to its inner end. With this condition, lateral wheel movement takes place during normal suspension travel - causing the car to lack directional stability and thus making it feel darty on uneven road surfaces. The cure for this is to raise the lower A-arms inner pivot point of the wishbone pivot by up to 2 inches, both maintaining the arm geometry parallel to the road and reducing body roll. However, the rotational axis of the swivel hub will be canted inward due the upward change of the angle of the suspension arms of the Armstrong lever arm damper, resulting in negative (-) Camber. This problem can be corrected by fabricating spacers to raise the Armstrong lever arm damper until the original suspension geometry is restored. This also helps to improve Camber gain with resulting improvements in grip.

Another consideration will be King Pin Inclination. This is the angle is described by a line drawn down through the swivel pin centers and a vertical line when viewed from the front of the car. Extended to ground level, the distance from here to the wheel / tire center line at ground level is the King Pin Offset. Ideally, the lines should intersect at ground level. This will give both lightness of steering feel and an effective absence of kickback through the steering wheel when hitting bumps known as center-point steering.

A further problem is that of Bump Steer. Although bump steer is both controlled and governed by the suspension and steering geometry, the King Pin Offset has a direct effect on the severity of it felt through the steering wheel. The greater the King Pin Offset, the more leverage the wheels have to exert on the suspension, thus magnifying the kickback that it can exert through the steering wheel upon the driver.

Adjusting Bump Steer is highly recommended to transform a car that has become a twitchy handful as a result of lowering into the proverbial handles-like-its-on-rails car. So exactly just what is Bump Steer? And what causes it? Bump Steer (often called Roll Steer by suspension designers) is the term used to describe how the steer wheels pivot in relation to the cars direction of travel as a steer wheel moves vertically. The steering arm, which is attached to the front suspension and to the steering rack by the tie rod, travels in an arc. When the connecting end of the tie rod travels in an arc that is different from that which is followed by the suspension, it changes the Toe of the suspension whenever the connected steer wheel rises and lowers. This is because the tie rod is a single point mount attached to the steering rack as well as to the steering arm for its particular wheel assembly. As a result, when the wheel assembly moves up and down, taking the attached end of the tie rod along with it, the effective length of the rod changes according to Pythagoras Theorem of right triangles (See, your old High School geometry teacher was right! There really is a real-world use for Plane Geometry!). This change in the effective length of the tie rod pulls on the steering arm as the wheel assembly moves up (and down), slightly changing the steering angle of the wheel. This is what pulls on the steering wheel and makes it seem as though someone other than the driver is steering whenever a bump is encountered by a single steer wheel, hence the term Bump Steer.

It is desirable to have the wheels Toe-in very slightly as the wheel ascends and Toe-out slightly as the wheel descends. This is called Roll Understeer and makes for precise and stable handling. Roll Oversteer, however, is an opposite effect that causes difficult-to-drive, squirrelly handling. Do not confuse this description of oversteer / understeer with the same result that stems from limited tire grip at one or the other end of the car. Obviously, when the mounting points of the suspension arms are relocated upwards, the tie rod of the steering rack assumes a steeper angle to the steering arm. The original geometry can be restored by heating the steering arms with a torch and slowly bending them downwards until the original angle of the tie rod is established. Under no circumstances should the steering arm be hammered into position, as this may create fracturing.

For those who find this conversion to be too challenging or expensive, there is a much simpler approach to the issue. Instead, install a set of swivel hubs with vertically offset stub axles. Lowering the rear suspension with blocks will increase the fulcrum effect of axle torque on the rear springs as a result of moving the axis of the pinion gear further from the rear leaf spring. This in turn will lead to the dreaded axle tramp at the moment of a vigorous application of power when pulling away from a stoplight, a result that is unacceptable in an enhanced-performance car. The use of decreased-arch rear leaf springs is also inadvisable as their Spring Rate will have to be stiffer in order to prevent the suspension from crashing into its bump stops and will not match that of the front springs, thus damaging both the handling and the ride quality of the car. You will swiftly learn to avoid railroad crossings and speed bumps. Removal of the front muffler (silencer) from the exhaust system and its replacement by a section of straight exhaust tubing in order to avoid scraping will be mandatory. Even with the front muffler (silencer) removed, the car will still tend to scrape on some speed bumps and railroad crossings. Instead, change both the rear springs and their rear spring mounts to those of the Chrome Bumper model, and be sure to use the earlier model 81/2 long limiting straps (BMC Part # AHH 6355) and 81/2 long damper linkages (BMC Part # 97H 2031) of the Chrome Bumper models. Although the front suspension of Chrome Bumper models can also be lowered using similar techniques and the rear suspension lowered by using decreased-arch rear leaf springs, such a modification is normally confined to cars that use very stiff springs and are normally driven only on a race track. As engineers are fond of saying, it is all a matter of finding a balance of priorities. In other words, you do not get something for nothing.
Stephen Strange

Paul,

Yep, Brunston was a right piece of work for many reasons. Just drove past one of his travelling cameras now. What a legacy ... great for tourism, locals have learnt to be vigilant, visitors get caught out all time.

Stephen,

Hey thanks a lot. There is plenty there to get me going for now ..... and some. I already have a chrome bumper cross-member, steering rack and steering column as well as dampers and springs.

Ta,

Iwan
Iwan Jones

An alternative way of lowering the rear is to use "reverse eye" springs. That loses about an inch.
Allan Reeling

Hi,

Allen - Reverse eye springs, do you mean special springs or is that with the top 'eyed' leaf turned over?

Paul, which suppliers have the flatter rear springs to lower the ride height you mention in your website? Do you replace the shorter drop arms, rebound straps etc..

Ta,

Iwan

Iwan Jones

Moss are showing lowered springs, MGOC a lowering kit. Brown & Gammons have also had them it appears.

If you just lower the springs you will hit the bump rubbers sooner, much as Stephen says for shortened front springs, unless they are also harder. If you fit the shorter bump pedestal to give you more upward travel of the axle you probably need to change the damper drop-links as well to avoid reaching the limit of the damper before the bump-rubber contacts. Then you need to fit the shorter rebound straps to prevent the other end of the damper being hit when unloading the axle. This is all from perusing various things over the years, it's not a job I've had to do, happy to leave my RB V8 as it is.
Paul Hunt

Iwan,
Got mine off the BHive, they use them on their race V8's.
Allan Reeling

Sorry, meant to say the top leaf is a special.
Allan Reeling

This thread was discussed between 21/01/2014 and 26/01/2014

MG MGB Technical index

This thread is from the archive. The Live MG MGB Technical BBS is active now.