What is the effect on the MGB front suspension geometry if the inner pivots for the lower A-arms are spaced down about 1/2" or so?

JB

Jim, I know you would change camber to positive. Dave Headly recommends going the other way. If you use lowering springs or cut 1" off the coil  spring like I did, the lower A arms slant up towards the wheel. So raising the mounting point would level the A arm and put the camber back. I plan on lowering the upper mount to increase negative camber gain also.( Tubular 1" A arms similar to Hoyle set up with 67-69 Camaro/big GM/S-10 5" travel shocks)Plus, Heim joint steering ends parallel with lower A arms to prevent bump steer.

I'll definatly be following this thread B4 I overhaul my front end!!

John, pretty simple really. The 4 bolt holes that bolt the lever shock on are used for a top plate with 2" DOM tube in middle of spring with top plate mount for Camaro shock. Bracket for bottom shock mount bolts to spring pan. Double shear brackets for lower A arms. 5/8" red urethane rod ends. IMHO the way MG should have done it! The shock must be set up 1" higher than mid point of travel at road height. You have to use a 2" hole saw from the bottom of top spring mount on crossmember. Speedway sells the Heim adapters for steering ends. It would look similar to this Fiero front suspension picture as far as upper shock mount ,not ball joint, not coil over,still MGB kingpin. Also pictures of Paul Walbran of New Zealand  suspension which uses stock A arms and geometry. Plus the rod ends would be adjustable for caster and camber like some Mustang II. You might do this before me . Life and work keep getting in the way!

Spaced straight down or using a spacer under the factory pivot?

A 1/2" spacer under the stock pivot will add negative camber because of the angle of attachment to the crossmember. The bonus is that it will impart progressive negative camber gain (or, so I've been told by a couple of people smarter than me).

Carl, You are right if you go straight out using the original mounting point. If you go down the crossmember, the opposite is true because of the angle on the crossmember.

I'm talking about sandwiching a 1/2" block between each of the mounts and the crossmember. The purpose for this is for more clearance between the (lowered) steering rack and the lower control arm, to avoid damaging the boot, etc at maximum suspension compression.

Both candidates for this mod (my roadster and the MG-Roadmaster) run wide rubber on the front, ~225-265. On my car, instead of lowering the front I find I need to raise it back up, possibly near the point where the lower a-arms will sit level, depending partly on tire diameter selected. To this end I have at long last found a supplier of air bags with a size that might fit the front crossmember. It will take 3 weeks to get them and then more time for fitting but I'm hopeful of soon having an easy height adjustment on the front end. Only testing will tell what the ride and handling will be like. I expect the ride to be good, but the handling may require a pretty big sway bar. The combination of the air bags at the front and the long travel Jag IRS at the rear should make for an amazingly good ride for an MG. Front suspension travel will be enhanced I think, as more of the suspension's range of motion will be used on the compression side without getting into the bump stops. Overall the car will probably be at or slightly above stock ride height but with a wider track should still handle pretty good.

You know it's funny, when I first got the car all I wanted to do was make it sit lower, ride stiffer, and corner faster. Now I want it to ride softer, I don't mind at all having it sit a little higher (easier to get in and out), and driving as fast as possible is nowhere near as important as it once was. I just don't get it.

JB

Jim B, I don't know what air bag you are looking at but ShockWave makes one with a built in shock for Mustang II street rod suspensions. 13" ride height, could work with my suspension idea. Pricey though at $1100 each.

I'm thinking about a tenth of that price. Far as I can tell there's nothing wrong with the Armstrongs. They are totally tunable and I've never had one fade. Plus with them on there's no need for a shock up the middle of the air bag, which to me just seems like a really bad idea for any kind of a compact suspension. The deal is, drop out the stock springs, trim the spring pans, slap in the air bags and go. Might put a remote fill line for convenience but that's about it. Then set the ride height and tune the shocks. But I'll know more in about 3 weeks. In the meantime I need to resolve this little issue about lower control arm clearance. At this point I'm thinking a 1/2" spacer will do no harm, does anyone have an idea of how much camber change it'll cause? Seems it'd move the bottom of the kingpin out slightly but I wouldn't expect it to change the angle by more than a degree or so just at a guess. It'd be really helpful if anyone knows for sure but I guess I'll just have to set up a rig to measure it.

JB

FYI., clan, sometimes it's easier to add more anti dive, upper control arm,(front to rear tilt of pivot axis).Making the "rear" of the pivot lower,(side veiw), will increase anti dive, add slightly to stiffness, and enhances "roll caster gain", which works in concert with increasing negative camber,(dynamic). These things are pluses for better adhesion and handling. Positive caster increases straight line stability , increases steering effort and tends to put the inside tire into positive camber,(another good thing).Hope this helps, roverman.

Art, are you talking about putting a wedge under the base of the shock? Might be a good thing but it doesn't help with my clearance problem. I will have some 1/2" flat bar stock the first of the week which it looks like that should take care of the clearance. I've decided to put them under the front pivots only, but I'm making up a set for Carl to try under both pivots. I know, I know... I'm an enabler. What can I say? I like to see him go fast.

The air springs came in but they are double convolute instead of the triple convolute that I was expecting so I had to re-order. It looks like they should fit. I'll have more on that in a few days.

JB

Well this idea has morphed a bit. In order to get adequate steering rack clearance I had to go much thicker on the spacers than I had originally intended (1-1/4") and it gave me way too much static camber. It's enough that I can look at the car and see that something isn't right. Obviously the angle of the attachment flange has caused the arms to move outward too far, so now I need to find a solution to that problem. An adjustable solution would be ideal, but first I need to measure the angle, do some trig calculations and see just how far the arm has moved. That will tell me, within a reasonable error, where it needs to be to go back to the starting point.



As you can see, the front spacer is 1-1/4" thick, and the rear one is 1/2". The idea was to get rack clearance at the front and reduce the caster with the rear spacer to get easier steering with the large front tires. I'll be giving some thought to the best solution for an adjustable bracket. It would have to move in and out, and if both front and rear are adjustable independently the rear could be the same height as the front and still have caster adjustment. Any comments or suggestions could only help.

JB

OK, I ran the numbers and it's a little under 1/2" of lateral displacement so no wonder it looked odd. But, that is fortunate. Using a 1-1/4" tall block attached to the pivot with capscrews and to the crossmember with slots and "T" bolts, it is possible to use the same diameter fasteners and get 1/8"+ of negative camber adjustment from the stock setting. A 1-1/2" block would allow about 3/16"+ and it might be possible to configure the parts to get more.  (adding in positive camber is not a problem) I haven't worked out what that means in degrees of camber and caster change yet but it's a step in the right direction. I'll have to take some more measurements and run some more numbers a bit later. With enough range I should be able to hold the camber stock and decrease the caster.

Considering that this moves the inner pivot downwards it also lowers the roll center so I'm not sure exactly what that'll do to the handling.

JB

Maybe a set of Bill's offset bushings might be used for the reverse effect.

At this point I'm thinking a fixed spacer that returns the camber to stock or near-stock may be the best approach. Adding adjustability is difficult because it would be hard to tighten the nuts from the back side of the crossmember, but with .567" offset on a 1-1/4" spacer, there is room to add a second set of bolts. Then camber tuning could be done in the conventional (though limited) manner. However, I'm still playing around with the caster idea. Using a thinner spacer at the rear is not the best idea, because of the fixed relationship between the LCA and the king pin. So both should really be the same thickness. However, their offsets do not have to be the same. In other words, maybe add an extra .100" to the front and subtract the same from the rear, in effect moving the kingpin pivot rearward and decreasing caster. I may be way off on the amount of movement needed though. More numbers to calculate. Sum fun. :-(

JB

More info. Essentially, the camber angle changes 1 degree with every 3/16" that the lower pivot is moved horizontally. The stock MG spec is 1 degree but I have found reference to cars coming with camber ranging from 1 degree plus or minus. 1 degree or less is suitable for street driven cars (Fast Cars IFS < 1*) and around 2* for track cars. So I have my range. Now on to Castor.

Stock caster is 7 or 7-1/2* and modern cars use around 2-1/2* Castor shims have been used for various settings and 3-4* seems most satisfactory. It looks like the B&G 3* reduction shims use about a 1/8" spacer judging from photos but I can't count on that for any accuracy. Calculating the angle change from relocating the inner pivots of the LCA shows that for a 3 degree caster change each mounting point has to be moved 3/16", moving the forward one outward and the rear one inward. This retains stock camber. In addition, since this moves the outer end of the LCA rearward  .471" or nearly 1/2", in order to avoid creating a bind on the kingpin pivot the rear mount has to be shorter than the front mount by .340" or about 3/8".

Does anyone have experience with these caster shims? Because the kingpin is 9" between pivots for this to match the photos the subframe mounts would have to be not much more than 3" apart so this does not correlate with what I saw in the photographs. Someone's estimate of the actual thickness of the caster shims would be very helpful here.

Now the question becomes, does this leave enough room to properly attach the front spacer. At the front, subtracting the 3/16" from the 9/16" that we have to relocate for a 1-1/4" spacer leaves 3/8" and means that the bolts are right up against each other. At the rear the reduction in thickness leaves us with.397" but the 3/16" is added to that so it isn't a problem. Going to a 1-1/2" front spacer gives us a half inch between centers for the fasteners and with steel spacers and socket head screws we're in the ballpark.

Another possibility is to use a 3/8" shim on the front only. This would move the front attachment point outwards roughly 3/16" and give about a 1-1/2* caster change and about a 1/2 degree camber increase. it puts a very light bind on the kingpin pivot but will not restrict suspension movement. (At least that sounds right.)

JB

Jim and clan, my post was generic in nature,(SLA with ball joints). It would be refresting for someone to use tranverse monoleaf, front suspension, like Opel and Corvette. I'm using a C5 Vette front suspension clip, in my 69' AMX. I"may" use composite/transverse leaf, with RX 7/Miata/Supra, front suspension.They have nice alum. "A' arms. 2 cents, roverman.

Jim, I've been rolling this one around in the old noggin and think I may have an idea you could use to get your caster you want without losing the camber. Nothing says that the lower A frame brackets have to bolt on the chassis at the same angle they do now so why not build an adaptor bracket that would move the mount inboard and verticle and you could then use shims to move either the front or rear outwards without getting the trunion bolts out of parallel too much or losing the camber adjustment.
Bracket could bolt on the crossmember using the original holes and probably another one to the inside.

Not a bad thought Bill, only problem is that turning the existing pivot shaft 90 degrees means the mounting tabs need 2-3/4" of vertical space. That problem can be solved by either using a different pivot shaft or possibly by drilling a pair of new mounting holes and gutting off the ears. (not necessarily enough beef for that option) Both less than ideal, but feasible. In my case I need the shaft and LCA moved down anyway for steering clearance.

JB

I got some 1-1/2" stock to work with and will cut and drill some new blocks, cutting the rear ones down to 1-1/8". I'll need to recheck my math before I get started but I thought I'd mention one other effect this will have on the suspension, the addition of some anti-dive due to the rear of the LCA being angled upwards. It'll take more calculations to know how much exactly, hopefully enough to be beneficial but not enough to cause other difficulties. I'm pretty sure there is enough give in the trunion cushions to handle it.

JB

Jim, to overcome the problem of the rather wide spaced mounting ears on the stock MGB part, perhaps you could find something from another car that would fit and have the more standard two bolt mount of a lot of American car upper A arms or even machine one from some bar stock.

Results are in. With the new blocks and with the car at about standard ride height the camber is right at zero, depending on where ride height is set. Under compression there is a gradual increase, for a total of maybe an extra half a degree or slightly more positive. But at full droop there is about 2 degrees negative camber. I'm afraid I did not check the specs with the stock mounting so I really can't say how much change there has been. Regardless, the car should be OK with these settings I think, I really won't know until I drive it. Caster is down in the 2-3 degree range, (my suspension angle measuring equipment isn't super accurate) and if this car had standard width tires I'd suspect, based on reports I've heard, that wheel return to center would be weak or non-existent. With the wider tires it will hopefully be acceptable. In any case the steering effort should be markedly reduced.

I'm not sure exactly what the angle of inclination of the kingpin plays in all of this... might be worth investigating. And I have good clearance to the steering rack.

JB

Jim, I think you want negative under compression and positive at droop.Also remember with the car on the ground,is the lower arm parrallel with tie rod to eliminate bump steer.Also you want a minimum of 4 degrees caster for the steering wheel to self center.  Just trying to help.

Hey Jim,

Nice work on the blocks.
What if you pie cut them so that the control arm mounting ears are parallel to the ground?
Slot them with "T" bolts and you can adjust the caster and camber to whatever you want.
You could then make them the same dimension and eliminate the angle induced into your lower arm.
I think that this angle is going to cause trouble with caster gain under compression.
The lower arm will move forward as it goes up and the upper arm won't.
The outside wheel is going to gain caster and the inner will lose some.
This will cause the car to want to turn harder as you turn in.
May be a non issue but I thought that I should bring it up.

Cheers
Fred

Thanks for the comments, some of those things I hadn't considered. Well, first let me say that the blocks haven't got any relation to the air bags, as the only reason they are there in the first place is because of the lowered steering rack. We didn't have to do any of this on the MG-Roadmaster, obviously we didn't lower the rack as far, so ultimately I may revisit the rack mounts and then have another look at things. But the air bags are a drop in swap for the coil springs. I'm going to try to order another set of them tomorrow, in case anyone wants to give them a try. It looks like about 80 psi should be the operating pressure.

At any rate, a lot was learned about the suspension by doing this and if I relocate the rack I'll take measurements with the stock mounts to see what the car normally does. But I do think there is some advantage to be had with decreasing the caster, just maybe not quite so much.

What would be real helpful at this point is some sort of a suspension program that will allow the pivot points and arm lengths and such to be varied and show the results. Preferably a shareware program, and preferably one that will allow the use of kingpins.

JB