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MG MGB GT V8 Factory Originals Technical - Front suspension modifacations for wider wheels and tires?
I have a 73MGB V8 conversion, (215) with 14x7wheels and 215/60x14 VR tires and flared,(Sebring type) fenders.
I want to reduce the cars sensitivity to wheel/tire imbalance/run out,(steering wheel shake).
I have tried different wheels and tires (same size)
The front suspension is stock, except for V8 lower control arm bushings and a 3/4" sway bar.
I am looking for information on which front suspension
modifcations work best on a MGB with wide wheel/tires
for street use.
thanks for your input
those are some quite wide tyres that you have there!
I have several suggestions:
The sheer weight of the wheel combination might be causing scuttle shake-especially with a roadster-ie the chassis legs are beginning to shake with extra forces from the rotating mass. I have found that tyre/wheel weighs a substantial percentage of the unsprung mass-try comparing the weight of a stock wheel tyre assembly in comparison to your current wheels.
Have you selected the wheels so that the King Pin axis passes somewhere through the centre of the tyre contact patch-you could find that excessive positive offset is accentuating any play in the suspension or causing the wishbones to move around.
Check the toe in settings carefully
Check the condition of the front x memeber mounting rubbers.(Change to poly)
The extra rubber will accentuate any slop in the suspension-the clearance in the bronze bushes must be bang on spec and you should be using poly bushes to counteract the extra loads imposed by the grip afforded by the wider rubber.I also like to have at least 0.5 deg negative camber but this will lead to inside edge tyte wear and increased sensitivity to cross winds/ruts.This is paid off by less initial understeer and much better turn in feel on corner entry-it's down to your choice....
Thanks for the info. I did some checking. The 14x7 wheel/tire combo weights 36 pounds. I measured the weight of a 175x14 tire/wire wheel off my 67B at 37 pounds. The spare for the 67B is an original 5.60x14 Dunlop on a wire wheel, weights 31 pounds.
The offset for the 14x7 wheel is 1/4 inch positive. I noticed that stock MGB wheels have a negitive off set to them. Do you know if there are steering knuckles with a higher king pin axis angle aviliable? Or maybe
a machine shop that can machine the king pin bores to an angle of 9 to 10 degrees? I think the factory angle is 8 degrees, though I have not done the math yet to see how much angle change is required.
Current alignment settings are, toe in 1/16 inches in, left camber 0 degrees right camber .5 negitive degrees caster is 7.5 degrees positive.
I have not tried poly x menber bushings yet. Ill check to see whats aviliable in the USA.
I did replace the steering rack, and king pins,bushes, due to wear. I have not done anything with the dampners other than check them for smooth operation through out their travel, while I had the suspension apart.
|Bill,I have a Sebring GT replica that currently runs 15x8 Revolution wheels with 235-60-15 tires.Years ago I ran the full "high performance set-up":negetive camber a-arms,full nylon bushings,1" sway bar in front,3/4"in rear,full race 600# front springs and 1/16 toe-in.Well after every filling was rattled out of my mouth from the stif ride I changed the set-up.The car was undriveable for long distance events(150-175 miles each way) which my club likes to attend.So now I run poly bushes,normal a-arms,stock springs and a 3/4" front sway bar.To tell the truth,the car didn't loose that much in the way of handling-FOR THE STREET,we're not building race cars for the street now.But after all this,I must say I still could not get rid of a little scuttle shake and steering wheel kick-back.This is just a result of the extreme positive off-set of the wide rims.Going to negetive camber a-arms will make the problem worse because the car will be directionally unstable due to up to 2deg of negetive camber the can give.The only solution will be as you suggested and offset bore the king pin angle to correct it and to get a more favorable scrub radius.I am on to my next project with narrower wheels so I will not spend any more time developing the Sebring car,it's handling is tolerable now with good quality tires on it...........................Good luck,dave|
|On my 67 gt (4 cyl) I have 15 x 6 alloy wheels with 195 55 tires. The wheels were designed for a Nissan and have a large negative offset. Rear I had to install a 1/4" wheel spacer to stop inside rubbing on cornering. Tires fit without having to touch the inside of the fender lips and the handling is superb. At full lock the front tires barely touch the stock 9/16" roll bar. my weight measurements of wheels was very close to the numbers quoted above. The 810 Datsun spacesaver spare only weighs 10 lbs. With alloy head, one battery, spacesaver tire and composite rear springs and above described wheels and tires my gt wieghs 2115 lbs with 3 gallons of gas,|
I can't see why you would want a different king pin inclination as there are other ways to sort the camber angles out.
Your data is very interesting if the measurements are accurate. What i tried to emphasise in my last post is that the increased rubber/grip gets to twist the lower wish bone a lot more with braking and cornering loads.
I don't think that even V8 spec lower wish bone bushes as sold by Moss et al will be any thing like good enough to handle your tyres and poly will be the only answer.
0.5 deg camber difference is actually a fair bit-it's the side to side diffence that I find interesting-either the bushes are distorting to give different readings-or the car has been moved during the measurement-OR the x member bushes are compressed differently either side(you have checked the chassis legs just behind the x-member for previous crash damage?). You want to aim to get the negative camber at about 0.5-0.75 deg negative for the street-any more will be too much of a trade off-with greater instability in ruts/crosswinds.
it's possible to change the camber in several ways-the normal way is to change to negative camber wishbone arms-these are longer and have the lower pivot bolt holes placed 6mm out board to give approx 0.75 deg negatve-more or less depending upon the set up. The trouble is they won't cure an unevenly mounted x member due to accident damage or dodgy x-memeber bushes.
Another way that the racers have used is offset nylatron upper trunnion bushes-rotation of the offset hole moves the kingpin inboard or outboard at the top to give the desired camber angle. The trouble with this is that nylatron is a bit of a compromise for the street.It requires lubrication like a bronze bush and transmits vibration.
One way to really fine tune the suspension is to use an adjustable upper suspension link in place of the lever arm damper-I'm using a modified Ron Hopkinson suspension kit-I have modified this so that I can shim the upper wish bone pivots to fine tune the camber settings. i think that it is worth it to chase .5 deg camber as you can actually feel the difference.
The castor angle at 7.5 sounds about bang on spec- the x memeber is symmetrical when on the bench-ie the wish bone pivot axis is in the same plane as the top mounting surface of the x member-its the lower surface of the chassis rails that it tilts the crossmember to give the castor angle. This is why it is important to get the cross member mounted evenly on the chassis rails.
I hope that this helps.
|Bill, I solved a great deal of my wheel vibration problems by machining an aluminum ring to fit between the wheel and the hub. I am running 14 x 6 minilite replicas with 195/65 Yokohama rubber. I had the front hubs machined to provide a "surface" for the ring and had the ring made to fit into the wheel rim. |
The rear axle is a Ford 9" and the Ford axles already had a machined surface. The rims are now located concentrically instead of being located on the wheel studs. This reallly helped my car a lot!
|John is obviously one hell of a lot more knowledgable about this matter than I am, but FWIW I fully agree with John about a bit of negative camber. I acquired a V8gt last winter. When I took it to the alignment shop, the fellow noticed there was a fair amount of negative camber -- not easily adjustable -- and warned me that it was outside spec and that the tires might wear prematurely. The printout they gave me said that each side had -0.5 degrees. Why this is is another story ... I have no idea ... the car's not lowered and I think they are just the standard lower A-arm pieces.|
It's too early to tell whether the tires will be ruined (I haven't noticed anything yet) but John is exactly right that the car dives much more smoothly into corners than my other bgt 4-pot with no + or - camber, which just as he says understeers into corners. Interestingly, the v8 just has the wimpy stock B sway bar and a-arm bushings, yet it handles much better and more neutrally -- especially on corner entry -- than my other car which has upgraded gear (and which other people think handles very well).
So I would say based on this one non-statistically proven study that a bit of negative camber seems to be a simply fantastic thing.
|Harry M. Peeters|
Thanks for the suggestions! Lots of good ideas! I connverted to five lug hubs (MGC),so I could use the style of wheels that liked. It is worth it for me to spend some time and money on the front end!
It looks like poly and adjustability is the way to go. I am also in the process of making some gauging tools to find out where the king pin axis point is in relation to the center of my tire contact patch.
John, the Ron Hopkinson kit you mentioned. Is it the type that uses Bilstein telescopic shocks and a fabricated upper wishbone? Does anyone know if there is a supplier in the USA for them?
Dave, I did reduce steering wheel kick back on my car by installing a steering dampner off of a Pontiac Feiro steering rack. It has the proper amount of stroke for a MGB rack. If want to try it let me know and I can give you some info on how it was done!
The Roadster Factory, in Armagh, Pa, lists the Ron Hopkinson kit, both the front tube shock conversion and the sway bars. I believe their net address is "www.theroadsterfactory.com". If not, you can find them by querying on the name.
Yes -it is a very early Ron Hopkinson kit. I have modified the upper wish bone pivot so that the axis of the mounting bolts is towards the engine rather than downwards-this means that you can use shims to fine tune the camber.I ahve also used poly bushes in the upper arms as the wishbone eyes are indentical to the lower arms.
I personally have tried and removed the Ron Hopkinson handling kit-ie the anti roll bars.7/8"F-5/8"R I consider these to be too stiff for the street. They are fine if all you do is drive on smooth race tracks with slick tyres. You will find that the car becomes very harsh-manhole covers throw the car off line and it becomes very sudden on the limit esp in the wet. People who run 1" bars on a street car are mad-the stiffness goes up four times for every increment of thickness-you will just end up twisting the chassis legs esp if you have weakened the inner wings by fitting RV8 style headers.
I reckon that the way forward is max 600lb springs on the front-say 520lbs with a 3/4 a/r bar in poly blocks. I think that the real way forward is variable rate springs-either taper wire wound to give a softer rate for samll street undulations yet greater roll resistance at larger displacements. I am going to try this out-possibly by using soft variable rate bump stops with near std spring rates.
You can see my modified setup on the v8 conversions site: www.mgcars.org.uk/v8_conversions
|Bill,how did you mount the Fiero steering damper to the MGB rack?The one pivot mounts to the rack,but on the Fiero the other end mounts to the actual rack gear on the chassis side of it's inner tie rod joint.What this does is dampen the sliding rack itself.If the damper was attached to the tie rod or the outer tie rod end,then as the suspension moved up of down while driving bump steer would result.Due to the resistance in the damper,it would push or pull the outer tie rod in or out as the suspension cycled.I have a couple Fieros at my shop that I keep for parts(seats,steering racks,etc..)and had that idea for a while,but without swapping in the complete Fiero rack I don't see how I could adapt the damper to the MGB rack easily or cost-effectivly from a machining standpoint.Keep in touch..........Dave D.|
To mount the moveable end of the dampner to the rack. I had a local machine shop make an adapter sleeve 2 inches outside diameter, 1 7/8 in long, an inside diameter of 1 3/8 in for a depth of 7/8 of an inch. From there the remaining inside diameter tapers out to 1 7/8 inches inside diameter at one end of the adapter sleeve.
I drilled and taped a 1.25mm x 10mm hole 1/2 inch from the outer edge of the sleeve.( The outer edge is the end with the tapered inside diameter). this threaded 10mm hole serves as mounting point for the 10mm x 2 7/8 inch mounting stud. This mounting stud is where you will attach the dampner to. A 1.25 X 10MM X 3 INCH premium grade bolt with the bolt head cut off and threaded on both ends works well for this. I also welded the mounting stud to the adapter sleeve from the inside of the sleeve, just to make sure things stay where their supose to!
The adapter sleeve is then slid over the right inner tie rod housing socket with the tapered inner end facing outwardes This is very important as it allows the inner tie rod to have full movement at maximun suspension travels! The adpater sleeve is secured to the inner tie rod housing by 2 allen set screws placed 90 degrees apart. I used an existing hole in the tie rod housing as an anchor point for one set screw and drilled a 3/16 3/32 deep hole for the other set screw with blue thread locker on the set screws. End page 1
|Dave Page 2|
This is taking up more room than I thought!
The adapter sleeve then becomes part of the rack gear and is not affected by suspension movement. No bump steer! I Cut a 1/8 inch hole between the 2nd and 3rd outer bellows ribs. stretch the bellows boot over thetie rod/stud assembly. The boot is sealed to the mounting stud by a 1/2 inch long sleeve over the mounting stud. this sleeve is concaved on one end to match the 2 inch dia. of the adapter. This will povide a postive seal to keep water and dirt out of the rack. I used a nut on each side of the dampner where it attaches to the stud.
The fixed end of the dampner is attached to the x member via a fabricated mounting bracket. I mounted the dampner as close to horizontal as possable. Side to side mounting is determind by centering the travel of rack and dampner. Dampner stroke is 3/8 inches longer than the rack stroke so dampner bottoming is not a problem. Tie rod housing dia. varys you may want to check yours with a micrometer. The machine shop charged me about 20$ to make the sleeve. End page 2
Dang out of room again!
This sounds more complex than it is! The dampner is very effective at reducing steering road shock! The adapter sleeve can be removed if rack service is required. The whole set up can be installed while the
rack is still on the car.
I did take some photos of the finished installation and I will try to scan/email them to you.
Hope this helps!
email@example.com ( new e mail address)
This thread was discussed between 19/10/2000 and 31/10/2000
This thread is from the archive. The Live MG MGB GT V8 Factory Originals Technical BBS is active now.