|The MGA With An Attitude
CAMBER CHANGE for the Rear Wheels - RA-301
At 03:29 AM 1/21/05 -0700, David Gills wrote:
"Year: 61 -- Model: ELVA Courier -- Is there an acceptable method for adding camber to the rear wheels by cold setting or heating and bending the axle tubes? I would like 1 degree negitive for autocross for the rear wheels."
Elva Courier, huh? Are you building a dedicated race car? This depends a lot on how often you are willing to replace the wheel bearings, half shafts and differential sun gears. It is sometimes done for a race car, but is not a good idea for a regularly driven street machine.
The wheel hub and half shaft must remain forever straight so the wheel will spin true and not wobble. So inducing negative camber for a rear wheel (while keeping the tire in contact with the ground) will put the inner end of the half shaft (and the differential sun gears) closer to the ground. The differential assembly must remain completely in tact with no modifications and will remain level. So this introduces an angular misalignment between the half shaft and the differential sun gear. The spline may tolerate up to about 1/2 degree misalignment and still run okay for a while, generally causing some accelerated wear in the splines.
If you introduce up to 1 degree misalignment at the splines it will cause greatly accellerated wear on the splines. It could also eventually lead to a fracture failure cause by fatigue stress due to slight bending of the halfshaft, with the fracture of the halfshaft occurring at the outer end of the sun gear, like this:
To reduce the probability of this type of fracture, you need to machine a bit of curvature on the male spline, on both flanks and on the top of each rib, introducing about 1 degree of curvature from the center toward both ends of the spline contact area. In simple terms, it means making a spherical male spline. A matching spherical female spline would be impossible for assembly without excess clearance, so you end up running the spherical male spline inside of the normal straight female spline. This leaves a very short line contact between the splines which also promotes accellerated wear, but at least it relieves the bending force on the halfshaft.
With the increased wear it becomes necessary to do occasional replacement of the splined halfshafts (and maybe also the sun gears). Halfshafts with special machined splines would be quite expensive. If you use halfshafts with standard splines and 1 degree misalignment it would be good to keep spare halfshafts on hand in the event of breakage. Long term service records might eventually yield sufficient data to predict approximate time of failure so you might change the parts before they fail. For a no holds barred race car you might consider changing the halfshaft once per year, or perhaps more often if you actually encounter a fracture failure.
The way to create this geometry in the rear axle is to heat the axle housing tube just outboard of the center housing and bend the ends of the housing upward slightly. Distance of upward displacement is the tangent of the desired camber angle times the distance between the sun gear and the wheel bearing. As distance between bearings is about 18 inches, the upward disposition of the wheel bearing would need to be 5/16 inch for each degree of camber (5/32 inch for 1/2 degree)
Since the bending point of the housing is not exactly in line with the sun gear, this introduces a slight angular misalignment of the wheel bearing relative to the axis of the halfshaft. The angular displacement of the bearing mount will be greater than the induced angle of the halfshaft. If you do not correct this misalignment it will lead to greatly accellerated wear of the wheel bearings.
If you were very creative you might heat and bend the housing slightly downward near the outer end to properly realign the wheel bearing to the now angled halfshaft, but it would be very difficult to achieve proper bearing alignment with heating and bending. It would be easier to re-machine the seat for the wheel bearing on the axle housing at the desired angle after bending the housing near the center. The critical surface is the inside shoulder where the inner face of the bearing will seat. Follow this by touching off a very small amount of the OD of the housing where the bearing bore will seat. This will result in a slightly undersize OD on the housing, so you then need to set the wheel bearing in place with bearing set adhesive ro fill the gap and keep the bearing inner race snug on the housing. In theory you should also re-machine the large thread for the bearing mount octagonal nut, but in practice the misalignment here will be minimal, and sufficient torque on the nut will pull the bearing tight against the inner shoulder for proper bearing alignment.
This intentional deformation of the axle housing works okay on the MGA and early MGB banjo type rear axle, as the bearing mounts for the differential are isolated in the separate removable differential carrier. For the later MGB tube type axle the bending of the housing must occur completely outside of the area of the differential bearing mounts. If the differential bearing mounts are misaligned in this process, these bearing mounts must also be re-machined for proper alignment of the differential bearings.
Question then is, are you willing to go to all this expense and bother and future maintenance problems for the advantage of rear wheel camber? This is not a modification to take lightly in a daily driver street car because of the long term maintenance problems.
If you autocross with SCCA, this modification would exclude you from Stock or Street Prepared class, putting your car in Prepared class along with the vintage racing cars (or maybe even Modified class). If you autocross with a local British car club where they run mostly stock or street prepared type cars, they just might not let your "prepared race car" run with the rest of the gang. If you intend to race with a vintage racing group you should read the rule book first to see in which racing class this might me allowed. Otherwise you have to test your morals and decide how long you might keep the modification secret. Do you think you can you get away with 1 degree camber without anyone noticing the odd tilt on the rear wheels?