Rally Car Rear Axles

[Hans]

This morning I was at Scanwest Autosport in Seattle. (Thanks to Rob Walden and everyone at Scanwest for supporting our Alcan 5000 efforts!)

Anyway, we were talking about his old rally cars and he mentioned how they strengthened c900 rear axles:

--Cut off the spindles.
--Drill holes along the axle, maybe 5/8-inch dia. (his recollection; been awhile).
--Insert a steel tube whose OD is the same as the ID of the axle into the axle.
--Rosette welds @ the holes.
--With whatever means necessary (he didn’t have details, as another shop did the fab work) modify the spindles so they have -1.5* camber and re-weld them to the axle.
--Weld gussets at the axle/spindle.

Most of this work was done by a frame/alignment shop. I don't know how they arrived at the -1.5* camber value. And, if it's not obvious, this is all second-hand, so yer on yer own for some details should you decide to try it out.

[Geoff]

cool
So drilling the holes was to allow the new tube to be welded to the existing one? Any idea on the hole pattern or spacing? It can probably be estimated...

Some modified rear axle shots from Hartman's car:
http://saabrally.com/member-uploads/geoff/June%208th/June%209th/Kinnekulle/IMG_0984.JPG
http://saabrally.com/member-uploads/geoff/June%208th/June%209th/Kinnekulle/IMG_0985.JPG

Note the extra layer around the bushing holder, the re-enforcement along the panhard rod mount, an upright brace opposite the panhard rod (under the exhaust pipe), and a gusset between the axle and the spindle

[Hans]

cool
So drilling the holes was to allow the new tube to be welded to the existing one?

Yes.

Any idea on the hole pattern or spacing? It can probably be estimated...

No idea; I assume every few inches?? There's probably a spec in an engineering book somewhere. I have to go back to SW this afternoon for some parts; I'll ask Rob if he's around then.

[Luke]

If going to all that trouble of cutting the castings off the stock tube, why bother to reuse the stock tube at all? Just get a peice of thick walled chrome-moly tube and use that.

[Rallyho]

Well, something has to be done to make the spindle-to-axle junction stronger...a stiffer pipe <heh,heh, heh> won't do it all. At least that's where all my Joey Chitwood specials have bent back in the rear axle area.

[Hans]

If going to all that trouble of cutting the castings off the stock tube, why bother to reuse the stock tube at all? Just get a peice of thick walled chrome-moly tube and use that.

Dunno. Availability? Cost? Not having done anything remotely similar to this, I can only guess that it's cheaper/easier/faster to cut/drill/weld in a second tube than it is to fab axles from scratch. Or maybe rules? I'm unfamiliar w/rally rules, especially ProRally G2 circa 1995, but I do know that in motosports in general, it's not uncommon to allow modification to stock components while prohibiting their outright replacement.

I'll ask...

[Luke]

Maybe if they were running production, but I think they ran group 2 where they definatly could have used a custom rear axle without issue. I'll probably add a taco gusset at the ends like Dennis Hartmans car.

[theoneGroo]

while it makes sense to re-inforce the rear axle beam i wonder what the best solution would be... my experience has been 2 kinds of failures from 2 very different forces... a side load from a hard lateral shot to a wheel (curb, rock etc...) and as mike and i discovered inadequate bump stops (sound familiar to lukes front end issues?) that causes the axle beam to bend right where the bump stops hit from a big jump/ hard landing situation (tall pines 2003) would a longer more progressive bump stop in the rear along with properly designed re-inforcement (sleeved axle) help out?

[Luke]

a side load from a hard lateral shot to a wheel (curb, rock etc...)

Thats whats done mine in before too, R3- /CR into ditch, woops!

[Crazyswede]

Luke and I discussed the whole monster rear axle idea at one point and we wondered what the danger would be. As in right now when you hit something hard you bend the rear axle. Well if the rear axle is indestructible do you run the risk of tearing the whole axle out of the car or severely damaging the axle mounts thus rendering your car undriveable? The car was a little squirrelly with axle as it was but it was still moving since the axle gave in.

[monte 283]

I had a similar discussion with Sean Tennis and Rob years ago before they built the NG900. They cut the axles and welded because they had a readily available rear axle supply, it was easy to do (his words, not mine) and above all cheap. He also said that if I didn't want to go to the trouble of cutting the ends off just welding a length of angle (with the v-facing down) would work better then stock. I never tried it since the stock one held up enough for chase car duties.

[Geoff]

Thats what Tom Bier did when he built my car. Angle steel welded as a forward facing "V". It helps deflect rocks

[ric]

Does anyone know if the rear axle from saab 9000 is stronger, it´s a heavier car. And would it be possible to make it fit (of course it is but I mean easy)?
It looks stronger, it´s also a little wider and it looks quite similar to the 900. Just thinking

[Sean Tennis]

Hans, how was Alcan?

Did Rob tell you that the axle they reinforced broke, yep right next to the welds... The problem they had with the axle/s bending was due to driving off course, ask Nate Tennis how many he's bent in all his rallies, 1 I know of (hit something-yep even Nate hit something).

I've never replaced the (82-900) rear axle on my 99 rally car (1994 G2 National Championship) since I installed it in 1993, has many, many rallies on it and a very nasty roll.

Axle tubing material tempering would be better then the added tube. I remember when they did this (stuffed the rear axle), we, the non Sam/Rob team, thought they were nuts, keeping the car on the stage seemed to be a better idea...

Unless, as Mike(?) said the axle takes a dead on side blow it won't give trouble and is good enough, better off to leave it alone. The other thing is the axles will bend upward at the lower control arm mount outward, all this does is give a little negative camber a degree or 2, which is a good thing, this happends on street cars sometimes too even on the factory rally car rear axles-I should post a photo so you could see...

IMHO,
Sean Tennis

[DeLorean]

Luke and I discussed the whole monster rear axle idea at one point and we wondered what the danger would be. As in right now when you hit something hard you bend the rear axle. Well if the rear axle is indestructible do you run the risk of tearing the whole axle out of the car or severely damaging the axle mounts thus rendering your car undriveable? The car was a little squirrelly with axle as it was but it was still moving since the axle gave in.

I have discussed similar with an old time saab racer that lives in palmerton, PA who insisted the same thing about the front control arms. he is a big fan of "just strong enough" so that the items that are attached to the car will bend and break before the car itself does. I guess there is logic to that?