I broke a subframe mount on my BMW and want some advice.



It’s not an uncommon issue and can be welded but the only guy that I know and trust that can weld it is packed up until spring with a house he is trying to sell. Should I look around and try and find someone else? A couple Of people have made brackets either by welding them, machining or hand forming(sawing and filing).

Here are some of the examples:







This is a lower mount that a guy made from a solid block and bolted on:





Should I use steel or aluminum? Find someone else to weld it? What size plots should I use if I make a bracket 6,8 or 10mm? Any advice or ideas are welcomed. I want to get it back together so I don’t have parts spread all over the shop.

Thanks
Ed

To me it looks like cast aluminum. I would fab a steel bracket and through bolt it on. Your equivalent thickness to 6mm,8mm and 10 mm would be 1/4", 5/16" and 3/8" thick plate. I would say that area is taking some stress and you could weld it back together but I don't know if it would be as strong as original..... Edited to add. I think you were asking about bolt size. A couple 10 mm bolts would be adequate. If it's a common issue maybe check with a BMW dealership. That may have a kit to repair the mount.

why not ask the person that you trust if he could recomend someone else who he would trust to do the work . good luck in getting repairs done in a timely manner.....lost my bike aug2016 to major flood event (2005 honda vtx 1800 f2) ..here in south Louisiana was able to ride year round just about 12 months out the year... ride safe ..... .......................drill sgt.

Hello Ed,

I have several BMW motorcycles, the newest being a ‘94 1100RS.

The casting is most likely aluminum and could be welded. It might even come out stronger than originally made. Having it fixed and look good/original would be the real challenge.

A BMW Shop could tell you, as could someone here on the web what the metal is. If it isn’t aluminum my next guess would be magnesium. Odds are very high that it’s aluminum.

My inclination (having done metalwork, welding, casting, machining, etc) would be something like the added, bolted bracket. Maybe even with a hidden reinforcing piece of steel in behind the aluminum. The “solid block” model looks better than the first all steel welded one. An all steel (stainless?) that looks like the solid block would be nice too.

What is going to keep this repair from breaking again? Have you figured out why it broke? Understanding why it broke would be good to share with the person doing the repair. He/she can then try to fix it to be as strong as needed.

Best would be some aluminum or stainless bent sheet brackets bolted above and below a nicely TIG welded cast boss.

I have no experience with BMW motorcycles. But from working in Harley Davidson dealerships if a bike had a damaged frame and came in for repairs it was either replace the frame or in the case of an insurance claim total loss the bike. Too much liability anymore. You can try a Beemer dealer but don't get disappointed if they decline to help.

If it was my bike I would find someone who does race car construction or aircraft repair for their opinions on how to proceed.

I'm also not too wild about the steel bracket in the first and second photos. I like the aluminum brackets much better, electrolysis between dissimilar metals is a concern of mine. I'm no engineer, just a old hot rodder/backyard hack.

I race motorcycles. We break stuff like this tab all the time. We weld them all the time. Its an easy area to weld and someone competent can do it in a mostly good looking way. But that solid block fix has a very nice aesthetic to it.

What BMW model is it?

04. R1100S

"Not an uncommon problem". Ok, maybe now would be the time to investigate what causes this and address it?

Usually when cast Aluminum breaks it's because the casting was porous when it was cast (and weak). You could weld it, an aircraft or ship certified welder could weld it. If your guy is busy, that's who I'd look for.

The crankcase of the BMW boxer engine is cast aluminum, but I don't know exctly which alloy. As per the interwebs this is not an uncommon problem, and it appears you can successfully weld directly to the case. As to cause I'm guessing that BMW under designed the stress point, which is why it's recommended that you gusset the repaierd part. Merry christmas!

quote:

Originally posted by Oldrider:
"Not an uncommon problem". Ok, maybe now would be the time to investigate what causes this and address it?

My guess would be that something is flexing side to side in addition to normal vibration, and a vibration dampening bushing at the bolt should have been designed into the system.
I like the craftsmanship on the machined replacement part even though the engineering of the bolted attachment leaves something to be desired. The welding on those bolt-on steel replacement parts, not so much.

Is there enough thread there after drilling and tapping?

quote:

Originally posted by Oldrider:
"Not an uncommon problem". Ok, maybe now would be the time to investigate what causes this and address it?

The subframe is made of a really stout tube steel so even a tip over has broken these tabs. Just not well engineered on BMW’s part.

Valpo, I lived in Crown Point until 10 years ago and became friends with a group that would meet every week at a hangar\shop that one of them had out at the Valpo airport. Super group of guys that would have both the tools and ability to assist. Guy I knew was Nick Saguros (sp) if you happen to know him. If you don't and are interested I think I have his card somewhere. Just let me know.
They are the type of guys that would be happy to help out.

quote:

Originally posted by Powers77:
Valpo, I lived in Crown Point until 10 years ago and became friends with a group that would meet every week at a hangar\shop that one of them had out at the Valpo airport. Super group of guys that would have both the tools and ability to assist. Guy I knew was Nick Saguros (sp) if you happen to know him. If you don't and are interested I think I have his card somewhere. Just let me know.
They are the type of guys that would be happy to help out.

Nick is the guy I was talking about! He built a Cobra frame using blue prints and it was a work of art. Nick is swamped with a house he is trying to get ready to sell and trips to Toronto to see his Grandkids he can’t get to it until the spring. Nick takes his time doing everything but it is done to perfection the first time!

quote:

Originally posted by crue-dell:
To me it looks like cast aluminum. I would fab a steel bracket and through bolt it on. Your equivalent thickness to 6mm,8mm and 10 mm would be 1/4", 5/16" and 3/8" thick plate. I would say that area is taking some stress and you could weld it back together but I don't know if it would be as strong as original..... Edited to add. I think you were asking about bolt size. A couple 10 mm bolts would be adequate. If it's a common issue maybe check with a BMW dealership. That may have a kit to repair the mount.

I agree with this completely. Looks like cast aluminum to me too and the repair won’t be as strong, the steel mount bolted in will be much better, not to mention easier and probably cheaper.

quote:

Originally posted by Valpo Fz:

quote:

Originally posted by Powers77:
Valpo, I lived in Crown Point until 10 years ago and became friends with a group that would meet every week at a hangar\shop that one of them had out at the Valpo airport. Super group of guys that would have both the tools and ability to assist. Guy I knew was Nick Saguros (sp) if you happen to know him. If you don't and are interested I think I have his card somewhere. Just let me know.
They are the type of guys that would be happy to help out.

Nick is the guy I was talking about! He built a Cobra frame using blue prints and it was a work of art. Nick is swamped with a house he is trying to get ready to sell and trips to Toronto to see his Grandkids he can’t get to it until the spring. Nick takes his time doing everything but it is done to perfection the first time!

Very cool. Glad to hear he's doing well. That was a really cool group of guys that would meet out at the airport. Some wickedly talented guys. Miss those get togethers.

It looks like a super weak design that would break again. I would go to a local machine shop and have them fab up a much stronger bracket designed to replace the one that is cast into the original part.

I would then grind the original off and have the new piece welded on. There are plenty of radiator shops who have competent aluminum welders. It will be a much easier wild than trying to repair what is there.