Montesa Frame Material

[montynut]

I hope someone can make a suggestion to assist.

What type of material should I use to repair a VR250 Monty frame. It has modified rear shock mounts and requires a section of tubing to be replaced. Yes its Nathan's bike 

The frame is 'Spanish' Chromoly of early '70's vintage so I assume it is not the top shelf stuff used on British unobtainium

High tensile steel, mild steel,  I hope someone may have some experience without sending samples off for analysis which would cost more than the bike

[hitatreeonce]

I'm guessing and possibly I'm wrong but knowing the spanish its just plain old mild steel.

[Slakewell]

I have a couple of old chrome molly frames I cut pieces from for jobs like this. If you take a photo I can give you advice.

[Captain Bilko]

Geezez Greg! Just get stuck in with the arc welder like i do.

You'll be posting up five hundred photos of your recent fly-fishing expedition next (or you train-spotting weekend/butterfly collection/trip to Science Works....)

[k2000x]

im in need of doing this to i got chrome molly tube ordered from local steel joint but havent done much else..you might want to check out dc plastics for there new black vr fenders thanks to john at rm montesa

[JC]

The magazines of the day said the front half of the VR frame (engine cradle, backbone etc) was chro-mo but the subframe was mild steel. Don't know if its true tho.

[montynut]

Geezez Greg! Just get stuck in with the arc welder like i do.

You'll be posting up five hundred photos of your recent fly-fishing expedition next (or you train-spotting weekend/butterfly collection/trip to Science Works....)

Geeze thanks Ross, I have noted your arc welding ability truely ummm amazing. The repair on your 360 looked like it was done by Foghorn Leghorn himself using very damp ‘50s welding rod executed without a welding helmet . The lap times dropped by 2 seconds once the daggs where removed. It was not only the weight reduction but there was an aerodynamic issue as well  .

You know very well my train spotting expedition is later this month and I visited the science display while in Canberra the other day . As far as butterflies, I hate the little buggers as they make a God awful mess on the windscreen.

You need to get back over to the Maico area you have ’em rolling in the isles .

You are supposed to be out in the shed searching for the VR250 info for me aren’t you

[montynut]

The magazines of the day said the front half of the VR frame (engine cradle, backbone etc) was chro-mo but the subframe was mild steel. Don't know if its true tho.

Interesting is there any way of easily testing the material? grinding, cutting, magnet etc

Ross ........ leave the magnet issue OK

[GMC]

The engine cradle is an important area as it’s the main part of the frame that holds everything together from the steering tube to the swingarm pivot..
The sub-frame up to the shock mounts is important but not as critical as the engine cradle.
Everything else is superficial.

I have used both 4130 Cro-Mo and ERW tube to repair sub frames and shock mounts, depending on the job and what the customer is prepared to pay.
4130 is very strong but suffers from brittleness around the weld area so if your not going to have the frame heat treated you may as well repair with ERW, but don’t use any thinner than 1.5mm wall
I have some 4130 if you want to go that way.

[GMC]

Interesting is there any way of easily testing the material? grinding, cutting, magnet etc

Not really.
The different steels will make a different 'ring' when tapped but you would need samples of each side by side to be able to tell.
4130 will have a higher pitch.
But old tubes with rust layers on the inside will sound different also.
Its doubtful that any of those early manufacturers used aircraft quality tube

Bottom line, don't sweat the small stuff as it doesn't matter that much, just find yoruself  a good tradesman with a TIG and all will be good.

[JC]

Interesting is there any way of easily testing the material? grinding, cutting, magnet etc

I have been told by a boilermaker on here (specifically wrt VR frame) that sparks given off from grinding Cro-mo are diff colour to mild steel, but again, I don't know if its true

GMC would know I would think

[montynut]

Thanks GMC
The area to be repaired is the tube running from the swingarm pivot to the top shock mount and also the outer plate of the top shock mount. I would guess this is the highest stressed area of the rear sub frame as it would be under consideable tension load when the rear wheel hits a bump. I will try to get some detailed photos.

If I was in Vic I would probably take it to you any suggestions for NSW. The Hunter Valley area would be ideal.



Ross restrain yourself

[GMC]

Can’t say I ever noticed so I just went out to the workshop and hit the linisher with some 4130 and mild steel and couldn’t see any real difference.
If you hit the linisher with a piece of Titanium you get all sorts of bright glittery sparks.
I believe they use Ti powder in fireworks for this effect.

[firko]

I'm with Geoff....I wouldn't sweat it too much. The steel used in Spanish bikes of the era comes from the same common market foundries that produced the steel for Italian cars so the quality is pretty poor when compared to off the shelf specialised 4130 chro-mo. In the end just use whatever you can find and it'll invariably work. I've repaired Maico frames using mild steel in the past and they're still giving good service.

[GMC]

Thanks GMC
The area to be repaired is the tube running from the swingarm pivot to the top shock mount and also the outer plate of the top shock mount. I would guess this is the highest stressed area of the rear sub frame as it would be under consideable tension load when the rear wheel hits a bump.

Mild steel will be fine for that area, it is a stressed area but not as critical as a swingarm. I wouldn’t use mild steel for a swingarm repair.

Cr-mo would make it a better job but then ideally you need to have the whole frame stripped and ‘normalised’ as the brittle weld area won’t be any stronger than the mild steel.

I have done a few repairs over the years in this area with mild steel and haven’t heard of any failures.