Commando Frame Straigtening

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lcrken

lcrken

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I finally got started putting my street MK3 back together, and the first order of business was straightening the frame. I rode it for a number of years with the bent frame, and to be honest, it seemed to work just fine, even when flogged hard in the twisties. But I could see that one of the front downtubes was significantly bent, so I thought I'd fix it while I had the bike apart. Aside from the bent front down tube, the front and rear isolastic mounting holes were about 1/4" too far apart and not on the same center line, and the steering head was twisted sideways by almost 4 degrees. I still had my home-made tooling, crude though it may be, from back when I was playing with steeper rakes on the race bikes, probably 25 - 30 years ago. My technique is pretty brute force. I clamp the frame to the table with the center line a fixed distance above the table, then jack and clamp the frame to put force in the directions needed, and then apply heat. I measure dimensions and heights as I go along, and when they are right, I let it cool in position.

So far, I've just used it to straighten this frame, but it will also be convenient for some other small mods I plan for the frame. The steel table in the shop is one I got for free 25 years ago, just for hauling it away, and it has been really useful for all sorts of fabrication stuff.

This method is not quite as sophisticated as making a purpose-build Commando frame jig, but it has worked pretty well for me so far.

Commando Frame Straigtening


Commando Frame Straigtening


Commando Frame Straigtening


Ken
 
One of the things my friend and I have done when making a frame is to put a bar through the pivot mount, equalise it and measure from it's ends to the ends of the bar through the steering head. How do you do that on your steel table ?
 
The long bolt through the rear iso mount holes (nearest thing to a "pivot mount" on a Commando frame) goes through the steel table and pulls the frame tight against a machined spacer on the table. That locates the frame with the rear iso mount axis perpendicular to the table. I use a fixture to locate the front and rear iso mount brackets in the proper relationship. I measure the appropriate frame tubes to make sure they are parallel to the table, and block and clamp them in place. I also check that the spine tube is parallel to the table. That lets me be sure that the central plane of the frame is parallel to the table. I measure (with the height gauge) the height at the center of the rear iso mounts, and take that as my horizontal datum. I fit a ground bar through machined adapters in the steering head, and measure to be sure the height at the center line of the steering head is correct, and that it is parallel to the table. Of course it never is at the start. I use adjustable supports and clamps (not all shown in the pictures) to put forces on the steering head to bring it into line, and then heat the appropriate frame tubes cherry red to let the frame settle into the proper geometry. That usually involves adjusting some of the clamps while heating the tubes to get everything right. In the case of this frame, one of the down tubes was bent, and I made a simple jig to straighten it before getting on with the steering head. The long threaded rod to the front down tubes applies pressure to bring the front iso mount to the proper distance from the rear iso mount, again while applying heat to the appropriate tube locations.

The whole point of the exercise is get the centers of all three iso mounts (top, front, and rear) in the same plane as the steering head, with the iso mount axes perpendicular to the steering head axis, and at the proper locations. If I get that right, and set up the iso mounts properly, the front and rear axles and the swingarm pivot axis will all be parallel, and the bike will hopefully go down the road in a straight line. The dimensions of other frame parts, like the rear hoop, are not as critical, but I get them as close to correct as I can, just for cosmetic reasons.

When I have the steering head and iso mount dimensions correct, I check the other dimensions of the frame against the factory drawings in the service manual, and make any other adjustments needed. In this case, I checked the steering head angle before starting, and it was right at the original 27 degrees, so I didn't have to worry about fixing that. The changes I've made to straighten the frame are not going to have a noticeable impact on the rake. I've considered steepening the rake a bit, to at least get back to the original trail dimension without having to bore the new triple clamps at an angle, as Norton did on the 850 frames. I've used steering head angles in the past on my race Commandos from the stock 26 degrees down to 24 degrees, with fork tube offsets as needed to keep a reasonable trail, and based on that experience,I think the stock 26 degree rake is most appropriate for a street bike like this one will be. I can probably get that just by raising the rear and dropping the front a bit, but I still haven't decided exactly how I want to go with wheel and tire sizes, so that's still up in the air a bit. I'd really prefer to avoid changing the rake, since that involves cutting and welding both the down tubes and the spine, and that's a bit of a pain.

I hope this explanation, along with the pics, is clear enough. Some quality drawings would be even better, but that's more work than I'm willing to do.

I will be making some other mods to the frame to accommodate the Ohlins rear shocks, and maybe fitting a horizontal brace in the front down tubes, but nothing in the way of drastic butchery. Monoshock and oil-in-frame conversions are pretty interesting, but they've already been done, and I need to get on with this project so I can be riding it at next year's rally.

Ken
 
lcrken said:
...............The whole point of the exercise is get the centers of all three iso mounts (top, front, and rear) in the same plane as the steering head, with the iso mount axes perpendicular to the steering head axis, and at the proper locations. ..............I hope this explanation, along with the pics, is clear enough. ............. and I need to get on with this project so I can be riding it at next year's rally......Ken
Click to expand...
Seems like the best use for that steel table :D good luck with your project and thx for posting, Will you be using stock triple clamps for this bike , or something like NYC Norton, or cNw's ?
 
cjandme said:
lcrken said:
...............The whole point of the exercise is get the centers of all three iso mounts (top, front, and rear) in the same plane as the steering head, with the iso mount axes perpendicular to the steering head axis, and at the proper locations. ..............I hope this explanation, along with the pics, is clear enough. ............. and I need to get on with this project so I can be riding it at next year's rally......Ken
Click to expand...
Seems like the best use for that steel table :D good luck with your project and thx for posting, Will you be using stock triple clamps for this bike , or something like NYC Norton, or cNw's ?
Click to expand...

I think I'll be making my own triple clamps. I'm using a set of conventional Ohlins forks that I got from Kenny Dreer, the same ones he used on his prototypes. I sourced a set of Yamaha triples that fit the tube diameter, but they are a bit wider than I need, and have too little offset. I was thinking of pulling the rake back to 24.5 degrees or so to get the right trail with them, but I don't think I really want to do that with this bike. At the moment, the plan is to make some triples with the right offset for a 26 degree rake, and just wide enough to fit the wheel and brake combo I end up with. Of course all these plans are subject to change. Sometimes in the evening I sit in front of the shop with the doors open looking at the bikes, and have brilliant flashes of illumination on which way the builds should go. Of course there are usually some beers involved, so some of those ideas don't survive after sober consideration the next day. But then, some do.

Ken
 
I note that you have heated the main frame tube behind the steering head. I found my 1970 frame was bent having ridden it like that for about 35 years. Never could get the front iso bolt through without a fight. I went down the line of adjusting the iso bolt holes along the lines of "the worlds straightest Commando" That didn't bring the iso lugs centred with an extended steering head axis. After some advice from a respected ex BSA frame builder, Martin Russel, he said that the steering headstock would not have moved but that the frame (down tube loops) would have parallelogrammed to one side. This means that the front and rear iso plates/lugs will not only be across to one side of the notional centreline, but because of the radius involved around the bend points, probably just below the headstock and the rear damper mounts, then one side will be higher than the other. This will effectively twist the whole gearbox cradle, swinging arm etc out of the vertical. I haven't got round to going any further with this frame but my plan is to anchor a scaffolding pole into a serious block of concrete and using a ratchet strop bend it all back into line. I may cut the cross tube to allow me to deal with one side of the frame at a time then re weld it.
 
Good comment, gripper. In my case, with the iso mount axes vertical, the center of the spine and the center of the headstock are at the same height above the table as the centers between the iso mounting plates. That indicates that I don't have the parallelogram effect you described. There was some of that effect at the front downtubes, but after straightening that has disappeared, so now all the center lines of iso mounts and headstock axis are in the same plane, and the iso mount axes are perpendicutlar to that plane. Therefore no parallelogram distortion remains. I did heat the spine behind the headstock while straightening, but I'm not really sure if that was necessary. I think most of the movement was in the front downtubes. I haven't checked to see if the rear shock mounting axis is perpendicular to the center plane, because I'm making some mods to those for the Ohlins rear shocks, and will make sure they are right before welding.

A better way to do this would have been to build a complete frame jig, as if building a frame from scratch, with all the key dimensions correct, but I didn't want to take that much time for a single repair job, and I already had most of the fixtures left from doing similar work years ago. Still, I was tempted to build the jig, but I managed to resist urge this time. I'm pretty sure the key dimensions will be very close, and the frame will be satisfactory for my use.

Ken
 
lcrken said:
cjandme said:
lcrken said:
...............The whole point of the exercise is get the centers of all three iso mounts (top, front, and rear) in the same plane as the steering head, with the iso mount axes perpendicular to the steering head axis, and at the proper locations. ..............I hope this explanation, along with the pics, is clear enough. ............. and I need to get on with this project so I can be riding it at next year's rally......Ken
Click to expand...
Seems like the best use for that steel table :D good luck with your project and thx for posting, Will you be using stock triple clamps for this bike , or something like NYC Norton, or cNw's ?
Click to expand...

I think I'll be making my own triple clamps. I'm using a set of conventional Ohlins forks that I got from Kenny Dreer, the same ones he used on his prototypes. I sourced a set of Yamaha triples that fit the tube diameter, but they are a bit wider than I need, and have too little offset. I was thinking of pulling the rake back to 24.5 degrees or so to get the right trail with them, but I don't think I really want to do that with this bike. At the moment, the plan is to make some triples with the right offset for a 26 degree rake, and just wide enough to fit the wheel and brake combo I end up with. Of course all these plans are subject to change. Sometimes in the evening I sit in front of the shop with the doors open looking at the bikes, and have brilliant flashes of illumination on which way the builds should go. Of course there are usually some beers involved, so some of those ideas don't survive after sober consideration the next day. But then, some do.

If the head angle is 26 degrees, 70s TZ350 Yamahas use that rake, the fork yokes off one of those should give slightly more positive than neutral oversteer on your commando. They also have pretty appropriate spacing and clamp dimensions for a Norton. You'll need to be careful the first time you ride the bike, in case it mishandles and grabs you.


Ken
Click to expand...
 
acotrel said:
If the head angle is 26 degrees, 70s TZ350 Yamahas use that rake, the fork yokes off one of those should give slightly more positive than neutral oversteer on your commando. They also have pretty appropriate spacing and clamp dimensions for a Norton. You'll need to be careful the first time you ride the bike, in case it mishandles and grabs you.
Click to expand...

That would be a good plan, since the offset and width would probably be about right. Unfortunately, the forks I am using have much larger stanchion diameters that the '70s TZ forks, and there is not enough material in the yokes to bore them for the larger tubes.

Making simple yokes is not all that difficult. It only gets hard if you get fancy with lightening cuts, engraving logos, and complex curves.

I'm not too worried about it mishandling. I'm staying within well-established design parameters in terms of the rake/trail/tire size/weight balance combination. In fact it will be very close to the setup I used on my PR at the end of it's racing career, and I know that worked very well. And it is only a street bike. If I were setting it up as a modern race or track day bike I'd do it quite differently, going with modern 17" race rubber, stiffening the chassis structure, with steeper rake and slightly less trail, giving up some high speed stability for quicker turn-in. Then again, if I wanted that I'd have kept the R1. My intention is to end up with a really comfortable, well-mannered (and fun!) Commando street bike, with a little added bling. Oh yeah, and fast too.

Ken
 
On every Commando frame I've seen the main backbone always has a slight bend, if you put a straight edge on the top. Any idea the reason for this? It seems odd that the factory would not use a straight piece of tubing.
 
seattle##gs said:
On every Commando frame I've seen the main backbone always has a slight bend, if you put a straight edge on the top. Any idea the reason for this?
It seems odd that the factory would not use a straight piece of tubing.
Click to expand...

It's distortion resulting from the welding process-as far as we know, the tube was probably straight beforehand?
 
Every Commando frame I've had, except one, had that bend in the spine. I don't know why the one exception had a straight tube. I don't recall if it was one of the frames made at Reynolds or one of the Italian ones, but I've seen both types with the usual welding distortion bend. I don't suppose it really matters, since they are welded in a fixture, so all the important dimensions should be correct.

Ken
 
Time for an update. After straightening the frame, I decided I wanted to reduce the steering head angle/rake from the stock 28 degrees to 26 degrees. The pre-850 Commandos had it set at 27 degrees, but on the 850 they increased it to 28 degrees, and used angled yokes get the wheel where they wanted it. I'm making my own yokes to hold Ohlin forks, and don't want to try machining them at an angle, so I'm pulling the rake back instead. I ran a race Commando years ago where I cut and welded the steering head back to 26 degrees, and used Yamaha forks and yokes with less offset to keep the trail correct, along with 18" wheels, and I really liked the way it handled, so I'm planning the same sort of geometry for this bike. I can make the yokes with the correct offset to get the right amount of trail.

To do the mod with some accuracy, I decided to use a frame jig made by a friend of mine for the construction of frames we used for Sound of Singles/Supermono racing. All it needed was some fixtures to hold the Commando frame with the iso mounting points in the correct locations. I already had a fixture I'd made years ago that located them the correct distance apart, and with their center line on axis with the frame. All I had to make were a couple of vertical pillars to clamp both ends of the fixture. These are pictures of the jig with frame and fixtures assembled. Next step is to cut off the steering head, change the angle on the jig, make the tubes line up, and weld it back together. I'll post more pictures when I get all that done, and probably start a new thread on the build.

Commando Frame Straigtening


Commando Frame Straigtening


Commando Frame Straigtening


Ken
 
Good stuff - I hope it all works out - you certainly seem to have all the right equipment and knowledge! I'm about to repair one of the early Commando frames - the ones that all snapped, so I've been working on my frame jig - this is based on a ten foot long surface table that looks a bit like a lathe bed. Although the frame will look original when it's finished, the replacement sections will all be cold-drawn seamless, and the top spine will have a second piece hidden inside it. I should make it clear that the finished result is not intended to be used as a regular road bike - if that were the case I'd simply use a later frame. I just felt that being such a rare beast it ought to go back together as it was first built. Once that's out of the way, my hot rod Commando frame will be next in the queue!
 
Thanks, Paddy. I also plan to fit a reinforcing tube into the spine at the weld. If I'd had a 750 frame with the 27 degree rake, I probably wouldn't have gone to the trouble to change it. And if I was using the stock 850 yokes and forks, I probably wouldn't have bothered either. But as long as I had to make new yokes anyhow, I decided to go the whole route. Besides, I just like challenge of trying to improve an already great bike. My other MK3 has the original geometry, and I really like the way it handles.

Ken
 
This is the frame after cutting and welding, with the swinging arm I plan to use along side it. I won't be using that cradle, just the swinging arm. It's one I made back in the '80s for the PR I was racing then, with a pair of tapered roller bearings on each side, and has a little more clearance for a wider tire. The shock mounts are also wider apart, and are located to give a slightly steeper swingarm angle, along with a longer shock. Next step is sandblasting and painting. After that I need to get on with making some fork yokes and finishing the 1007 engine build. I need to machine the Maney cases to take the pre-MK3 alternator mount, and to do so I need to mock up the crankcase in the frame with some sort of primary covers to make sure it all fits. It WILL be finished in time for the rally next year. I've got the major design features sorted out, but still a lot of detail stuff to decide.

As I mentioned before, I put a doubler tube inside the main spine tube at the weld for extra strength. I think this frame was hit from the side at some point. The steering head was way out of line, and the spine is bent slightly to one side, with a bow in the middle. One of the nice features of cutting the steering head off and welding it back in a jig is that I can get all the critical parts lined up properly without trying to heat and bend the large diameter spine tube. I was originally able to get the steering head lined up properly by clamping to the table and heating tubes, but it put a lot of stress in some of the tubes. When I cut the steering head out, the down tubes popped out of line by 1/8" or so.

Commando Frame Straigtening


Ken
 
Excellent - I plan to build a swing arm similar to that one myself - it will also be using paired taper rollers!
 
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