Commando Frame Straigtening

Ken,

Is there any concern about frame strength after heating and bending the tubes?

illf8ed said:

Ken,

Is there any concern about frame strength after heating and bending the tubes?

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No. It's just mild steel tube, not heat treated. As long as I don't overheat it, there's no change to the material properties.

The most difficult part of it is getting decent welds in the tight area around the steering head and down tubes.

Ken

Looking good Ken :mrgreen:

Time to end this thread. I'll start a new one for the actual bike build, as soon as I make some more progress, but the straightening process is done.

Frame after sandblasting:



And after some of my favorite frame paint, Rustoleum gloss black.



Ken

Great job Ken, wouldn't even know it was ever bent.

Ashley

lcrken said:

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

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How much yoke offset is used with the 750 commando frame which has the 27 degree rake, and what is the wheelbase ? Are the wheels 18 or 19 inch ?

Ken - how did you go about setting the gap between the swing arm taper rollers and the engine cradle - was it shimmed, or did one side slide and then get clamped?

Paddy_SP said:

Ken - how did you go about setting the gap between the swing arm taper rollers and the engine cradle - was it shimmed, or did one side slide and then get clamped?

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The tapered bearing pairs are each pre-loaded by a precision machined spacer. There are grease/oil seals at both inner and outer bearings, with short spacers that fit into the seals. The spacers butt up against the engine cradle, and a one piece shaft holds it all in place. A tight fit against the cradle is achieved by machining the inner seal spacers to whatever length is required. I don't recall how easy that was, but I suspect I ended up making the spacers a little short, using a feeler gauge to measure the clearance, and then making new spacers the appropriate bit longer. I'm a patient sort, so I would have kept at it until I got the fit I wanted. I'll try to take a picture today that shows a little more detail.

Ken

acotrel said:

How much yoke offset is used with the 750 commando frame which has the 27 degree rake, and what is the wheelbase ? Are the wheels 18 or 19 inch ?

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Stock Commando wheelbase is listed as 56.75" (1447 mm) for 750 and 57" (1448 mm) for 850. Stock 750 yoke offset is 2.75" (70 mm). Not sure what stock 850 offset is, but it is effectively less than the 750. The 850 yokes were bored differently to bring the front wheel back a bit so the wheelbase wasn't stretched so much by the change from 27 to 28 degree rake, which also increased the trail measurement. Commando wheels are 19". I will be using 18" wheels on this bike.

FWIW, by my calculations, the stock 750 Commando has something like 3.75" (95 mm) of trail. Depends a little on the choice of front (19") tire size (90 or 100). Not sure what trail the 850 has. but it's certainly more than the 750. I would have to measure a set of 850 yokes pretty carefully to figure that out.

In the past, I played around a fair bit with different rakes and trails on a 750 Commando frame, by varying front and rear heights, wheel sizes, steering head angles, and yoke offsets. I didn't find it to be terribly fussy about the settings. The long wheelbase and large rake (relative to modern race bikes) has something to do with that. The shorter the wheelbase, the more sensitive the bike is to front end changes. For me, with my PR, the setup I ended up with (and liked a lot) was 18" racing slicks, longer rear shocks and lower front to give between 26 degrees of rake, and 42.5 mm (1.67") offset in the yokes. Depending on the front tire, the gave a trail of around 109 mm. That's a little long by today's racing standards, but I liked the way it worked. The long wheelbase and trail numbers gave it really good stability on high speed tracks like Daytona. For this street bike, I'll probably design for 100 mm of trail.

Ken

lcrken said:

Paddy_SP said:

Ken - how did you go about setting the gap between the swing arm taper rollers and the engine cradle - was it shimmed, or did one side slide and then get clamped?

Click to expand...

The tapered bearing pairs are each pre-loaded by a precision machined spacer. There are grease/oil seals at both inner and outer bearings, with short spacers that fit into the seals. The spacers butt up against the engine cradle, and a one piece shaft holds it all in place. A tight fit against the cradle is achieved by machining the inner seal spacers to whatever length is required. I don't recall how easy that was, but I suspect I ended up making the spacers a little short, using a feeler gauge to measure the clearance, and then making new spacers the appropriate bit longer. I'm a patient sort, so I would have kept at it until I got the fit I wanted. I'll try to take a picture today that shows a little more detail.

Ken

Click to expand...

Thanks for the helpful reply - that's pretty much how I saw it being done! I couldn't make up my mind as to whether it'd be better to do this or to make the bearings on the right hand side slide inside the housing - that way the spindle would pull them up tight, and a clamp arrangement would then lock it all in place. Pragmatism vs. elegance would have me do it the same way you did!

Ken,
my 1970 750 has the rear wheel fully at the forward end of the adjustment slot and I'm getting a good weave on above 75mph. I'm pretty light and the bike is light also with alloy 18" rims and alloy tank. Do you think I would benefit from putting a couple of extra links in the chain and moving the wheel to the rear to improve stability? (I've been through the tyres, new, iso's and rose jointed head steady)
Dave Evans

my 1970 750 has the rear wheel fully at the forward end of the adjustment slot and I'm getting a good weave on above 75mph.

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I have the minimum links in the chain and wheel is also right forward in the grooves. Mine also weaved at 75+ with 18 x 110 rear and 19 x 100 or 90 front. What cured it was getting the engine and swingarm assy pointing forwards and not off to the left. 060" off the front mount right tube and an equivalent washer added to the left side fixed it. Since then it has had both AM26 and BT45 and the weave has not returned. I didn't expect the mod to solve it as it was just the one thing I did after reading the 'Worlds straightest Commando' article. I never did do all the rest and I'm sure mine doesn't track fully straight but its lots better.

gripper said:

Ken,
my 1970 750 has the rear wheel fully at the forward end of the adjustment slot and I'm getting a good weave on above 75mph. I'm pretty light and the bike is light also with alloy 18" rims and alloy tank. Do you think I would benefit from putting a couple of extra links in the chain and moving the wheel to the rear to improve stability? (I've been through the tyres, new, iso's and rose jointed head steady)
Dave Evans

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I wish I had a good answer for you, but I don't. Of all the Commandos I've had, only one, a MK3 Roadster, had the infamous weave problem. It went away after I modified the bike for road racing, and then later converted it back to a street bike. I don't know which of the changes fixed the weave (wider rims, 18" tires, different shocks, Norvil top mount, lower bars, etc.). I've read all sorts of theories about the Commando weave/wobble problem over the years, and you've probably already seen most of them here on the forum, so your guess is as good as mine. Using extra chain links to put the axle as far back as possible sounds like a good experiment in your case. It wouldn't cost much to try, and might help. I would also suggest checking for alignment problems as Keith1069 described, if you haven't already done so. The other problem I've seen that is pretty common on older Commandos, is worn out swinging arm pivot. That will definitely affect handling. I'm guessing that since you've already gone to the trouble of new tires and isos, you've also checked for swinging arm slop. Same sort of comment regarding checking steering head bearings.

When I made the new swinging arm for my PR race bike, I extended it an extra inch, and extended the adjustment slot so that I had the stock range of adjustment, plus an additional inch to the rear. The actual location varied with different gearing combinations, but was typically further back than it would have been with a stock swingarm. Conventional wisdom at the time was that the Commando didn't have enough weight on the front end for optimium handling, and extending the swinging arm put a little more weight on the front. I was also running a steeper rake and less fork yoke offset than stock, which also put more weight on the front, as well as keeping the wheelbase close to stock. I never experienced any kind of speed wobbles or weaves with that setup. That leads me to believe that trying to keep the axle as far back as you can in a Commando is a good idea.

Ken

On my Ducati ST4S a common tweak is to adjust the rear shock mount to raise the ride height an inch or more this steeped ingredients the rake for a bit quicker steering. Would adding a taller rear shock on the 850 Commando be worthwhile for this? If so? How tall?

lcrken said:

When I made the new swinging arm for my PR race bike, I extended it an extra inch, and extended the adjustment slot so that I had the stock range of adjustment, plus an additional inch to the rear.

Ken

Click to expand...

I shall steal that idea and claim it was my own.

A full tank of fuel and rear suspension set to medium from soft has improved things a little. I had recently done some touring with luggage and suspension on medium, no problem. Seems a combination of a few elements all gang up to screw the handling. Maybe those hog riders with the tool roll hanging under the headlight are onto something as well.