swingarms and frame bracing?

Paul LeClair

Paul LeClair

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Calgary, Alberta, Canada
I have purchased a used JMC swing arm for my 79 1200.

the reason for the purchase is to fit a 4 inch wide Astralite wheel to the rear of the bike. The 4 inch wide Astralite does not fit in a stock 1200 swing arm, although it does fit in an RGS swing arm.

I have been fiddling with fitting an RGS swing arm to the 79 1200, and the RGS swing arm is modified and I have had bushings machined for it, but I have not fitted it yet.

then the used JMC swing arm came available..... I do not yet know, but am hoping, that the JMC swing arm for the 1200 will accommodate a 4 inch wide rim, we will see. Does anyone know?

I did a total comprehensive rebuild and restoration on the 79 1200 between 1999 and 2001, then some further improvements over the past 18 years, but the bike has now done roughly 100,000 kms since the rebuild ( 99,685 kms, to be exact ). The original stock wiring harness is struggling and I have patched it in several places, and the turn signal switch has broken for the 4th or 5th time. I have obtained a brand new wiring harness, new handlebar switch gear, etc.

so, the bike is coming apart again for a refreshing and re wiring, new steering head bearings, swing arm hardware, full rebuilds by a pro of the speedo and the tach which both are exhibiting wildly swinging needles, and whatever else it may turn out to need. While apart I am thinking about tackling bracing the head stock, and bracing the swing arm pivot area.

Advice, drawings, instructions, for the frame bracing front and rear? Best practices?

Paul LeClair
 
Been discussed many times.
If a search did not show what you need then I'll recap.

Swingarm bracing: I have seen double skinned type (like SFC 750) or boxed in type.

Frame bracing : I've seen this across twin down tubes just below exhaust curve as they exit the head, some triangulation behind airbox, and reinforcement of battery tray area. I have also seen bracing the length of the spine tube from steering head to battery area.

Head bracing: Triangular plates between downtubes and headstock or 6cm extension below steering head and then tubular or plate bracing back to the frame. The latter fully reinforces one of the weakest areas and does away with the need for a fork brace as the lower yoke is 6cm closer to the axle. Needs a 6cm longer steering head spindle though.

Guessing the JMC may need cutting to remove the torque arm lug like my Corsa swing arm and POP's swingarm did.

It's possible to fit a 4 inch rim and 160/70 tyre on a 180 swingarm but a couple of flats need to be made/welded in at the point where the tyre is closest to the swingarm.
 
When Red bought or at least attempted to buy a bunch of these, I seem to remember one that he got wasn't as wide as he wanted.I think someone bought it but not sure who.
 
Are SF swingarms the same length as pre-RGS triples? I can easily fit a 4.25 inch rim and a 160 x 17 tyre (measures 167mm) with plenty of clearance b/w/tyre and swingarm - the problem is the chainline. I am running a 4mm offset drive sprocket, but would prefer not to. Not sure a 160 is recommended for a 4 inch rim? Maybe 18s have different requirements to 17s?  I would have preferred a 150 on my 4.25 but choices are more limited for heavier bikes.

Or are triple swingarms narrower at the tyre area to fit inside the frame parts (pegs, muffler brackets etc)?

Grant's mod with the longer steering tube is an excellent one. Makes sense in so many areas. The length of unsupported triple fork has always troubled me.
 
A couple of photos of the headstock/downtube bracing I recently had done.

 

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Piranha Brother 2 said:
Are SF swingarms the same length as pre-RGS triples? I can easily fit a 4.25 inch rim and a 160 x 17 tyre (measures 167mm) with plenty of clearance b/w/tyre and swingarm - the problem is the chainline. I am running a 4mm offset drive sprocket, but would prefer not to. Not sure a 160 is recommended for a 4 inch rim? Maybe 18s have different requirements to 17s?  I would have preferred a 150 on my 4.25 but choices are more limited for heavier bikes.

Or are triple swingarms narrower at the tyre area to fit inside the frame parts (pegs, muffler brackets etc)?

Grant's mod with the longer steering tube is an excellent one. Makes sense in so many areas. The length of unsupported triple fork has always troubled me.
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1. Yes

2. Yes

The 750 swingarm slots straight into the triple.  Drum-brake swingarms require some sort of adapter for the RH axle mount to accomodate disc-braked wheels.  Disc-braked SF3 swingarm is actually a triple item, so no use swapping these.  RGS swingarms are about 3cm longer than the 180?/twin items.  I run a 150/4.00" rim or 160/3.50" rim in the RGS swingarm with a #520 chain, no sprocket off-set required.  The design of the front sprocket fixing does not lend itself to off-setting, premature damage to both sprocket and shaft are inevitable.  Seen this to accomodate over-wide 17 inchers, shafts knackered beyond repair.

piet
 
I have a 160 rear with 520 chain, on the SF, no front sprocket offset required. Have been toying with the idea of larger dia tubing swingarm, similar to Ducati Sport Classic 1000, but in aluminium, will talk to my welder, and see what he thinks.

Was curious about the Motociclia SFC swingarm posted elsewhere here, showed their SFC job beside a red SF swingarm, which was actually a triple or possibly a SF3 as Piet pointed out in his post above. (No bend in the arms, as per SF1 & SF2)
 
I have a jmc on the  gelati as does Red or should I say did on no2 , both running gsxr 4.25 rears. The trouble is that the widest possible tyre you can fit is a 140, nothing to do with the chain clearance, the swing arm bends in too close to the wheel, bt 45 140 has about 4mm clearance each side, maybe the new Dunlop gp300 radials may fit a 150 , will know in a few weeks when I test fit one, but you define fly won't be fitting a 160 or a 150 I doubt either, and they are a prick to get the eccentric right as you need to adjust the ride height once you determine sprocket size and chain length. Then there is the fitting of it, talk toRed about what you need and how to fit it, h has all the dimensions of the stuff that needs to be turned up
In summary, I am usd to It now and  how it works it's on the bike cost a fortune and looks good,  in reality I don't really know if does anymore that look good, you definetly wouldn't put it on a race bike, you be readjusting the whole rear every time you changed a sprocket size, just bling in my opinion, I like bling :D
 
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Seems like a hell of a lot of work and then you cannot even fit a 150, whats the chance of doing a cut and shut to make that possible.
 
Grant said:
Sure you got these rims sizes the right way round?
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Yes. :D

The 160 is too wide on the 4.00", but tucks in nicely on the narrower rim to give just enough clearance.  Was a matter of suck-it-and-see.  Works excellently!

piet
 

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Vince said:
Seems like a hell of a lot of work and then you cannot even fit a 150, whats the chance of doing a cut and shut to make that possible.
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none, be easier to make a new swingarm rather than cut open a JMC to widen it
and I'll never ever buy an eccentric adjustment swingarm ever again, pain in the butt

 
I was going to say something derogatory about the eccentric adjusters on the swing arm, but Red beat me to it.  :D

Ant Carberry has a braced alloy swinger on his 1200 (dunno whether it's JMC or some other brand). Chatting to him at one of the Broadford Bike Bonanza piss-ups gatherings a few years ago, I asked him whether it was an improvement over the original as far as handling was concerned, and he said there was no difference. Bike felt exactly the same.

Pros and cons evaluation ...
Pros - It looks flashy.
Cons - It costs a small fortune, won't accommodate bigger tyre, offers no performance improvement, and there are technical deficiencies (that Cosi and Red mentioned).

So from an engineering point of view it's inferior to the original swing arm.
 
I always found it easier to modify an existing gadget than making one from scratch but this gadget seems so compromised in basic design. My only experience with an eccentric adjustment swingarm was on my brothers 860 Bevel, we had real issues aligning the wheels as it didnt have anything to adjust at the end of the swingarm only the eccentric at the swingarm pivot and that could not be adjusted individually for each side.
BTW Bridgestone makes a 140mm Radial tyre that would work in this case and in my case, T30 Evo, the latest version of the old BT 020 sports touring tyre. Now made but not imported into OZ, I have been asking the local distributor to bring a few in, they order them and then they just don't turn up in the container. He said keep ringing so time for another call.
 
Vince said:
... we had real issues aligning the wheels as it didnt have anything to adjust at the end of the swingarm only the eccentric at the swingarm pivot and that could not be adjusted individually for each side.
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But that is the main "advantage" of eccentric adjusters, keeping both sides of the wheel in line while adjusting.  Turns into a disadvantage if the frame or swingarm get out of alignment.... 

Same here, eccentrics at the rear axle are a solution in search of a problem, unnecessary and often fiddly.  Not even good for race bikes, tyre/wheel changes can take ages.  The Ducati/Rickman method of moving the swingarm spindle at least keeps hassles of installing the rear wheel to a minimum.

piet
 
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