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Twins Technical Talk Technical Talk for Hinckley Triumph Twins: Bonneville, T100, Speedmaster, America, Thruxton, and Scrambler.

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Old 03-13-2011, 09:26 PM   #1 (permalink)
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New JMC swingarm

I installed my new JMC swing arm a few weeks ago. It took about 2 hours to remove the OEM swing arm, install the new swingarm, torque everything, go for a test ride and re-torque all bolts . JMC provided two spacers which would not fit so I only have one spacer on the brake side. I had to find another socket for the axle nut owing to the recessed block which houses the axle bolts on each side. The chain seems to be aligned. When I went for a ride, the change was noticeable – a more lively feel from the rear end – much more feedback. The weight difference is significant.

A few questions. Is a change in preload required? Also note the length of the swing arm and the forward position of the axle. I used my chain, which is OEM, so there is not much room to adjust forward. JMC have provided the ability to extend the rear axle to the rear - any benefit in this? I assume a moving the axle backwards would increase the stability of the bike but reduce its ability to change direction quickly.
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New JMC swingarm-lmc2.jpg   New JMC swingarm-jmc3.jpg   New JMC swingarm-jmc4.jpg   New JMC swingarm-jmc-1.jpg  
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Old 03-13-2011, 10:10 PM   #2 (permalink)
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Well, obviously if your chain has the same tension as before, then you have the same wheelbase, so no worries there. A longer wheelbase would give a smoother but harder to toss ride.

If you subscribe to the concept of static sag, then you don't need to change preload. You haven't changed the sprung weight, so the sag has not changed. That said, the lighter swing arm does allow the suspension to do its job better ( less inertia), so you might want to play around, particularly with rebound damping if you can.
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Old 03-14-2011, 12:38 AM   #3 (permalink)
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Thx NT. What I omitted to say was at 18,000 miles I am looking to replace the chain in the next few thousand miles so wondering the impact of adding a few links to the new chain so I can move the axle an inch or so towards the rear. I have no problems with the stability of the Thruxton - just thinking about the handling impacts. For some reason JMC have built the swingarm longer than the stock unit
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Old 03-14-2011, 01:30 AM   #4 (permalink)
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That thing is funky looking as long as it is. I like JMC's stuff, but I've never noticed that extra length before (or they haven't done it to the ones I've seen).
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Old 03-14-2011, 04:04 AM   #5 (permalink)
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They will make a swingarm to your specs. It can be extended or shortened. It can have top- or bottom- or both bracing. It can have normal or eccentric adjusters. It can be polished or powdercoated.

Your look like it has a mild extension. Good if you want to do some dragracing
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Old 03-14-2011, 08:28 AM   #6 (permalink)
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Is there a possibility the a new standard chain won't fit? I mean if you are fully forward now on a standard chain (and sprockets I'm assuming) which is due to expire a new one may not fit.
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Old 03-14-2011, 09:55 AM   #7 (permalink)
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Wow, someone who has actually got one!!!!! How long did it take from ordering to delivery?
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Old 03-14-2011, 10:26 AM   #8 (permalink)
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I can't keep that front wheel on the deck I was planning to get the 904 plus head work so now I have an excuse to go to the 1087...

To my knowledge it was standard JMC swingarm so you'd think the axle would sit in the middle of the adjustment range. Once I installed the chain, I was surprised how forward the axle sits. Overall its not that noticeable when you look at the bike.

How long did it take? I did not have to wait as this was a cancelled order which I managed to pick up (anyone out there cancel their JMC swingarm order??).
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Last edited by Aussiematt; 03-14-2011 at 10:39 AM.
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Old 03-14-2011, 12:40 PM   #9 (permalink)
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I dont think adding a link will change the bike to much when you go to a new chain it looks like its only less then a inch longer.The drag race guys go way longer to keep the front wheel on the ground and yes they dont turn to good lol.But in your case I dont think it will hurt you much at all.I would just add whats needed to get the new chain to go on right the new chain will get longer as it wears anyway.
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Old 03-14-2011, 01:17 PM   #10 (permalink)
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Thumbs up

Hey Matt looks great. I told you it wouldn't be a big deal. The preload is strictly dependant upon the length of the rear axle in relationship to the front pivot and the shock top mounting point. If you are in the standard position with the stock chain everything will remain the same and you can adjust as normal. If you decide to lengthen the chain and change the axle center, some additional shock length and preload will be in order.

A couple of other points to think about. Lengthening the rear axle point will also require the use of a longer rear shocks and will extend the wheel base of the bike. In doing so, you'll slow the handling down. There will be some necessary adjustment to the front you would also have to make to keep everything in order.

Another question; Did you remember to prelube the caged roller bearings? What I did is to drill and tap then add zerk fittings to the bottom of the front mounting points for regular maintenance.

I'm just about ready for reassembly so when I get to that point, I'll share some photos with you. Nice job!

With your chain you might want to change to Supersprox and an RK GB525GXW- They've for different lengths available and are a really good addition to the bike. You really don't need to go to a 530 setup unless your planning to build the motor. Even then it's a bit of over kill.

Cheers
Jeff




Last edited by Skull Crusher; 03-14-2011 at 01:28 PM.
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