Swing Arm Design/Chain Adjuster/Geometry

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Swing Arm Design/Chain Adjuster/Geometry

Post by johnu »

I'm planing on building a swingarm for my tr5mx and I was also going to move the swingarm pivot forward as far as possible to a fixed position then have the chain adjustment at the rear wheel axle. I could then lengthen the swing arm by the amount that I move the pivot point forward. I also intend to increase rear wheel travel slightly by increasing the swing arm angle a bit and lowering and moving the top shock mount forward. This would also help reduce the rake angle on the fork slightly. My question is why does tr5/b50 have the snail cam adjustment at the pivot point? Seems that if you go from one end of the adjustment to the other you are going to change the rear suspension geometry to some degree. Probably not significant but it would change the shock angle slightly.
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Re: Swing Arm Design/Chain Adjuster/Geometry

Post by minetymenace »

johnu wrote:Probably not significant
Probably right, I certainly wouldn't notice but my riding style is more about hanging on rather than worrying about suspension geometry!
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Re: Swing Arm Design/Chain Adjuster/Geometry

Post by Grouty »

You would never notice it John. The B50MX turns a little like a barge anyway. I have raced a standard B50MX frame with std swingarm, a longer swingarm, longer shocks … etc etc. None of it made a great deal of difference. The motor is the best bit.
The 73 CCM seemed to have got it right. Not that it looks a great deal different. But then, I am biased :-)
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Re: Swing Arm Design/Chain Adjuster/Geometry

Post by johnu »

Grouty wrote: Tue Mar 24, 2020 10:10 am You would never notice it John. The B50MX turns a little like a barge anyway. I have raced a standard B50MX frame with std swingarm, a longer swingarm, longer shocks … etc etc. None of it made a great deal of difference. The motor is the best bit.
The 73 CCM seemed to have got it right. Not that it looks a great deal different. But then, I am biased :-)
Ok thanks, your bike is certainly a beauty! I've never ridden a b50 so it will be a bit of an eye opener, I'm so use to my modern mx bikes. I will still probably go ahead with my plan anyway as I like put my own touch on stuff. A lttle more rear travel is usually welcome!
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Re: Swing Arm Design/Chain Adjuster/Geometry

Post by koncretekid »

"My question is why does tr5/b50 have the snail cam adjustment at the pivot point? Seems that if you go from one end of the adjustment to the other you are going to change the rear suspension geometry to some degree. Probably not significant but it would change the shock angle slightly."

The total amount of movement from one extreme to the other is only about 1/2", which is not enough unless you use a half-link to compensate for chain wear. I (as well as PES) have made up new snails that have the required 5/8" movement to eliminate the need for a half-link. That being said, there is no more movement in length, less actually, than the standard slotted drop-out at the back of most early swing arms, so no more change in geometry than any other system of adjustment.

As for the reason, I think BSA wanted to make the rear axle more rigid, as the ferrules welded to the back of the swing arm make what is known as a "moment connection" at each side. This is much stiffer than the standard 1/4" plate used on all the previous British bikes. You can actually verify this by just placing the rear axle into one side of the swing arm and pulling back on it. You can't move it more than the clearance allows. Then try this on a standard plate type swing arm, by fitting the axle with a nut on each side of the flange. When you pull back on the axle, you will be able to flex that rear flange more than you can on the B50. So it is an excellent design. The swing arm also uses Torrington needle bearings on the outer ends, so again, a very good design if the previous owner has bothered to keep it greased.

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Re: Swing Arm Design/Chain Adjuster/Geometry

Post by johnu »

Tom, I haven't taken my swing arm out yet but it appears that the slot at the arm pivot point is longer than 0.5". Kind of interesting that the original snail cams weren't made to allow more adjustment!
I do understand what you are saying about the strength/flex at the axle. If I decided to have the adjustment at the axle I wouldn't be using the flat slotted plate arrangement.
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Re: Swing Arm Design/Chain Adjuster/Geometry

Post by Shawn_Mc »

That snail adjuster was probably a result of the night terrors left over from the Titanium frame and Swing arm attempts of factory BSA for the Factory race bike. The swing arm flexed so much they couldn't keep the chain on.
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Re: Swing Arm Design/Chain Adjuster/Geometry

Post by koncretekid »

Bonneville B50 rear suspension
Bonneville B50 rear suspension
Here is the rear swing arm I fabricated for my land speed B50, which as you suggest, has a much better supported rear axle than is possible with the flanged type swing arms.

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Re: Swing Arm Design/Chain Adjuster/Geometry

Post by johnu »

koncretekid wrote: Thu Mar 26, 2020 3:17 pm Bonneville bike rear suspension 2.jpg

Here is the rear swing arm I fabricated for my land speed B50, which as you suggest, has a much better supported rear axle than is possible with the flanged type swing arms.

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Wow do you have a thread for this bike build? And yes that is exactly the type of adjustment I was thinking :thumb
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Re: Swing Arm Design/Chain Adjuster/Geometry

Post by koncretekid »

Yes, but it's been going on for 9 years, so you'll need some time to read it!

The full build is here: http://www.landracing.com/forum/index.php?topic=9341.0

The suspension alone is here: http://www.landracing.com/forum/index.php?topic=16962.0
Last edited by koncretekid on Fri Mar 27, 2020 1:29 am, edited 1 time in total.
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Re: Swing Arm Design/Chain Adjuster/Geometry

Post by johnu »

koncretekid wrote: Fri Mar 27, 2020 1:07 am Yes, but it's been going on for 9 years, so you'll need some time to read it!

The full build is here: http://www.landracing.com/forum/index.php?topic=9341.0

The suspension alone is here: http://www.landracing.com/forum/index.php?topic=16962.0
:shock: ok thanks! I'll check it out :thumb
Last edited by minetymenace on Fri Mar 27, 2020 9:59 am, edited 2 times in total.
Reason: Correcting the copy of the edited post...you know what I mean, see KcK's post below!
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Re: Swing Arm Design/Chain Adjuster/Geometry

Post by koncretekid »

I had to edit my post as I posted the same link twice. It is now corrected.
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Re: Swing Arm Design/Chain Adjuster/Geometry

Post by eebtr7 »

I, wholeheartedly, agree with Koncretekid! For nearly the past half century my B50 frame has been a pillar of strength and stability keeping me upright over all kinds of paved and unpaved terrain. That swingarm design is brilliant in keeping the rear wheel aligned and the chain quickly adjusted.

If any of you good folks want to butcher a B25/B50 frame, please don't tell me. If you want to build your own frame, please save the stock frame for when good sense returns to your head.

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Re: Swing Arm Design/Chain Adjuster/Geometry

Post by johnu »

eebtr7 wrote: Fri Mar 27, 2020 2:19 pm I, wholeheartedly, agree with Koncretekid! For nearly the past half century my B50 frame has been a pillar of strength and stability keeping me upright over all kinds of paved and unpaved terrain. That swingarm design is brilliant in keeping the rear wheel aligned and the chain quickly adjusted.

If any of you good folks want to butcher a B25/B50 frame, please don't tell me. If you want to build your own frame, please save the stock frame for when good sense returns to your head.

God love you, all!

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Probably more stock frames around than will ever be built up, just sayin :thumb I actually am thinking of sticking to the original idea of the snail cam and slot at the pivot point just moving it as far forward as possible. I will still build my own modified swing arm.
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Re: Swing Arm Design/Chain Adjuster/Geometry

Post by Momus »

I heard this from Len Boulter, Rolls aero mechanic.

Management were so alarmed at the cost of producing the oil in frame chassis for the B50 that when they realised the tooling and material involved in the submitted swinging arm design, a B40 style refinement with a clevis pull chain adjuster and a built in guide and tensioner, they ordered it back for a costing haircut. The engineer/draughtsman went down to the pub and got sodden.
The only bloke up to it left was a sparky cleaner and he was told that design and material had to be economical. Being the cleaner he new his way around the works and reckoned that the unused sheets of 3/16" and 1/8" mild in the material store were good, there were a thousand rejected -because of a machining error - high tensile pivot axles for another bike in a stillage and a snail cam stamping die somewhere else. Lengths of machined bar from another F--K up (axle spacers wrong length) could be used for the swinger ends. With the tube squashed to suit they looked ok but were too short because the expensive slotted flange and clevis were gone. No problem the cleaner said, we just make the flanges longer-it will look like shit but who cares?

That is a true story about end times for Beeze.
Last edited by Momus on Fri Apr 24, 2020 2:34 pm, edited 1 time in total.
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Re: Swing Arm Design/Chain Adjuster/Geometry

Post by minetymenace »

I hope Brad reads this, maybe it should be an addendum in his book and included in the next edition! :laugh
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