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Old 08-03-17, 04:50 PM   #1
magnesium
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Swingarm Pivot is Crank Axle

I am interested in a rear suspension frame where the swingarm pivot is the crank axle, for a more rigid frame and better mechanical peddling efficiency. The bottom bracket would only be as wide as the seat and down tubes with sealed bearings screwed into threads in the known manner, then the swingarm fits over the bottom tube with two more sealed bearings screwed into threads in the known manner, then the crank axle goes through all four bearing holding the swingarm onto the frame, maybe held in place latterally by circlips.

I am attaching two photos of bikes, the fendt shaft drive, which has a more appealing geometry, and the blackmarket killswitch which although less comfortable to ride has the more accessible sprockets and chain drive.
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File Type: jpg 1983 Fendt Cardano.jpg (99.4 KB, 86 views)
File Type: jpg Blackmarket Bikes Killswitch.jpg (101.2 KB, 86 views)
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Old 08-03-17, 05:03 PM   #2
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I'm no suspension expert, but I believe there are much better designs
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Old 08-04-17, 04:45 AM   #3
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The only benefit to that design is not having any suspension related chain growth, so it is possible to run it as a single speed without a tensioner. Otherwise there are much better locations for the suspension pivot.
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Old 08-09-17, 07:26 AM   #4
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Don't confuse pivot location with frame stiffness. Any pivot location can be made with solid/stiff designs or flexie ones.


As an aside I find the goals of a "stiff" bike that has a purposeful, built in, flex (as in the suspension's movement) somewhat questionable. After enough frame/pivot stiffness so that the steering remains predictable and the components remain in spatial relationships (the rear der stays vertical during shifting as example) how much more stiffness is really needed? After all if the goal is power transfer to the ground efficiency then start with tires that aren't soft.


The above is said with some tongue in cheek but still with some real meaning.


I just finished a 2000 mile tour and was very glad to have 559 x 37 tires (26 x 1.5) at only 70psi. They made the ride so much more comfy then the 95psi 622 x 25s I'll be on later today. Tires are really the #1 factor is the perception of stiffness, IMO. Wheels are #2. The frame comes into play much later. Andy.
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Old 08-10-17, 01:40 PM   #5
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Well, it would save weight to have this one assembly perform these two functions.

Also, from the looks of the Cannondale SE 1000 there is stress on the frame at the pivot point (note the auxilliary top bar adding strength to the frame at the pivot point) and the junction of the seat and down tube is possibly the sturdiest part of the frame.

I think it would be better if this bike had the pivot point at the crank axle.
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Old 08-10-17, 05:26 PM   #6
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I would look at more current frames. There were patents in force that kept people from doing the smart thing, and a number of older bikes just used stupid pivot schemes because they didn't know what they were doing
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Old 08-10-17, 08:26 PM   #7
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Well, it would save weight to have this one assembly perform these two functions.

Also, from the looks of the Cannondale SE 1000 there is stress on the frame at the pivot point (note the auxilliary top bar adding strength to the frame at the pivot point) and the junction of the seat and down tube is possibly the sturdiest part of the frame.

I think it would be better if this bike had the pivot point at the crank axle.

I'll fall back on my pat answer about the this kind of reinventing what's already been done for years. If the OP's ideas were really better, as proved in the market place, then we would see these designs all over the place. That we don't might suggest some things... But I also am happy to see the OP trying old stuff again. No reason not to reconfirm what others have found out. Andy
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Old 08-10-17, 09:21 PM   #8
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Specialized tried this back in 2012

https://www.pinkbike.com/news/Specia...Crankworx.html

Flow Mountain bikes also had a downhill rig with a multi link suspension that pivoted around the bottom bracket. That was around 4 years ago.

If you're really interested in suspension design, then look into what's been done with motorcycles. Most of what you see as a successful design in bicycles has been inspired by what happens in the motorcycle world. Suspension geometry has been studied to a much larger degree in motorcycles than bicycles. The forces and speeds are greater, therefore amplifying small changes in suspension geometry. You can learn quite a bit from these guys. It might narrow down some ideas you have when/if you decide to do something with your ideas.

Last edited by taras0000; 08-10-17 at 09:31 PM.
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Old 08-11-17, 10:57 AM   #9
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If you're really interested in suspension design, then look into what's been done with motorcycles.

Why? It's not as though motorcycle suspension is sophisticated or technically advanced. It's still very primitive, just like the brakes. Those motorhead guys don't actually understand geometry.







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Old 08-11-17, 05:07 PM   #10
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I dunno.... Looks like a lot of slide rules went into that design, lol.
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Old 08-13-17, 12:17 AM   #11
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Well, it would save weight to have this one assembly perform these two functions.
The loads of the two sets of bearings are very different. Not sure why it would save weight.
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Old 08-18-17, 05:12 PM   #12
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Having the pivot concentric with the bottom bracket would be beneficial if the bike was a single speed or internally geared hub, there'd be no chain tension issue. Although, that is not what Magnesium is proposing. The thought was simplicity and lightness.


I think it will be tricky to design the bottom bracket with a pivot that will presumably fit over it. The spacing will be tight and it might require a removable element on one side to fit into the bottom bracket.
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