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Beemerguru 02-04-2011 08:22 PM

G/S swing arm extension suspension question
 
OK beemerphiles.

What kind and length shock did you use when you extended the swingarm that silly little 100mm longer?

Stock mounts on the frame and swingarm.

Thanks

StephenB 02-05-2011 05:15 AM

For the 30mm extension I did I moved the swingarm mount in order to stay with a shock with stock G/S dimensions. The original swingarm mount is a flimsy bracket anyways so not a big loss. I would do the same for a 100mm extension.


<center> http://www.stephenbottcher.net/BMW/R.../_DSCF3182.jpg http://www.stephenbottcher.net/BMW/R.../_DSCF3184.jpg http://www.stephenbottcher.net/BMW/R.../_DSCF3185.jpg </center>

One Less Harley 02-05-2011 09:59 AM

Greg , if your building something radical I think you need to share w/ the collective!!!!

Stephen- you'd have to use a stiffer shock than normal due to the differing angles....right???

StephenB 02-05-2011 10:41 AM

Please proceed with caution, no guarantees for any of the following being correct or applicable for your application:

Stiffer spring yes, but not for the angle. May I elaborate:

Applicable lever law (thanks Wikipedia):
The force applied (at end points of the lever) is proportional to the ratio of the length of the lever arm measured between the fulcrum (pivoting point) and application point of the force applied at each end of the lever.
Mathematically, this is expressed by M = Fd, where F is the force, d is the perpendicular distance between the force and the fulcrum, and M is the turning force known as the moment or torque.

Translated to a rear swingarm (I am totally simplifying): the distance between the (swingarm) pivot point and the moving mass (final drive) is extended, hence the loads (torque and perpendicular forces) around the pivot point is increased -> hence more damping (stiffer spring) for the same amount of spring action (=comfort) is required. The increased perpendicular forces on the pivot pin are also the reason why at the same time, you may have to strengthen the frame tube around the swingarm pivot point.

Using rudimentary calculation methods (just to get an idea) for my 30mm extension I calculated the perpendicular force at the swingarm pivot point to increase by about 30%. Since the Paralever GS (with a longer swingarm than the G/S) uses the same frame, I took my chances and did not reinforce the area around the swingarm pivot pin. At the same time, the 30mm extension also requires a 17% higher spring rate of the rear shock, because of the change of the levers and dynamics. So my spring is a 95kg/mm as opposed to the 85kg/mm which would be the correct one for a 220lbs rider.

My university lecture days are way back in the past, this is only a rudimentary analysis and should in no way being taken as gospel. But I think I got the right idea and proportions. My road tests including sections of the TAT on a loaded bike seem to prove me right though.

Beemerguru 02-05-2011 11:17 AM

My Physics and Math degree expired about 40 years ago back in the dark ages..when they just discovered quarks and "Sears and Zemanski" was the bible. :rofl:rofl

So in simple terms...stiffer spring, re-enforced mounts.

Still no comments of the brand of shock for a twin shock rear setup?

StephenB 02-05-2011 11:34 AM

Ohlins here ... Wilbers would be my 2nd choice.

Desertpistons 02-05-2011 01:57 PM

lenght
 
Quote:

Originally Posted by Beemerguru (Post 15113857)
OK beemerphiles.

What kind and length shock did you use when you extended the swingarm that silly little 100mm longer?

Stock mounts on the frame and swingarm.

Thanks


Yes, hpn dudes prepped bikes with that lenght. With the stock steering front angle, the bike already feels long so I never ventured in anything longer. I would go minus 1.5% steering front an 150 mm back extension. Ohlins or worksperformance customs shocks

Desertpistons 02-05-2011 01:59 PM

Quote:

Originally Posted by Beemerguru (Post 15117273)
My Physics and Math degree expired about 40 years ago back in the dark ages..when they just discovered quarks and "Sears and Zemanski" was the bible. :rofl:rofl

So in simple terms...stiffer spring, re-enforced mounts.

Still no comments of the brand of shock for a twin shock rear setup?

twin shocks : white power like the hpn bikes see my frankenkrauser

Beemerguru 02-05-2011 06:35 PM

And length..eye to eye?

Desertpistons 02-05-2011 07:26 PM

Quote:

Originally Posted by Beemerguru (Post 15120015)
And length..eye to eye?

I will measure it and get back. dont recall

Clay Spinner 10-03-2012 01:45 PM

Wanted to bump this... Wondering what length (eye to eye) shock people are using on their 100mm extended swingarms.

adventure950 10-04-2012 05:23 AM

on my twin shocked dakar replica I am using 420mm eye to eye length shocks, these have 100mm max compression, a spring length of 272 by 50mm wide. I have tried the following spring weights 50kg/cm, 45kg/cm ad a variable 25 / 35 / 40 progressive spring. I found the 50s rock hard, the progressives to soft and the 45s are firm but allow about 70mm of suspension travel in normal road and gravel track riding ( so leaving 30 mm for harder use before bottoming out onto the rubber grommets in much harder use) and give a fairly comfortable ride that is with 10 mm of pre compression on the spring settup length. They can be wound up a bit more for luggage etc and also have 22 clicks for each of the option of high low speed compression as well as rebound. I did consider trying a shorter 200mm 45kg/cm mixed up with a 70mm 40kg/cm starter spring to see if that takes a little of the initial firmness out of the ride but have not got round to that at the moment. I am not sire of the exact working here but i assume that 2 x 70 / 45/kgcm come in at 90/ kg /cm but that would be plus or minus some equation for there being two shocks as opposed to one and associated drag / friction/ damping / acceleration and deceleration or whatever- so then its back to the boffins and the blackboard. You also have to take into acount as the longer swing arm gives much more travel and leverage on the whole back end and the therefore sits at a steeper angle ( ie 13.5 mm from centre line in the case of my bike giving 230mm travel at the axle) then the exact positioning on the swing arm of the mount is critical but you also need to take into account that changing angle of the shock absorber as it compresses and the swing arm moves up changing the angle the suspension is moving and operating in throughout the arc of travel, get it wrong and you will end up bending your shock absorber or not maximising the shock absorber in its best operating area. There will be a pay off at the extreme ends of the travel where the shock is working at a limited amount of its ideal damping range.

Clay Spinner 10-04-2012 07:23 AM

Good reply Jake. I see you've tried all kinds of settings and I'm not sure my ass is dialled in to be able to tell me what is okay...versus what is perfect. The reason for asking is that when I extend my swing arm by 100mm, I am unsure what shock length I should be using. I have an absolutely wonderful Maxton on the g/s now and something very similar on the ST. I know the ST shock is going to be too short and will need replacing but I am unsure whether or not I'll need to replace the maxton. I'm curious, a question to those who has done it... with the 100mm extension, have people changed their top and bottom bracket location for the rear shock and if so, where to/by how much. I'm also curious about the leverage ratio (also the open and closed length for that matter) for the rear with the swingarm extension... I'm sure this comes down to placement and such but If I'm looking at getting a shock built its quite important.

Rucksta 10-04-2012 07:37 AM

Quote:

Originally Posted by Clay Spinner (Post 19737907)
Wanted to bump this... Wondering what length (eye to eye) shock people are using on their 100mm extended swingarms.

Try standard length.

Maintain the std distance between swingarm pivot and lower shock mount
Reinforce the frame at the swing arm pivot and the upper shock mount
The cantilever effect of a "central" lower pivot attempts to increase the
distance between the swing arm pivot and the top shock mount.
Eventually it will.

With the same spring as you used before the extension the ride hight of the extended setup will be the same

Zero Sum.


Increasing the spring rate and / or preload will raise the ride hight.
Experiment with preload to get the ride hight you require and use that to calculate a more optimum spring rate.

This takes the trigonomertry out of the equations and reduces the process to low tech scantling on linear values.

If it doesn't work out all you've invested in is a spring.
Once you get in the ball park you can refine shock length , ride hight, spring rate & mount positions.

Have fun.

adventure950 10-04-2012 11:37 AM

I think Ruksta is right as long as you are not altering the front end too much but you do need to to take into account the length of your front forks as this sets the geometry ( ie longer forks alter the rake and trail )for instance on my bike from the bottom face of the bottom steering head bearing to the axle bolt is 71 cm (28 inch) My forks have 300mm of fork travel, had I kept the rear suspension mount in the same place as standard (I think about 34 cm) the swing arm even with added 100 mm of travel would have sat at a much lesser angle and gave much less travel ( I have not worked it out sorry) but the rear of the bike would also be sitting lower, the frame sloped back more than it should and the rake at the front slightly increased - unless of course you are using a shorter front end set up - which then starts to beg the question why are you bothering at all to do any alterations. I set my rear suspension mount points 28 cm (11inch) back from the swing arm mount ( to give a steeper drop 13.5 degrees on the swing arm and give like I said 230 mm of travel on the rear axle ( HPN push that to 240 mm of travel and 14 degrees off centre line on the shaft).Of course its all horses for courses everyone needs to do whats right for them but these figures and calculations are from my set up and seem to work well.


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