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Discussion in 'Airheads' started by Beemerguru, Feb 4, 2011.
I'm not using a paralever. So far no explosions
Hi Prutser, when I did the calculations regarding the driveshaft length and the amount of travel tha Maximum on a 100mm extension was theoretically 260mm doing ( a rough bit of working out 244 at 50mm extension) really would be the maximum travel so to achieve 248 you need to be carefull your not pushing two areas too far. Firstly the uj will be running very tight at those angles, and even though these are extremes if the shaft is revolving at any speed the wear and stress on the joint will be very high. Also the internal dimensions of the shaft tube mixed up with the offset pivot between the uj and the swing arm pivot mean the more extreme the angle the closer the shaft gets to the edge of the internal area of the tube which in turn puts pressure on the UJ and swing arm pivots. This clearance I believe runs out before the above measurements. Which would indicate your drive shaft could well be rubbing on the internal metal of the tube. Both these factors are part of the reason why HPN run a maximum of 240 travel on a 100mm extension and take into account they also machine the internal area of the tube as well as machine part of the the bevel box cup end of the shaft to lighten and reduce the diameter of the shaft which may give them a slight advantage over the standard sizes. I wanted to go a bit safer again and so reduced that travel to 234mm travel in real terms but doubt it would ever get beyond 230. I am not being critical here at all just hope you are aware of the problems that may be encountered and ( i hope are not a problem for you) if your informed you can prevent a failure of the components or damage to your bike or even worse yourself. Jake.
Thank you Jake,
I'm am aware of those things and using the thin drive shaft.(not the one with the demper)
When I measured the clearance there was still enough room. And not to much (extra) stress on the u joints.
The shock I'm using is even shorter than what I could actually fit on the bike.
I have measured a lot of those swingarms and checked the clearances. Al the bikes had a different amount of travel before something would touch on the inside or the u-joint would be under to much stress.
At the moment I'm putting a bike together with the same length swing arm. And this one will never be able to make the same wheel travel because of the swing and shaft combi...!
I think I was just lucky with the shaft and the swingarm combination i'm using.
What I did do is give the bike more sag. To keep the monolever from jacking(lifting) until the shock is fully extended.
But thanks again for all the data. That might be helpful to a lot of us.
How are you checking clearance inside with it assembled? Snaking a bore scope in from the trans side?
Hey Bas, ta for the reply and glad you did not take it the wrong way as I know some folk could easily have mis-interepreted my way of putting it across. I reckon your doing really well I am pretty impressed that you managed to get a few various parts to mix and match so to speak - its interesting as it starts to show the big tolerance variations of the original manufactured parts from BMW.
What adventure950 says is correct and the measurements ring true with my experience..
With the standard rear end geometry, a standard(ish) shock with 90mm stroke
and a 100mmm swing arm extension resulted in a 60mm increase in travel or 230mm total measured at the rear wheel. (no bump stop)
Increasing the drive shaft angle particularly the down angle (range of arc) will result in more travel.
A longer stroke shock will allow taking advantage of the longer travel and clearnace problems do arise
There are slightly different approaches to the solution of clearnace and shock length while still retaining
a cush drive shaft rather than going to the solid shaft prefered by HPN.
Hogging out extra metal from where the U- Joint / shaft passes through the curve in the swing arm is a help.
You can really go to town with the die grinder and remove everything you can get to.
This solves the problem with the U-Joint grinding.
The next point of grinding is the cush drive assembly.
A beveled edge on the inboard end of the cush drive spring helps clearance at the rear of the tube.
Preloading the spring with one or two 42mm hardened washers again on the inboard end of the cush
(GS front end has a couple) moves the fouling point and carefull alignment of the swing arm position in the
frame (left / right) lets you find a sweet spot.
The way the swing arm is extended will have an influence on how much travel can be achieved.
Extending straight back will produce less max travel.
Extending back and to the left along the existing line of the swing arm will give more clearance for the cush drive.
Coming back and to the left allows you to run a wider rear tyre (140) but increases the rear wheel offset.
Dave Kellet the frame chiropractor tweaked the offset for me so the wheels align.
How he does it is his secret.
At this point you are chasing the last (practical) 5-10mm of travel.
Maximum shaft angle can be determined by experiment you will feel the point where it grinds and whatever grinds
will leave a witness mark visible on disassembly - modify, assemble, test, disassemble inspect, modify recursively.
The lower shock mount can moved forward approx 20mm to achive max travel on a 90mm stroke shock.
Removal of part of the top lip of the rear footpeg mounting plate is required to accomodate the spring.
This change in geometry maximised the down angle and limits the up angle. (shock bottoms)
Raising the subframe was no longer required.
The angle formed between the swing arm axis and the shock body is now closer to 90 degrees at
full compression resulting in a bit more progression to a stiffer effective spring rate on the big jumps.
I'm back to a std length swing arm with 210 mm travel.
The 100mm extnsion resulted in 270 mm.
I have 80-100,000 Kms of hard use on a driveshaft running at this (extreme?) angle.
No problems with wear in the U-Joint, bell coupling or the cush drive so far.
I'm using a 680 lbs/inch spring.
Std was 390 lbs/inch.
I'm 250lbs + riding gear
For me the whole rationale of jacking up the rear was to level the ride hight with a replacement front end.
I'm finding I use only 225-230 mm travel up front of the available 270mm.
I do use all of the available rear travel and would like to have a little more in reserve for those landings when I don't want to lead with the front.
With the 270/270 suspension setup to use all the available travel the changes in geometry could be extreme requiring a very smooth riding style to control the beast especially at speed.
The point of the suspension upgrade was to allow me to chuck the bike around.
I'm thinking a 250/250 setup on a 75mm extended swing arm,a slightly steeper front end (not so long forks) with a triple clamp offset in the 28-32 mm range would produce a great all round package,
I hope these results and observations help you achieve your own solution that matches your needs and capabilities.
As they say in the infomercial - results may vary.
I used a frame that I did cut in half ones to shorten a swingarm. And now used it to mount a swingarm and gearbox and see wat kind of room there was for the shaft to move around. This way I could have a good look.
So I did say "measured" but it was more "looked" at the clearances..
Have seen some bikes that had a lot of machining being done on the shaft and the inside of the swingarm just like Rucksta is explaining about. Done by The Boxertoko in The Netherlands.
For riding off road and not leaving the country to far I wouldn't mind the machined bits.
But I tried to build something with the weak parts like the u-joint original. So when necessary I can just order that part nr.and replace it.
Thanks guy's for explaning and sharing all the info.
Can you share your swing arm shortening experience?
This is all really useful stuff.... though its not my thread I for one certainly appreciate the time people have spent writing in it. It'll take me a while before it all starts to make sense absolute sense but it is interesting to hear everyone's experiences.
I don't mind to share that. But it might belong in a Hack thread.
I shortened the swingarm for a R100 engine in a R69S frame.
The original 69S drive shaft is expensive and scarce. And I don't want to ruin original classic parts.(frame was already ruined by the previous owner) The mono swing costs 50 with the shaft in it.
Thats why I shortened a mono swingarm so the shaft sticks out enough to reach the gearbox.
When I extensively researched my build I learnt all of this stuff and given my anal lifestyle made extensive notes. I was going to run twin shocks but was overwhelmingly assured that it is not successful, does not last and suffers from spline issues and hub bearings flogging out continuously.
When selecting a shock absorber, the 'piggy back' style is superior in that it hold more oil, lasts longer and is cheaper to rebuild.
YSS make a good shock as do Ohlins.
The ohlins range to suit the G/S are as follows
BM836 for years 1989-94 is 495mm eye to eye with a 80mm stroke
BM541 for years 1989-94 is 498mm eye to eye with a 78.5mm stroke
BM317 for years 1981-88 is 361mm eye to eye with 89mm stroke
for the 100mm swingarm extension on a R80G/S you need to order the Ohlins BM317 with the standard shim stack BUT it requires a 20mm longer stroke and 20mm longer eye to eye. Spring stays standard.
For the mod to work the suspension is actually softer than you might usually think. It is the nature of the beast and the rear end but it works a treat.
(I was going to run twin shocks but was overwhelmingly assured that it is not successful, does not last and suffers from spline issues and hub bearings flogging out continuously.)
Kaijb I am interested in your comment above - I am not doubting nor agreeing with it but would love to know the basis of the statement. HPN sell the twin shock Dakar as the Indestrucatable HPn and the one with the extremelly durable double ( twin shock ) swing arm under all conditions. I have also spoke with many engineers and a few racers of classic BMWs who again rate the twin shock bevel box as the least complicated, longest lasting and strongest / most reliable set up. On that basis I built a twin shock set up. Its disadvantage is weight - unsprung weight and maybe slower suspension reaction in responding to changing conditions, all part of that classic bike feel maybe. But I find it hard to figure out how such a strong and balanced set up would be less reliable the wheel is supported on its own bearings on an axle so taking the stress off the final drive bearing and internals as all they have to do is turn the wheel after turning the power through 90 degrees ie a very simple diff unlike single siders who have to support the wheel weight and all the stresses from the road through the bevel box bearing. Just asking as your the very first person I have ever heard of discounting the twin shock back end over the mono or paralever - BMW back end on the grounds of reliability. Interesting.
I hope you don't mind me replying with some comment.have been thinking about a twinshock setup to.
But a few things i liked a lot more about the mono. One thing is how easy it is to swap wheels.(i'm using different wheels with different types of tires)
Using 2 shocks is more expensive and has more friction. + the prefered bolt on hub insted of the spline ones.
For off road with al the mud water and sand getting into everything the splines might wear quite fast.
As for unsprung weight, does any body know how much difference there is between all the options ?
I have been weighing some wheels ,swing arms and final drives last week to compare.
How about bracing the swing arm? I would think it will weigh less than a twin shock, certainly not more, and you'll then have less friction with only one shock.
Interestingly, HPN shows two single shock bikes on their gallery page that are only partially braced:
I can only assume they were not intended for the type of riding their Dakar bikes are built for, but I've been curious about these.
Take another look. Those frames aren't "partially braced." They're completely custom frames made from CroMo. They're much stronger and lighter than stock frames. Second, not all the dakar bikes were twin shock models. The '82 factory bikes were extended monolevers. Too bad they DNFed with gearbox mechanicals.
When you say "the suspension is actually softer" thats not that strange IMO!!
You didn't mention where the bottom shock mount is on the swing arm.(distance from pivot to shock mount)
The longer swing will change the wheel/shock travel ratio.(depending on where the shock is mounted)
The shim stack is based on that ratio.So is the spring ! I'm not saying it couldn't work !!!
The piston inside the shock will travel on a different speed than it was meant to do in the original setting.
There for the low and high speed oil flow could feel really different.
Using the same spring could be kind of tricky. Did you mean same spring rate or the exact same spring as on the shorter shock ?
Adding 20 mm of travel in the shock does mean the spring must be able to make THAT amount of travel before it blocks!!!
The ratio change with the softer feeling might people want to add more pre-load to the spring.
Than there will be even less usable travel left in the spring before it blocks.(block is compress the spring until all the coils touch each other so it forms a metal tube. And that could be a bit hard to compress even further.)
I'm not trying to judge your setup !! I'm sure its great. It's all so personal what people prefer !!!!
Maybe this pile of letters can help someone.
I'm not mainly interested in a swing arm extension for more wheel travel. I want a longer wheel base. If I could get my boxes between my axles I think it'd take the twisties loaded down better. The wheelbase on the G/S is crazy short compared to almost anything out there. It makes wheelies easy though
Thanks for the info!
I know the frames were one off's, and I was referring to the swing arms, which is what this thread is about. It's pretty clear they are not stock components. I just thought it was interesting that the swing arm bracing didn't wrap all the way around to the left side of the axle. The two picks I linked were just what HPN had on their gallery page for non-mono or paralever single shock bikes. I'm sure there is plenty more out there like the one you have there (which is a very rad looking!)
I see little point entering a debate about why twin shocks are more prone to wear. I simply shared what I learnt from those who raced BMW airheads and had done the conversion numerous times and settled on a formula that worked. I was intent on running twin shocks but if I was honest with myself I was after a 'look'.
Similarly i wanten inverted forks and purchased some but again learnt that inverted shocks, while offering many benefits, also blow seals with regular monotony and when doing so, lose their oil rapidly by virtue of the design.
So I changed back to a conventional design and took the advice of those who knew better and offered advice for free but had no time for debate. take it or do your own thing.
I wanted longevity and I wanted reliability and I wanted to limp home if needed. So i hope the logic assists those questions.
As for the mounting points of the shock, it is 155mm from the diff flange. That is where the alloy diff bolts to the steel swing arm.
As for the specs of the shock, again my source was an Ohlins tech, who worked for Ohlins and also raced airheads in safaris and engineered his own bikes with the same mod being a 100mm or 4" extension.
Who am I to question him? He builds them, he races them, he was also a suspension engineer and he said to do what I wrote in answer to the question........I did it. I was not about to debate his free advice on forks, shocks and mods,
Nice looking set up.
If you have not reached the point where the bell of the swing arm (where the rubber boot attaches)
is fouling the upper cross member of the frame then you haven't approached the limit of angle the U-joint can handle.
Squashing the cross member will get you there.
With the pictured setup the weak spot is the top shock mount.
Consider extra bracing in the area where the mount joins the frame as is curves toward the back bone.
Gussetts in the angles where the rear downtubes meet the backbone will help reduce some of the imbalences the offset single shock imparts on the frame.
(one of the advantages of twin shock)
The bolt holding the subframe stays looks a bit soft.
Hardened steel is the go there to prevent shearing if you give the setup a punishment with a load on.
The other subframe bolts should be of hardend variety also.