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Old 10-06-2012, 05:50 PM   #31
kaijb
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Here is what I know

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.
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Old 10-08-2012, 06:20 AM   #32
adventure950
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(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.
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Old 10-08-2012, 07:53 AM   #33
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Quote:
Originally Posted by adventure950 View Post
(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.
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Old 10-08-2012, 11:19 AM   #34
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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.
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Old 10-08-2012, 11:51 AM   #35
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Quote:
Originally Posted by LoJack View Post
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.

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Old 10-08-2012, 03:03 PM   #36
Prutser
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Quote:
Originally Posted by kaijb View Post
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.
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.
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Old 10-08-2012, 03:13 PM   #37
bgoodsoil
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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!
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Old 10-08-2012, 05:24 PM   #38
LoJack
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Quote:
Originally Posted by Airhead Wrangler View Post
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.

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!)
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Old 10-09-2012, 05:08 AM   #39
kaijb
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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,

Enjoy.
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Old 10-09-2012, 07:41 AM   #40
Rucksta
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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.
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Old 10-09-2012, 08:43 AM   #41
adventure950
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Its nice to see the points raised and have to say I agree with many of them - If I was to do my bike again with the intent of mainly off road riding I would probably go for the single side set up, I would also go for right way up forks and will probably change the forks eventually in any case as the oil seals on the usd are a pain. However my bike was built more for travelling and carrying luggage / spreading the load of the back end and strength / looks etc so the twin shock is a nice strong rear end set up which looks nice and I am going to stick with. It is i agree heavy - i did not weigh the swing arm wheel and bevel box but maybe should have however I have weighed the bike, with about 3 gallon fuel, tools battery etc and it comes in at 223kg. I had not I suppose thought of ingress of sand and crap into the bevel splines and suppose its something I will need to check for a bit more often than with a single side set up so agree that could be a potential problem but am not so sure how big a problem in real terms. I notice the latest range of gs 1200 etc run a single sider with the wheel driven on splined interface between wheel and hub - maybe another backward step then ?.Cheers for the comments Jake.
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Old 10-09-2012, 02:20 PM   #42
kaijb
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Quote:
Originally Posted by Rucksta View Post

Nice looking set up.

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.
.
The top mount is not an issue and not a weak spot historically. You can over do it with gusseting that ultimately places more stress.

You are correct on the bolt as that was a mockup shot from years ago when I just had to put that shock in place.

I have braced the frame and sub frame again mimicking the race bikes. We have one here in Australia that has done numerous Aus Safaris without failure and so I took that as as good a reference as I could hope for. If you go to the 'Racing' section here and look at the Australian Safari thread you will see the airhead raced in that again only last month. That is where I have sought most of my advice.

I am an engineer and like a lot of guys have researched every angle, read books, trolled sites to death and yet, at the end of the day, the mods were very straight forward.

Lengthen swingarm 100mm

Use two shafts to make one longer ones ensuring the same metal

Shock as described

Brace frame

Longer forks (and for another debate, I went XR650R although 50mm Extreme forks are the popular ones)

Big arse, fat as tank (and that was a global event resulting in a 40 litre alloy job that I treasure)

Replica fairing and rear guard etc (again lucky find i really should replicate)

Most time goes in sourcing. The engineering works and given we are not racing them there is no point over engineering. As an engineer and a racer I know things break regardless. It is par for the course.

Mine is a R80G/S so my whole plan has been to build a PD race replica of sorts.

If I was to go again and probably will, I would start with a R65, do it all the same but use the 1000 barrels etc on that for a narrower motor.

Enjoy.
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Old 10-09-2012, 02:42 PM   #43
Rob Farmer
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One of Richie Moores has just been sold on Ebay. Went for big money

Ebay link
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Old 10-09-2012, 03:14 PM   #44
adventure950
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One of Richie Moores has just been sold on Ebay

I notice it has no triangulation brace between the extension and the cross member - maybe not needed at 50mm extension but something I would rather have in place once the pipework has been cut and re worked. But the seller is right to buy the whole kit including the shocker etc would be a whole lot more new.
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Old 10-09-2012, 08:40 PM   #45
Rucksta
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Quote:
Originally Posted by kaijb View Post
The top mount is not an issue and not a weak spot historically. You can over do it with gusseting that ultimately places more stress.
Good to know you have engineers on the job.
Advice was offered for free you can place whatever value on it you choose.
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