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Old 10-09-2012, 07:53 PM   #46
Airhead Wrangler
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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.

I haven't broken one yet, but I'm guessing they don't go to all this trouble for nothing.





Notice all welds are moved to the neutral axis of the tube and spread over a longer length.
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R80ST Gets The HPN Treatment
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Old 10-09-2012, 08:19 PM   #47
kaijb
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Quote:
Originally Posted by Airhead Wrangler View Post
I haven't broken one yet, but I'm guessing they don't go to all this trouble for nothing.





Notice all welds are moved to the neutral axis of the tube and spread over a longer length.

I see little difference in the two and no greater srength. It is essentially the same although relocated. It is always going to be the case that some engineers will do it their way because in their opinion it is better. Mostly some of the things i do are as much ego as well because I am happier in myself with the result.

Something that I found with this project is that the internet is full of opinion, very little experience and even less fact. Most of it is offered as "advice" and there is a reason those who are actually doing it stay out of the public arena because they have some IP and can make a living out of it and all power to them.

I followed anothers experience and paid for his expertise. I have not seen a broken top mount from the mod but I am sure it has happened. If you fully compress the shock and look at the angles, mounting on a radius makes good sense. I can see how relocating it might help but we then come back to just how often the rider is using the full extent of the lever action.

I understand the known 'weak spot' is around the swing arm pivot so the bracing here is important. There are two schools of thought on if it should be fully welded or just above and below.

The rest is essentially around flex and frames need to flex. It can be an art more than a science at times which is why opinions differ and some engineers will do it a bit differently.

I am interested in what other people are doing and why. The image here looks like a HPN job and and patch they run behind the mount looks to me like it would allow moisture to build behind it. I understand why they would not weld all the way but I would expect that to rust. I am sure they have a very good reason.

Nice welds.
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Old 10-09-2012, 08:32 PM   #48
Airhead Wrangler
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Quote:
Originally Posted by kaijb View Post
I see little difference in the two and no greater srength.
Take another look. The mount is taller and is welded across a longer length. Then it's not even welded directly to the tube. It's welded to a plate wrapped around the tube which is then stitched up both sides of the tube along it's neutral axis rather than getting welded to the most stressed side of the tube. Furthermore, by not moving the lower shock mount back when you lengthened the swingarm you're effectively multiplying the force applied to the stock upper shock mount. So, effectively you're taking a stock frame and making it weaker - granted, not at a common point of failure, but still not something I'd want to try.
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R80ST Gets The HPN Treatment
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Current rides: HPN #834, '93 R100GSPD "red rocket", '73 R75/5 Toaster mongrel, '80 Ducati Pantah 500SL, '92 DR350, '67 Honda SS50, '80 Honda Chaly.

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Old 10-09-2012, 09:05 PM   #49
kaijb
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Quote:
Originally Posted by Airhead Wrangler View Post
Take another look. The mount is taller and is welded across a longer length. Then it's not even welded directly to the tube. It's welded to a plate wrapped around the tube which is then stitched up both sides of the tube along it's neutral axis rather than getting welded to the most stressed side of the tube. Furthermore, by not moving the lower shock mount back when you lengthened the swingarm you're effectively multiplying the force applied to the stock upper shock mount. So, effectively you're taking a stock frame and making it weaker - granted, not at a common point of failure, but still not something I'd want to try.

I did note al of those things you mention. The plate I call a 'patch' is where I see the moisture issue. I feel it would sweat.

The mount is taller but i suspect this is so to allow the rear subframe to mount at its normal position. They have lowered the top shock mount and this could be for any number of reasons I imagine.

Ultimately though, the welds are not moved becuse the mount is not cast into that plate. It is weled exactly the same. This is why i see little difference. Maybe HPN do not want to weld over the same place twice as the metal recrystalises with the heat. It could be that by utilising the plate, you cannot put the bracket in the wrong place becuase the profile forces you to follow the contour...It would make sense if they sell the mounts commercially.

kaijb screwed with this post 10-09-2012 at 09:28 PM
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Old 10-10-2012, 08:57 AM   #50
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If you check out a paralever top mount you will see a similar yet different approach than the HPN solution using what looks
(to the non engineer) like a forged component that adresses the re-crystalisation, water trapping & nuetral axis issues

There are other solutions to the same problem develped either independantly or as adaptations of existing solutions.
I wonder why so much R&D has been expended on a problem that doesn't historicly exist.
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Old 10-10-2012, 09:16 AM   #51
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Quote:
Originally Posted by Airhead Wrangler View Post
Furthermore, by not moving the lower shock mount back when you lengthened the swingarm you're effectively multiplying the force applied to the stock upper shock mount. So, effectively you're taking a stock frame and making it weaker - granted, not at a common point of failure, but still not something I'd want to try.
For the benefit of anyone who is contemplating moving the lower mount forward, as descibed previously please take note of this. AW is right the cantilever forces acting on the frame are increased as the amount of swing arm behind the lower mount compared to forward of it increases. Frequency of failure will depend on configuration and usage (abusage?).

I've seen 'stock' G/Ss with BM317 Ohlins crack frame tubes above the top mount. Not common but not rare either.
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Old 10-10-2012, 10:57 AM   #52
Airhead Wrangler
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Quote:
Originally Posted by Rucksta View Post
I've seen 'stock' G/Ss with BM317 Ohlins crack frame tubes above the top mount. Not common but not rare either.
Or also right above the swingarm pivot like this (stock length swingarm):

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R80ST Gets The HPN Treatment
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Current rides: HPN #834, '93 R100GSPD "red rocket", '73 R75/5 Toaster mongrel, '80 Ducati Pantah 500SL, '92 DR350, '67 Honda SS50, '80 Honda Chaly.
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Old 10-10-2012, 11:12 AM   #53
Airhead Wrangler
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Quote:
Originally Posted by kaijb View Post
Ultimately though, the welds are not moved becuse the mount is not cast into that plate. It is weled exactly the same. This is why i see little difference. Maybe HPN do not want to weld over the same place twice as the metal recrystalises with the heat. It could be that by utilising the plate, you cannot put the bracket in the wrong place becuase the profile forces you to follow the contour...It would make sense if they sell the mounts commercially.
The outside of the the bend in the tube is put under tension when the tube bends with supension inputs. The section of the tube with the shock mount welded directly to it will be less elastic than the portion immediately below it leading to a stress concentration right at the bottom of the shock mount where the bead ends. This point would be a likely spot for a crack to start. The HPN modified mount moves the connection TO THE TUBE to the neutral axis and minimizes stress concentrations on the face of the tube. So although the connection of the shock mount is more or less in the same spot, the connection to the tube itself and it's effect on the most stressed portion of the tube wall is entirely different. The corrosion issue had occured to me, but the gap is so small that it was easily sealed off when the frame was powder coated. I expect that a little bit of movement in use could cause the powdercoat to crack in that spot, but again the space is so tight I could easily rinse out any corrosion with an acid wash and fill the gap with black silicone.
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R80ST Gets The HPN Treatment
Seattle to TDF on an airhead

Current rides: HPN #834, '93 R100GSPD "red rocket", '73 R75/5 Toaster mongrel, '80 Ducati Pantah 500SL, '92 DR350, '67 Honda SS50, '80 Honda Chaly.
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Old 10-10-2012, 12:12 PM   #54
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AW .... nice photos of the top mount. Have they moved the mount on the swing arm at all? I'd also be interested in hearing about your shock measurement.. eye to eye and any shock specs that you have. I want to get a maxton built but need a few specifics... very keen to get this right the first time! Many thanks for everyones input as it is a very interesting read.
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Old 10-10-2012, 01:09 PM   #55
Airhead Wrangler
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Quote:
Originally Posted by Clay Spinner View Post
AW .... nice photos of the top mount. Have they moved the mount on the swing arm at all? I'd also be interested in hearing about your shock measurement.. eye to eye and any shock specs that you have. I want to get a maxton built but need a few specifics... very keen to get this right the first time! Many thanks for everyones input as it is a very interesting read.
Unfortunately, I'm in mexico and my bike is in the US, so I can't give you measurements. I don't have the specs on the shock either so, I'm afraid I can't help you much until my next trip home. The lower mount looks like this. Similar, but not the SAME exact position as original. Also, it is no longer centered laterally on the swingarm. It's moved farther away from the wheel to give the coil a bit more space off the tire.







Notice also the weld where they fitted the extra length to the arm. They grind it flat and also further pinch in the portion of the swingarm to create a flat spot to give a bit more tire clearance. This is one small detail I'd never seen mentioned anywhere before. I didn't even notice it myself until I assembled the bike. The glare makes it a bit more obvious here:



Just be warned that if you start exploring the limits of travel, HPN pretty much has their design pushed to the limit. Klaus warned me several times in stern German fashion not to adjust the ride height of the shock at all even though it is adjustable. I am not to touch it. Ever. Shorten it and the swingarm will swing too high when the suspension bottoms out. Longer and it will be too low at top out.
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R80ST Gets The HPN Treatment
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Current rides: HPN #834, '93 R100GSPD "red rocket", '73 R75/5 Toaster mongrel, '80 Ducati Pantah 500SL, '92 DR350, '67 Honda SS50, '80 Honda Chaly.

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Old 10-10-2012, 01:19 PM   #56
Clay Spinner
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Very well spotted. Its the minor details like this that makes the difference I guess. I can work out a good estimate of the shock from the photos I think. I'll have a crack and post up here and see what people think. Many many thanks for your help. Just got a set of wp4860s, 1 year old for 84 quid... very chuffed!
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Old 10-10-2012, 01:44 PM   #57
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Quote:
Originally Posted by Airhead Wrangler View Post
The outside of the the bend in the tube is put under tension when the tube bends with supension inputs. The section of the tube with the shock mount welded directly to it will be less elastic than the portion immediately below it leading to a stress concentration right at the bottom of the shock mount where the bead ends. This point would be a likely spot for a crack to start. The HPN modified mount moves the connection TO THE TUBE to the neutral axis and minimizes stress concentrations on the face of the tube. So although the connection of the shock mount is more or less in the same spot, the connection to the tube itself and it's effect on the most stressed portion of the tube wall is entirely different. The corrosion issue had occured to me, but the gap is so small that it was easily sealed off when the frame was powder coated. I expect that a little bit of movement in use could cause the powdercoat to crack in that spot, but again the space is so tight I could easily rinse out any corrosion with an acid wash and fill the gap with black silicone.
I do follow your reasoning and clearly it works otherwise they wouldn't be doing it. Watch me grimace when my top mount cracks......

I see that the plate is not welded top and bottom and this would be to allow movement/flex and this is where I am not convinced. Based on their engineering logic, a larger mount (shaped like an original) that meets the frame where that plate welds would make good sense. Anyone making their own bracing might want to consider that.
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Old 10-10-2012, 01:48 PM   #58
Airhead Wrangler
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Quote:
Originally Posted by Clay Spinner View Post
Very well spotted. Its the minor details like this that makes the difference I guess. I can work out a good estimate of the shock from the photos I think. I'll have a crack and post up here and see what people think. Many many thanks for your help. Just got a set of wp4860s, 1 year old for 84 quid... very chuffed!
I just remembered that German ebay until very recently was littered with ads from Wilbers for shocks to fit HPN models. They had one model made to fit the HPN Adventure which is one of their monolever models. You might try contacting WIlbers in Germany to see if they have the specs you need.
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R80ST Gets The HPN Treatment
Seattle to TDF on an airhead

Current rides: HPN #834, '93 R100GSPD "red rocket", '73 R75/5 Toaster mongrel, '80 Ducati Pantah 500SL, '92 DR350, '67 Honda SS50, '80 Honda Chaly.
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Old 10-10-2012, 02:02 PM   #59
Airhead Wrangler
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Quote:
Originally Posted by kaijb View Post
Based on their engineering logic, a larger mount (shaped like an original) that meets the frame where that plate welds would make good sense. Anyone making their own bracing might want to consider that.
Agreed. That would be a bit cleaner solution and certainly easier to execute. The later paralever GSs had exactly that.

G/S (for comparison)


Later GS (photo doesn't show it very well though):



Until we break a statistically significant number of frames, I guess we just have to take all the theory with big grain of salt. To some extent it always has to come down to faith in intuition because nobody has enough experience to be an authority.
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R80ST Gets The HPN Treatment
Seattle to TDF on an airhead

Current rides: HPN #834, '93 R100GSPD "red rocket", '73 R75/5 Toaster mongrel, '80 Ducati Pantah 500SL, '92 DR350, '67 Honda SS50, '80 Honda Chaly.
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Old 10-10-2012, 04:48 PM   #60
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Thanx to all for this lively discussion. I have an '86 R80gs and I am so wanting to Dakar HPN replica the bike. Thanx again
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