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Old 11-18-08, 06:00   #1
[FLUX]
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Default Rear suspension linkage plates - anyone modified them?

Edit: For those interested in these plates, there's a group buy for them here: [ame="http://www.triumph675.net/forum/showthread.php?t=30701"]*OPEN* GB #2: '06-'08 D675 Rear Suspension Correction and/or Ride Height Plates - Triumph675.Net Forums[/ame]


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Talking about the triangular linkage plates (see attached image). The image is not taken from my bike btw.

I've often speculated that the 675's stock suspension action is too harsh, in that it rises too quickly.

For a quick explanation of what's going on, the swingarm, when the suspension is compressed, rises up. This effectively lengthens the distance between the dogbone mount point to the bike frame, and the top-triangle swingarm mount point. The pivot point between the dogbone and the triangle will move up. The further you compress the suspension, the faster it'll move up, effectively squeezing the rear spring even faster.

This is what makes the bike's rear suspension a rising rate system.

Now, the issue seems to be that you need to give a fair amount of pre-load to the 675 to get it to sit far enough down in the rising rate stroke to make the rear feel more compliant. This is exactly why we have that strange behavior with the 675's rear suspension. To actually make it feel softer to ride, you need to increase the preload. The problem with this of course is that in doing this, you're also eating into your top-out range. This reduces the rear suspension's ability to extend far enough to deal with going over rises well without the rear feeling all "wobbly", or even leaving the road when it normally shouldn't. In a nutshell, some top-out range is a good thing.

So, it would seem to me that the way to "fix" the 675's rear suspension issues would be to lengthen the distance between the two right-hand triangle pivots (the triangle-swingarm pivot, and the triangle-dogbone pivot). This will reduce the rate at which the suspension will rise. We would also need to lengthen the dogbone-triangle pivot and triangle-shock pivot distance correspondingly.

I need to pull out the ruler and do the proper trigonometry math, but it would appear that in leaving the top two triangle pivots alone, but dropping down the dogbone pivot about 2mm along the arc of the dogbone swing should be pretty close to what's needed to soften the action of the rear suspension without altering the ride height of the bike. This should allow for a wider range of supple action before the rate rises and becomes harsh near the top of the stroke.

Anyone messed around with anything like this before? Experiences?
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Old 11-18-08, 18:03   #2
ggillies
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[FLUX] Lad.... strangely enough, my local Triumph dealership here in SF has a set that they had made as a test... billet aluminium, for sale at $59.00 US... They can have them made in almost any length needed. I almost bought them for my Bike since it is oing to be track only next year. My debate is whether to try them (they are cheap enough not to matter) or whether to try the Ohlins/Penske rear shock route, or BOTH...

What do you think?

I could try to get a picture and some specs from Munroe Motors the next time I'm down there...



P.S. my Stator is fried.... waiting for a replacement.
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Old 11-18-08, 18:33   #3
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Thanks ggillies. I had figured that someone else out there must've been thinking down the same lines.

I'd say to do both. As in, shock + linkage plates. The reason being is that even if you change the rear shock to a quality item, it's still going to have to work hard to soften the rate's action out. Better to get the action sorted first, IMO, and on this project 675 that I've just bought, that's what I intend to try first.

So yeah, I'm definitely interested in those plates mate. Some details on the specs of them would be handy, otherwise I've got some local machine shop guys here who could make me up a few different sets to try out.

Will try to get the trig done for it in the next 24hrs to get a good idea of the required added length needed.
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Old 11-18-08, 19:20   #4
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I have the Aella Casuno low down plates I did this cause Im short 5'5" I see you have STOCK spring change it out for a race tech one for your weight it helped ALOT with the harshness of the rear

http://www.triumph675.net/forum/show...1.html?t=25161
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Old 11-18-08, 19:22   #5
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It's also on the list of things to do for me...

If you get something worked out, I'd be interested to see what you come up with...(but I'll still end up doing the math myself)

And you don't need to alter the shock to swingarm distances (that would alter spring rate overall), but only the dogbone length and relative position of the dogbone pivot (on triangle) if you want to maintain the ride height.

Or longer dogbone and ride height adjustable shock for a "quick fix"

Last edited by ccsracer989; 11-18-08 at 19:30. Reason: Added ideas
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Old 11-19-08, 01:05   #6
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A longer dogbone would be the other option, but the issue with that is that it will alter your ride height, so yes, you would need to then use a ride height adjustable shock.

Thing is though is that lengthening the dogbone will also alter the spring rate, so I'm unsure where you're going with that one if you believe that by lengthening the dogbone that the rising rate will remain the same. It won't.

So, the very issue here is to alter the rising rate. The problem is that it is too harsh and rises too quickly. That IS the issue that needs to be fixed.

Just got done with all my math. Dug into ye olde trig from days of yore, and some quick application of Perl to simulate the 675's rear suspension action.

In summary, from full extension to full compression the spring rate rises about 2.3x. ie. you have to push 2.3x as hard to compress the rear by 1mm when its (almost) fully compressed as opposed to when it's fully extended. There's a nice gentle rising rate plateau for about the first half of the swingarm movement range, and then it starts to asymptote up strongly in the second half of the range, which is what gives it its harshness, and why it seems to ride better if you give it plenty of preload to try to keep it in the upper half of the travel.

On the triangle, the distance from the swingarm pivot to the dogbone pivot is 72mm as stock. The distance from the dogbone pivot to the shock pivot is 66mm.

From what I'm working out with the suspension calculator I wrote, I'm thinking that with the same stock dogbone length, these two triangle distances need to both be increased by around 5mm for a good compromise. A 3mm increase would still be a touch harsh, and a 7mm increase would suit lighter riders and/or deal with bumpy roads better.

I know it reduces the spring rate, but that's the point. I don't reckon even a 230lb rider with gear could bottom out the rear even if they used the pillion seat as a trampoline. It just gets too harsh/stiff before it gets that far.
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Old 11-19-08, 01:39   #7
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Quote:
Originally Posted by [FLUX] View Post
Anyone messed around with anything like this before? Experiences?
hmmm VERY interesting read.

Ive lengthened my dogbone as you might know (by 1.25"), because I WANTED to alter the ride height (make it shorter). What it did do though was soften the suspension (by pulling the spring out), so it then required an increase in preload (which again increases the ride height slightly). What it did was force the spring into the more subtle part of its travel, but I agree that it also diminishes the range of motion somewhat.

OK so to maintain ride height AND set the spring up so it doesn't need all that preload you'd just have to either put a stiffer spring in...

OR

If you were to make the triangle 'taller' though, wouldn't you be decreasing the spring's range of motion during compression- because you'd have less leverage?

P.S. I didnt even know you can get custom triangles!! This might be a better way for those of us actual wanting some ride-height reduction...

Here's a pic of my lowered setup- notice the angle between the dogbone and the triangle is less than the stock setup. Sorry if this just confuses things.
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Old 11-19-08, 02:02   #8
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Flux , i saw a set of plates on ebay the other day , it was from a german seller and once i looked i lost interest as i dont spruken de deutsch. they looked a little odd so i thought they may be playing with ride height for all those suffering from ducks diesease . i ll try to find the seller again and try to translate to get an idea of what they were for.


p.s. your street naked bike looks great , great starting point i mean
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Old 11-19-08, 05:08   #9
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i found it flux http://cgi.ebay.com.au/Heckhoeherleg...3A1|240%3A1318

it is a set of triangles i guess to lower the bike 25mm (1 inch). not a bad price ??? cough . local engineer could knock some out of aluminum cheaper i reckon.

p.s. if your street tripling the new red addition i saw a second had striple loom on ebay , but it was in italy , may save a shitload of wiring mods just ask canyondancer if he has any hair left ????
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so many roads,twists and turns. Where do i start? I know , on a D 675


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NAAH i think i like the honda better ????
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Thought you were gunna tame the big duke
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Old 11-19-08, 07:22   #10
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Thanks for that Tim. Am not really interested in anything that raises or lowers the ride height though.

For purposes of clarification, let's apply some labels to the triangle distances we're talking about here. All distances are center-of-hole distances.

A = distance between shock pivot and swingarm pivot.
B = distance between shock pivot, and dogbone pivot
C = distance between dogbone pivot, and swingarm pivot

The stock distances are: A = 69mm, B = 66mm, C = 72mm

Assuming that we leave the dogbone length alone.

If we want to lower the ride height but leave the spring rate alone, we would decrease B and lengthen A to compensate. C is left alone.

If we want to increase the ride height, but keep the same spring rate, we would increase B, and decrease A minorly to compensate (to keep the shock moving along the same path)

If we want to decrease the rising rate, but keep the same ride height, we keep A constant, and increase C. B is increased too, by as much as is required to ensure that the shock remains at the same height. Due to the decreased spring rate though, some preload may need to be added to compensate, but the overall spring rate will be more supple.

Thinking on it more, and analysing the rate ratios, it looks to me like an ideal sort of linkage plate would be:

A = 69mm
B = 73mm
C = 79mm

That should keep the ride height about the same, without needing to add more preload (or maybe just a small amount). That will also give about an extra 15mm of swingarm travel before the action starts to rise rapidly again, with the action in the main working range being more supple.

I reckon that'd go a long, long way to fixing the rear action issues for most users, and heck, even for the race-track. Racers will just choose rear spring rates as they require, but even they would still appreciate a more supple rising rate response, rather than being kicked off bumps.
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