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just sayin but you are getting ahead of yurself
you need the shocks now to fabricate your suspension geometry
if the shocks end up to be vertical you arrive at having a falling rate
which means the swingarm gains mechanical adbvantage the further the suspension compresses
its the opposite of good
laying them down gets you a slight rising rate\ swingarm looses leverage
i am trying to help its a mistake to keep building without your chosen suspension units
you could actyually figure what springs you n eed with them now as well
 

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Discussion Starter #342
just sayin but you are getting ahead of yurself
you need the shocks now to fabricate your suspension geometry
if the shocks end up to be vertical you arrive at having a falling rate
which means the swingarm gains mechanical adbvantage the further the suspension compresses
its the opposite of good
laying them down gets you a slight rising rate\ swingarm looses leverage
i am trying to help its a mistake to keep building without your chosen suspension units
you could actyually figure what springs you n eed with them now as well
I have the swingarm and frame suspension mounts approximately equal distance from the swingarm pivot, to provide a consistent angle and negligible change in leverage/rate, regardless of suspension travel.
 

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I have the swingarm and frame suspension mounts approximately equal distance from the swingarm pivot, to provide a consistent angle and negligible change in leverage/rate, regardless of suspension travel.
but that does not make good suspension
if at all possible the change in rate ,rising,is exactly what allows sdlightly softer springing which gives better compliance on small bumps
helps maintain traction on corner exit ,is more comfortable and resists bottoming better
yioun have the oportunity to dom it right ,now that you are aware
as you have it the shock mount on that tube and gusset ,relying solely on the gusset and its attachment to the vertical tube is just totally contrary to good design practice ,
 

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This is what XB was getting at, but illustrated.

You have a range of angles to lay the shocks at so that you have a progressive rate suspension without needing progressive springs/damping.

Basically, as the rear suspension compresses, the angle of the shocks vs the swingarm need to approach 90 degrees as that is where the shocks will be applying their full force against the direction travel.

If the angle is moving away from 90 degrees you end up with a regressive suspension. No good.

Of course the longer the SA is and the longer the shocks are the changes in angles are reduced for the same amount of travel.
 

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Discussion Starter #345 (Edited)
This is what XB was getting at, but illustrated.

You have a range of angles to lay the shocks at so that you have a progressive rate suspension without needing progressive springs/damping.

Basically, as the rear suspension compresses, the angle of the shocks vs the swingarm need to approach 90 degrees as that is where the shocks will be applying their full force against the direction travel.

If the angle is moving away from 90 degrees you end up with a regressive suspension. No good.

Of course the longer the SA is and the longer the shocks are the changes in angles are reduced for the same amount of travel.
Thanks for that illustration. I'm aware of what you guys are saying. To further clarify what I was trying to say but expressing very poorly: when the top and bottom suspension mounts are equidistant from the swingarm pivot, it puts the rate right in the center of the "progressive" area in the illustration you posted. I added some arcs to the diagram to illustrate this:



I have actually tried to do an appropriate amount of research on this topic, and while I admit that my head can get to spinning pretty easily, I think I have a decent layman's grasp of the concepts. I created this diagram that compares the change in the shock length, from 30 through 90 degrees included angle, when the bottom of the shock is mounted either 60%, EQUAL, or 140% as far from the swingarm pivot as the upper shock mount. It confirmed for me that the equal distance provides the most favorable rate through the entire range.

 

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Discussion Starter #349 (Edited)
I want to sincerely thank XB33BSA for calling out my initial shock mounts as crap. They were, for all the reasons he mentioned. Also, I remembered that the overall shape of the gusset should be a concave curve, to avoid concentrating undo stress on the thinner-walled upper frame tube.

IMG00004.GIF

Challenging me to redo my design helped me remember and change that. The crowd here gets a lot of criticism for being blunt and not putting up with stupid crap, but that's why I'm here. I cannot tell you how many times members of this forum have forced me to recognize the shortcomings of my work, and do (re-do) things the right way. When criticism is accurate and constructive, it's worth more than gold.
 

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thanks for recognising my intent is to help out. another design aspect is the swingarm angle and i ask because your drawings seem to indicate a level swingarm with no downward angle
i raced a pursang back in the era of the supension revolution
every year the bikes had more wheel travel i modified my model 121 bultaco 360 pursang twice to increase wheel travel
bultacos have an issue with excess chain slack variation because of the long distance pivot to c/s
you want to have the suspension travel both below and above the 3 axis line
if all or most of the travel is above the 3 axis line ,as in what you will have with a level swinger angle ,the amount of slack becomes an issue that is difficult to address
have you got a steering head angle worked out with the forks and front wheel you have chosen ?
because that will detrermine what the swingarm angle will be
ie the static ride height both front and rear suspension topped out
with both ends topped out and the machine on a level surface with the final tire choice is where you set the steer tube angle
 

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Discussion Starter #352 (Edited)
thanks for recognising my intent is to help out. another design aspect is the swingarm angle and i ask because your drawings seem to indicate a level swingarm with no downward angle...
...if all or most of the travel is above the 3 axis line ,as in what you will have with a level swinger angle ,the amount of slack becomes an issue that is difficult to address
Once you increase swingarm travel much over stock, you have to start considering an automatic chain tensioner (spring-loaded).
Here's what I had the last time I dummied up the major components. I expect that the dual-shock DT swingarm will be positioned similarly to the YZ arm in the photo. My intention is to definitely have less travel than a stock Pursang (something more typical of a lightweight street bike – maybe 5 inches or so of rear suspension travel) and divide that travel on either side of the countershaft sprocket—swingarm pivot axis.



have you got a steering head angle worked out with the forks and front wheel you have chosen ?
Depending on the length of the rear shocks and the amount of static sag, I should have a steering head angle within about 1° of a stock Pursang 360's 28 degree rake. The fork tubes are fairly long, but I've slid them up in the triple clamps some, since I am using Interceptor 250 handlebars that clamp on above the top clamp. As it sits in the photo, the front end has about 3.4" of trail, which I wasn't comfortable with. Since that photo was taken, I've switched to a set of '82 GS750EZ forks, which are the same length and diameter as the GS650L forks shown in the mock-up photo, but not leading axle to increase trail.


 

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Discussion Starter #353
I picked up the frame from the welder last night after work. I'm pleased. Even though the whole point this project is about learning the skills to do things myself, I think it was wise to have a pro weld the frame tubes. It's well put together and accurately aligned. There's a lot of beautification to do, but it's nice to have something that can be called a complete frame, not the chopped-up assortment of tubes and fittings I've had for the past two years.





A cross-brace will need to go somewhere in between the shock mounts, but that will have to wait to get sorted out along with the seat mount.
With the DT175 swingarm, the top and bottom shock mounts should be within 1/2" of the same distance from the swingarm pivot. A quick string measurement indicates that I will want shocks that are around 15" – 15-1/2" in length, but again, the exact length is TBD.

My progress from this point needs to be:
  • Finish the rear engine mount (which fits to the swingarm pivot),
  • Modify the swingarm slots for the larger Suzuki rear axle and then add gusset plates to the openings,
  • With the swingarm installed, get the rear wheel properly aligned with the centerline of the frame,
  • Assemble the whole chassis and see how it all sits, to make a final determination on rear shocks.
 

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Discussion Starter #354
I wanted to get all my current measurements down before I start messing around with the twin-shock swingarm, so I assembled the chassis with the YZ swingarm and snapped a couple of photos. I then took a side view and superimposed all the suspension angles as they are now. It looks as though ≈4" of travel will be what I'll be shooting for. That puts the travel centered on the longest chain distance with my current ride height, and that's also very conveniently the limit of tire clearance.





 

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You mention that you ended up with 3.4" of trail, so you switched forks and it appears you ended up with 4.4". Just a general question for anyone..... What sort of trail number should be the target for this motorcycle or the average well behaved street bike?
What would be the minimum number to prevent the bike from misbehaving?

Some fodder for conversation:

"The state of North Dakota (USA) actually has minimum and maximum requirements on rake and trail for "manufacture, sale, and safe operation of a motorcycle upon public highways."[25]

"All motorcycles, except three-wheel motorcycles, must meet the following specifications in relationship to front wheel geometry:

MAXIMUM: Rake: 45 degrees - Trail: 14 inches [35.56 centimeters] positive
MINIMUM: Rake: 20 degrees - Trail: 2 inches [5.08 centimeters] positive
Manufacturer's specifications must include the specific rake and trail for each motorcycle or class of motorcycles and the terms "rake" and "trail" must be defined by the director by rules adopted pursuant to chapter 28-32."
 

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Discussion Starter #356 (Edited)
Just to give you a range for street bikes, an Aprilia RS250 has 4” of trail, a Harley dresser has about 6.2”. Below 3.5 is a one-way ticket to Tankslapper City, unless you’re a GP setup guru.

EDIT: I found mention of earlier Aprilia RS250s having 3.6" of trail.
 

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Discussion Starter #357 (Edited)
Well, I am discovering that affordable dual shocks suitable to this project are just as problematic to source as the short monoshock I was originally trying to find. Given the approximate 15.5" – 15-3/4" (395–400mm) length needed I have three alternatives:

The same crappy Chinese RFY shocks every other half-assed builder uses, with mystery-meat spring and damping rates...

s-l450.jpg

Equally Crappy Chinese Honda XL125/XL185S replacement shocks, which would at least have reasonable spring rates for the weight of my bike, but most likely damping like a pogo stick and too long a stroke...

s-l450.jpg


And finally, overly-huge shocks that are sprung for a Harley, have clevises too big to fit my swingarm, and are still about 3/4" too short of optimal.

s-l450.jpg

At this point I'm leaning toward the RFY shocks just to get something rideable, and attempt the recommended rebuild/mods, should I actually get to the point of being able to ride this someday.
 

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yes you need long shocks but do you need the travel of a long shock
what i suggest is determine what minimum closed length shock you can use
put a half inch thick lil block of wood ontop rear tire and wire the swingarm up so that block is at whatever seat base fender whatever
then measure the mounts csarefully to get a mmin closed leng\th
then return swinger to normalk ride heihht
and measure mounts distance c to c
these 2 numbers will tell us a critical deal that is the limit of shock travel you can have
you actually have a bonus in the longer shock but cant use long shock travel and that is just do RHA on the rod ends
ride heihght adjusters
when measuring shock stroke be sure and include well over half the bumper length
its simple to fab using a 1/2'' female threaded rod end then a piece of good thread-all welded to the shock rod end with eye cut off
bottom line is i can either make you a budget pair of boge mulhollands or just sell you the parts to play with
they are good shocks for twin tubes
but need the shock travel numbers and lengths
 

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Just to give you a range for street bikes, an Aprilia RS250 has 4” of trail, a Harley dresser has about 6.2”. Below 3.5 is a one-way ticket to Tankslapper City, unless you’re a GP setup guru.

EDIT: I found mention of earlier Aprilia RS250s having 3.6" of trail.
Thanks for the reply.

My ADD limits the amount of time I can spend searching for info (maybe thats my problem), but from what I have read...... it seems there is a lot of discussion about what it is and how you arrive at it, but little discussion about what the limits are. You are not alone thinking that less than 3.5 is not a happy place.

There is this: ("2006 Buell Ulysses XB12X has only 20 degrees of rake with a trail of 2.8”) I copied this from an article on the web, so can't speak for the accuracy.

Why does North Dakota specify a minimum of 2"? Not that you should necessarily trust the government.

I fully understand how to arrive at the trail # and know why you need some... I just find it odd that there seems to be a lot of discussion about what it is, but thats where it ends.

Maybe its the 5 whys training I was subjected to.

I don't expect you to spoon feed me the answer... just venting and wondering if anyone else has an opinion..
 

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Thanks for the reply.

My ADD limits the amount of time I can spend searching for info (maybe thats my problem), but from what I have read...... it seems there is a lot of discussion about what it is and how you arrive at it, but little discussion about what the limits are. You are not alone thinking that less than 3.5 is not a happy place.

There is this: ("2006 Buell Ulysses XB12X has only 20 degrees of rake with a trail of 2.8”) I copied this from an article on the web, so can't speak for the accuracy.

Why does North Dakota specify a minimum of 2"? Not that you should necessarily trust the government.

I fully understand how to arrive at the trail # and know why you need some... I just find it odd that there seems to be a lot of discussion about what it is, but thats where it ends.

Maybe its the 5 whys training I was subjected to.

I don't expect you to spoon feed me the answer... just venting and wondering if anyone else has an opinion..
That’s because a lot of the “why do i need x amount?” Depends on the rest of the motorcycle.

The weight on the front wheel
The stiffness of the frame
The width of the front tire
Etc etc

What the trail determines, all else being equal, is the amount of force required to perturb the equilibrium state of the front wheel alignment.

How?
Well the trail is the distance between the tire contact patch and where the extended steering axis would touch the ground.
When moving forward, the is some resistance to rolling by the front wheel and that applies a force away from the steering axis. When you turn, you move the wheel out of the line between the rear wheel and the steering axis.
In order for a force to be applied to te bike to make the turn, the wheel must apply a force to the ground and so the ground must apply an equal opposite force back to the wheel.
This force acts to push the wheel back into alignment with the rear and steering axis.
As you increase the trail, you increase the leverage of this force.

To little, trail and you can get unstable equilibrium states with significant oscillations due external perturbations (tank slapper).

To much trail it becomes harder to turn.
 
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