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erockson
02-06-2011, 10:06 PM
So I'm finally starting on the 81 XS400 that I picked up for $300 and have had sitting torn apart in my garage while I finished my GS450. Because of the XS400 frame I didn't really want to monkey around with trying to make a custom hardtail unless I had to, so I decided to chop then shorten the rear section to keep the shocks but move the mount a good 6" forward, increasing the angle and dropping the bike. I like where it will sit now (about 4" of clearance), but I'm worried the shock angle may be too steep for it to work and/or cause issues? Anyone have any experience with doing a bike this way or know about suspension geometry and can give me a heads up as to what the max angle a shock can be placed at and still function properly?

https://lh4.googleusercontent.com/_54euraRhXr8/TU9t7Zjn9CI/AAAAAAAABYs/KI9giXkZOxQ/s640/Photo0467.jpg
https://lh5.googleusercontent.com/_54euraRhXr8/TU9t6rBHzeI/AAAAAAAABYk/rBa28QY_4Dk/s640/Photo0462.jpg


I sat on it without the front end on (I have the forks torn apart to lower them), and the shocks definitely aren't as springy as they felt at the original angle... actually feels almost rigid which is what got me worrying, but I'm not sure without a front end I'm actually going to get the proper feel anyway.

Thanks!

Ember
02-06-2011, 10:49 PM
I know very little about suspension geometry, but I do know that the greater the angle of the shock, the higher the spring rate required. Even a few degrees makes the spring much stiffer to compress.
Yours are at a pretty steep angle, so you may need to switch to a progressive spring/shock to help compensate for the increased spring rate.
I'm sure there are conversion/correction factor charts on the web some place to factor your spring rates.
But just a quick example, if the springs are rated at 100lbs (lb/in) at vertical, then say at 45 degrees that same spring rate may increase to 200lbs/in.

Hopefully someone else will chime in that knows a thing or two on the subject.

erockson
02-06-2011, 10:59 PM
Shit, good stuff! Those were some soft ass shocks when they were mounted near vertical, and now like I said when I sit on it and try to bounce em they feel the same as the 10' heavy coil sporty shocks I have on my GS (1 step above riding a hardtail). I'm not really worried about having a soft ride. Mostly I'm just worried that if they can't still move as designed due to an excessive angle that they will basically become a really weak rigid strut.

https://lh5.googleusercontent.com/_54euraRhXr8/TU99u2lSRAI/AAAAAAAABZI/65y80zx-dbE/s640/Photo0467%20copy.jpg

Basically I''m afraid that when force is exerted in the direction of the red arrow (direct downward force) if the shock can't react properly in the direction of the green arrow (the shock angle), it will create a stress point where the red and green lines intersect, and could fail?

P.S. Should I keep the sweet spray can white walls the bike came with?

Ember
02-06-2011, 11:25 PM
I meant to mention that fact as well, but slipped my mind. Personally, if it was my bike, I may look into shorter shocks and extending the shock mounts to decrease the angle a bit.
Not really sure, man. It is a cause for concern, though.

erockson
02-06-2011, 11:47 PM
I'm definitely not sold on it after feeling how rigid it was, but again I'm not sure if I'm really getting the right feel without the front end on it. I was really just trying to use what I had on hand and make it work to slam the bike with 13" shocks!

If I have to scrap it and drop some coin I'll probably just go with a hardtail and buy some tube and have it bent and source out someone to make axle plates (Can't buy the builder kits because the xs400 rear axle is17mm and all the kits come with 20mm axle plates).

Hooligans
02-07-2011, 12:13 AM
You always have the option of running Honda Rebel or Suzuki Savage shocks.... approx 10.5" eye to eye allowing you to move the lower mount forward

erockson
02-07-2011, 12:42 AM
I had looked at going that route before but decided if I was going to spend money on the frame/suspension I would just go ahead and do a hardtail. I just wish the xs400 frame wasn't so funky and so much different from the 650.

So for shits and giggles before I dropped any money on it I figured I'd try this route using the 13" shocks I had in my garage. It looks cool as far as the stance and the lines of the bike... just worried about safety/functionality since I don't know shit about proper suspension geometry.

miller0613
02-07-2011, 8:14 PM
I am planning on doing something similar on my xs650 so I am interested to see what direction you go. Honestly I was more worried about making sure to get good weld rather than the geometry of the thing. Post your decision whichever route you go. thanks

erockson
02-07-2011, 8:54 PM
I did some more research and what Ember said was exactly true, and it seems up to a 45 degree angle is the steepest listed (and should be right about where I am), and at that steep of an angle it increases the stiffness of the shock by 2x. So the soft shock I started out with is now stiff as it gets, which is fine with me, I just care how it looks and that I won't hit a bump and have them explode and put the tire up my ass at 70mph.

erockson
02-07-2011, 9:04 PM
I am planning on doing something similar on my xs650 so I am interested to see what direction you go. Honestly I was more worried about making sure to get good weld rather than the geometry of the thing. Post your decision whichever route you go. thanks

Since the 400 has a completely different frame I had to cut and shorten the existing rear subframe to re-use it, and made my own slugs out of some bar I had that just happened to be the exact size of the ID of the frame tube. If I had a 650 I would probably just go with the brat kit from these guys since it comes with everything: http://www.visualimpact2.com/Merchandice.html

At the very least that link will give you a perfect how-to (it's exactly how I plugged and re-welded mine).

miller0613
02-07-2011, 11:35 PM
I may end up going that route and I did actually order some of the shock mounting tabs from them but I am trying to keep the rear loop kinda like this. Plus I kinda want to do it on my own if possible

LOCKARD666
02-11-2011, 10:22 AM
I did some more research and what Ember said was exactly true, and it seems up to a 45 degree angle is the steepest listed (and should be right about where I am), and at that steep of an angle it increases the stiffness of the shock by 2x. So the soft shock I started out with is now stiff as it gets, which is fine with me, I just care how it looks and that I won't hit a bump and have them explode and put the tire up my ass at 70mph.

i don't know why the spring feels stiffer at an angle, it should feel softer... the only thing i can figure is that during the shock relocation, the bike also maybe got torn apart some, decreasing the weight, although i'm not sure that would make all that much difference.

i'm not tryin' to be a dick, but you're looking at this backwards

a spring will carry the most weight & feel the stiffest @ 90*

the more you angle a spring away from 90*, the more pressure is put on the spring, per the same amount of downforce, decreasing it's load capacity & making it feel softer.


at that steep of an angle it increases the stiffness of the shock by 2x.

as you angle the shock, it does not change the rate of the spring itself, but it increases the necessary rate to acheive the same result.

a 100lb load @ 90* requires a 100lb spring rate.

a 100lb load @ 45* requires a 200lb spring rate.

here's a chart, just punch in the numbers & it will tell you what you wanna know http://www.proshocks.com/calcs/anglefirst.htm

it's a car spring company, but the same mechanical rules apply.

does this make sense?... i'm not sure how well i'm explaining it, but i know this is how it works.

OgdenCityMurderCycles
02-11-2011, 11:40 AM
Don't forget, changing the shock to 45 also changes leverage. Leverage wins...

Ever used a prybar to move something you couldnt do by hand?

OgdenCityMurderCycles
02-11-2011, 11:43 AM
Sounds to me like your shocks are mounted on an incorrect angle which is not allowing them to compress properly. Take as look at some of my soft-tail builds and the shock angles. It takes some engineering to make them work properly.

Blackjack
02-11-2011, 1:49 PM
Takes very little engineering to get them to work properly, you just need to be able to use a tape.

The top shock eye (though, more accurately the static one....) shouldn't be nearer the swinging arm pivot than the bottom (or moving) one.

That's easy enough, explaining why that's so might need some engineering.

Yeah it's softer, chances are that in this case it FEELS stiffer because the shock eyes now need to rotate significantly to move, and it'll feel stiffer right up to the point where the bonded rubber bush rips apart....

Also, it's got progressively wound springs. This alters things somewhat, but without measuring it who knows.

Other thing to remember is that the softer spring rate means that the rear wheel travel has increased. Usually folks doing this shit lower the bike. Lower bike + more travel = potential loss of control when frame hits deck travelling in a straight line.

Oh yeah, I was mildly concerned that the point of closest approach between the top and bottom mounts, might not be greater than the compressed length of the shock. It's unlikely because so much shit would get in the way, but having your rear suspension go over centre is highly unlikely to prove amusing....

BillyT
02-11-2011, 5:10 PM
http://www.chopcult.com/uploads_user/10000/9872/48167.jpg

I ride the piss out of this thing... No issues...

And as was already mentioned, because it's stiffer, you can run tighter tollerances on the fender to wheel.

The reason rebel shocks are so stiff when people run them on anything beside a rebel, is that they are mounted so much further forward on the swing arm of the rebel, they get my leverage acting on them. If you mounted them further up on the swing arm of your bike, you would not only accomplish making them feel softer, but you would get rid of the angle if you feel like it is really a problem(I don't)

Judging by where the savage shocks mount on the swing arm of the savage, I'd say they are good bit softer than the rebel shocks, as well as have more travel, and would be a bad choice to mount further up on the swingarm of your bike.

Shacknasty
02-12-2011, 4:03 AM
Are the shocks adjusted to the softest preload setting? It does not appear so in the picture...

Blackjack
02-12-2011, 4:31 AM
Are the shocks adjusted to the softest preload setting? It does not appear so in the picture...

Pre load doesn't alter spring rate, just the ride height.

erockson
02-12-2011, 1:31 PM
Takes very little engineering to get them to work properly, you just need to be able to use a tape.

The top shock eye (though, more accurately the static one....) shouldn't be nearer the swinging arm pivot than the bottom (or moving) one.

That's easy enough, explaining why that's so might need some engineering.

Yeah it's softer, chances are that in this case it FEELS stiffer because the shock eyes now need to rotate significantly to move, and it'll feel stiffer right up to the point where the bonded rubber bush rips apart....

Also, it's got progressively wound springs. This alters things somewhat, but without measuring it who knows.

Other thing to remember is that the softer spring rate means that the rear wheel travel has increased. Usually folks doing this shit lower the bike. Lower bike + more travel = potential loss of control when frame hits deck travelling in a straight line.

Oh yeah, I was mildly concerned that the point of closest approach between the top and bottom mounts, might not be greater than the compressed length of the shock. It's unlikely because so much shit would get in the way, but having your rear suspension go over centre is highly unlikely to prove amusing....

See I knew there had to be some easy "suspension triangle" geometry! And that looks like that is my problem (since I took what I read backwards and the angle should have been making my soft shocks 2x softer). I just went into the garage and from the top shock mount to the swingarm pivot is 13", from the bottom shock mount to the pivot is 16". So it looks like I'm going to have to spend money on the rear one way or another.... looks like she's getting that hardtail.

Anyone know who makes/sells 17mm axle plates, or even better a hardtail Kit with 17mm plates (every one I've seen is 20mm)?

erockson
02-12-2011, 1:50 PM
Sounds to me like your shocks are mounted on an incorrect angle which is not allowing them to compress properly. Take as look at some of my soft-tail builds and the shock angles. It takes some engineering to make them work properly.

Looked at your photos thanks, but now I'm confused cause in photo #9 (the picture of just the rear shock) it looks like the top mount would be closer to the swingarm pivot than the bottom mount, just like in my case? Not sure if you remember that bike or what you did and how it rode?

Jasonisdico
02-12-2011, 11:23 PM
It actually appears in Ogden's picture that the two point are the same distance from the swingarm pivot. I think the camera angle is what is throwing you off.

And as for preload, it is not just for 'ride height.' The more you compress a spring, the stiffer it gets.

Blackjack
02-13-2011, 6:46 AM
It actually appears in Ogden's picture that the two point are the same distance from the swingarm pivot. I think the camera angle is what is throwing you off.

And as for preload, it is not just for 'ride height.' The more you compress a spring, the stiffer it gets.

The only way a spring gets stiffer as you compress it is if it's progressively wound.

That out of the way, here's what happens with a constant rate coil spring.

Say, it's 100 lb/in. You install the spring with 1/2" of preload on it, and the subject it to 150 lbs of load. The sping will compress 1 1/2" under 150lbs of load, but its already compressed 1/2", so the shock assembly compresses 1" and the vehicle drops 1". Add another 100[bs spring compresses another 1", vehicle drops 1". Spring rate is 100lb/in

Wind the preload up to 1", subect the shock to 150 lbs of load, it's only going to compress 1/2", vehicle drops 1/2". Add another 100lbs, spring compresses 1", vehicle drops 1". Spring rate is STILL 100lb/in.

In that example, with 1/2" of preload, fully extended it takes 50lbs to start moving the shock, with 1" of preload, it takes 100lbs to start moving the shock, But in all cases the spring rate stays the same and it doesn't get any stiffer.