SR 250 Mono Build

Looking good. I would 100% brace that swingarm. Dual shock requires far less strength in the swing-arm because the loads are translated pretty much straight from the axle into the frame via the shocks. That poor little noodle of a swingarm is just not up to the torsional loads that a mono shock will place on it, plus now the wheel has all that length of leverage on the swingarm where it had very little before.
 
+1 on the spindly swing arm. Also, you should seriously consider the geometry or the suspension will behave very poorly. The angle in the picture below should arrive at 90o when the shock is maxed out. As is, the angle is already past that and will get much worse as the suspension compresses. This will result in a very harsh ride over tiny bumps and severe bottoming on larger ones.
 

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SR 250 Mono Build

I have personally witnessed someone literally break an Ohlins shock because their geometry was wrong. They slowly rode (5 - 10mph) to a spot to load it up on a trailer with a 2x4 shoved between their swingarm and frame to keep the back wheel off the hoop.
 
jpmobius said:
+1 on the spindly swing arm. Also, you should seriously consider the geometry or the suspension will behave very poorly. The angle in the picture below should arrive at 90o when the shock is maxed out. As is, the angle is already past that and will get much worse as the suspension compresses. This will result in a very harsh ride over tiny bumps and severe bottoming on larger ones.
what are you referencing the lower line to? Will go back to the drawing boards on the shock mount. Some harvest a swing arm from another mono bike.
 
2Planks said:
Some harvest a swing arm from another mono bike.

There is a lot of good in this statement. Engineers spend mucho time and monies into designing a suspension that works. Harvesting a donor off of an already monoshock swing arm bike and copying dimensions could get you a lot closer to where you need to be.

MOST of them also have linkage setup and the bottom mount isn't hooked directly to the swinger like you have it. The only one ive ever personally seen (not saying that there aren't others) is a tw200 that has the shock mounted directly over the swing arm like that without linkage
 
2Planks said:
what are you referencing the lower line to? Will go back to the drawing boards on the shock mount. Some harvest a swing arm from another mono bike.

The lower line on mobius' diagram is drawn between your lower shock mount bolt and the swingarm pivot bolt.
 
You should source a Yamaha TT250 swing arm. Should be a very close fit if not direct.
 
advCo said:
The lower line on mobius' diagram is drawn between your lower shock mount bolt and the swingarm pivot bolt.

Correct of course sir, thanks.

Obviously there is more to figuring out any suspension scheme than may seem apparent at first glance. However, that doesn't mean that it has to be hopelessly difficult. Most any sort of linkage scheme will be taxing to properly design from scratch for a couple of reasons. First, calculating the motion and resulting accelerations of the parts would be in order, and second, it is fairly easy to generate very high loads in the various parts making "eagle eye" engineering a bit risky. And then of course, the shock/spring will almost certainly not ideal as it was engineered for a likely much different application. So the value of grafting a complete system from another bike starts looking very attractive. Picking something of similar weight would be good, and replicating the original configuration very precisely is super important. Angling the shock differently to the original application will have just as profound a result as in the simple no-linkage system you have now. If you are designing your own system, keeping it simple will pay off at the end. And you can eagle eye things reasonably safely if you keep a few things in mind. In your case, assuming you will need to keep your current basic shock location and orientation, the lower mount would need to be a bit below a line drawn from swing arm pivot and rear axle. This will make the spring (and dampening) have more mechanical advantage over the swing arm the further it travels, so the suspension will be softer over small bumps, but offer more resistance over bigger ones. Obviously, this makes for a much more complex swing arm fabrication. In selecting an appropriate shock, you can get in the ballpark simply by calculating what the stroke length will need to be, which you can directly measure by moving the suspension through the full travel you want and measuring the distance the lower shock travels along the intended shock orientation. Getting a shock that has this same amount of travel will insure that it will work in the new application presuming the original application weight is similar.
The strength of the swingarm itself is another story altogether - another reason to start with a system off of another bike. It isn't hard to make the arm vastly stronger, but as all things in motorcycle world, the additional framing often conflicts with other things like exhausts, chains, fenders, and of course the suspension which is also taking up a lot of nearby real estate!
I think the simplest "do it yourself" single shock scheme is to copy Yamaha's original monoshock dirt bike design with the triangulated
swing arm and long travel shock assembly mounted high in the frame more horizontally over the carb.
There is another important issue (though less so) which is the very large increase in loading of the swing arm pivot in the frame. Originally, most of the load for the back of the bike was transmitted through the shocks and into the rear subframe and directly to the seat where the biggest single component on the bike is located - the rider. Without the twin shocks, all those loads are transferred into the frame behind the engine. These loads are also often greatly amplified due the leverage incurred by the long swingarm arms working on much shorter distances between the pivot and suspension components. I think often this is does not become a safety issue, but certainly has to have an impact on stiffness and keeping the suspension under good control.
 
jpmobius said:
Correct of course sir, thanks.

Obviously there is more to figuring out any suspension scheme than may seem apparent at first glance. However, that doesn't mean that it has to be hopelessly difficult. Most any sort of linkage scheme will be taxing to properly design from scratch for a couple of reasons. First, calculating the motion and resulting accelerations of the parts would be in order, and second, it is fairly easy to generate very high loads in the various parts making "eagle eye" engineering a bit risky. And then of course, the shock/spring will almost certainly not ideal as it was engineered for a likely much different application. So the value of grafting a complete system from another bike starts looking very attractive. Picking something of similar weight would be good, and replicating the original configuration very precisely is super important. Angling the shock differently to the original application will have just as profound a result as in the simple no-linkage system you have now. If you are designing your own system, keeping it simple will pay off at the end. And you can eagle eye things reasonably safely if you keep a few things in mind. In your case, assuming you will need to keep your current basic shock location and orientation, the lower mount would need to be a bit below a line drawn from swing arm pivot and rear axle. This will make the spring (and dampening) have more mechanical advantage over the swing arm the further it travels, so the suspension will be softer over small bumps, but offer more resistance over bigger ones. Obviously, this makes for a much more complex swing arm fabrication. In selecting an appropriate shock, you can get in the ballpark simply by calculating what the stroke length will need to be, which you can directly measure by moving the suspension through the full travel you want and measuring the distance the lower shock travels along the intended shock orientation. Getting a shock that has this same amount of travel will insure that it will work in the new application presuming the original application weight is similar.
The strength of the swingarm itself is another story altogether - another reason to start with a system off of another bike. It isn't hard to make the arm vastly stronger, but as all things in motorcycle world, the additional framing often conflicts with other things like exhausts, chains, fenders, and of course the suspension which is also taking up a lot of nearby real estate!
I think the simplest "do it yourself" single shock scheme is to copy Yamaha's original monoshock dirt bike design with the triangulated
swing arm and long travel shock assembly mounted high in the frame more horizontally over the carb.
There is another important issue (though less so) which is the very large increase in loading of the swing arm pivot in the frame. Originally, most of the load for the back of the bike was transmitted through the shocks and into the rear subframe and directly to the seat where the biggest single component on the bike is located - the rider. Without the twin shocks, all those loads are transferred into the frame behind the engine. These loads are also often greatly amplified due the leverage incurred by the long swingarm arms working on much shorter distances between the pivot and suspension components. I think often this is does not become a safety issue, but certainly has to have an impact on stiffness and keeping the suspension under good control.

Thanks Mobius!!! Im glad I didn't just say fuck it haha. Appreciate the right up am learning a bunch on this first build. Thanks to all the other responses!! I think I will shift to getting a mono shock designed swing arm.

Maybe something like this
 

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Got busy finally got to work on the bike again!
Got a swing are from a dt 250 bike and went back to the drawing board for my rear shock design. Finished the subframe and gave it some bracing. Made the shock mount now I just have to make some tank mounts and it can be powder coated.
 

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After a little over a year, I am excited to start working on this build again! Money and three jobs kept me busy but after working on the bike the past few days I am excited about it again.


I laced up the wheels and tried truing them myself. ( harder than I expected ) I took the wheels to Woody Wheels Works in Denver, CO. The guys there are legends, with great turn around time. No I have a roller, put the engine back in as well as the forks. Tank is just sitting there currently that is my next task.

Tank mounts
Build a seat
figure out carb fitment
Jet/Tune for high altitude
turn some foot pegs
All the wiring ( not to much keeping this simple )


excited to ride her

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I think your swingarm angle is too steep. I believe that around 12* would be better. I am also concerned that the shock is too short and you will have very little wheel travel. Have you checked it?
You have plenty of clearance between the tire and swingarm upper loop, so I would shorten the upper arms and braces to give more distance between the upper and lower shock mount. I would also move the upper shock mount on the frame up and forward as much as possible. It will be tight, but fractions of an inch there will pay off with getting the swingarm angle right, creating chain clearance and increasing wheel travel.
All in all, I think you have a good solution working and with some tweaks, you will be there.
 
I think your swingarm angle is too steep. I believe that around 12* would be better. I am also concerned that the shock is too short and you will have very little wheel travel. Have you checked it?
You have plenty of clearance between the tire and swingarm upper loop, so I would shorten the upper arms and braces to give more distance between the upper and lower shock mount. I would also move the upper shock mount on the frame up and forward as much as possible. It will be tight, but fractions of an inch there will pay off with getting the swingarm angle right, creating chain clearance and increasing wheel travel.
All in all, I think you have a good solution working and with some tweaks, you will be there.
Thanks! The upper shock mount is positioned to direct the force from shock compression thru the backbone. The swing arm angle isn't my favorite. I have sat on it and its pretty stiff. This is my first time messing with suspension. Iv'e had a ton of useful help on this site! I'm going to ride for a bit and see how it feels. Appreciate the feedback
 
Riding it and seeing where you're at is not a bad idea, as long as you are prepared to make the changes if it isn't right. I am concerned that the excessive swing arm angle will cause issues with the chain clearance and the top of the swingarm. If you have the clearance, you may be able run a bigger front and rear sprocket and gain a little. Like I said, fractions of inches may make a difference.
How much travel do you have at the rear axle?
 
The problem with a swingarm angle that steep is you are going to have poor anti-squat characteristics...Because the rear axle is so much lower than the output shaft when you roll on the gas the chain will pull the wheel downwards, potentially topping out the shock and limiting the effectiveness of the shock while accelerating, not to mention the issues of chain droop and you will need chain sliders so you dont saw your swing arm in half.
 
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