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Author Topic: SR 250 Mono Build  (Read 2167 times)

Offline doc_rot

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Re: SR 250 Mono Build
« Reply #20 on: Nov 13, 2018, 12:01:11 »
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.

Offline jpmobius

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Re: SR 250 Mono Build
« Reply #21 on: Nov 13, 2018, 12:28:06 »
+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.
Mobius


On a long enough timeline, the survival rate for everyone drops to zero.

1973 RD350 Yamaha build  http://www.dotheton.com/forum/index.php?topic=66498.0

Offline irk miller

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SR 250 Mono Build
« Reply #22 on: Nov 13, 2018, 15:01:36 »
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.

Offline 2Planks

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Re: SR 250 Mono Build
« Reply #23 on: Nov 13, 2018, 17:13:50 »
+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.
"I'd take years off my life, before life off my years"

Offline Hurco550

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Re: SR 250 Mono Build
« Reply #24 on: Nov 13, 2018, 17:24:09 »
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
"If you just picked up a drug habbit instead of a kawasaki triple at least you could get high while you waste money"
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1988 tw200, the swamp donkey: http://www.dotheton.com/forum/index.php?topic=75455.0
2006 Suzuki DR650 Adventure monowheel trailer build: http://www.dotheton.com/forum/index.php?topic=75033.0
1976 BMW R90/6 farkilitious: http://www.dotheton.com/forum/index.php?topic=74349.msg882477#msg882477
1975 RD350 Road Racer: http://www.dotheton.com/forum/index.php?topic=70652.msg833688#msg833688
1976 GT250 Rebuild: http://www.dotheton.com/forum/index.php?topic=64973.new#new
Full Custom Pit Bike: http://www.dotheton.com/forum/index.php?topic=66954.new#new
1971 Yamaha 90cc twin HS1: http://www.dotheton.com/forum/index.php?topic=70498.0
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Offline advCo

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Re: SR 250 Mono Build
« Reply #25 on: Nov 13, 2018, 17:42:32 »
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.

Offline Brodie

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Re: SR 250 Mono Build
« Reply #26 on: Nov 13, 2018, 21:39:11 »
You should source a Yamaha TT250 swing arm. Should be a very close fit if not direct.
I'm not sure, but don't ask Brodie.

Way to many build and half done projects to list here.
Sr250, Gs750, Z50A, XV1100, A10, Z160A.

Offline jpmobius

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Re: SR 250 Mono Build
« Reply #27 on: Nov 13, 2018, 22:11:19 »
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.
Mobius


On a long enough timeline, the survival rate for everyone drops to zero.

1973 RD350 Yamaha build  http://www.dotheton.com/forum/index.php?topic=66498.0

Offline 2Planks

  • Posts: 21
Re: SR 250 Mono Build
« Reply #28 on: Nov 14, 2018, 10:22:54 »
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
"I'd take years off my life, before life off my years"

Offline 2Planks

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Re: SR 250 Mono Build
« Reply #29 on: Jan 04, 2019, 01:23:31 »
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.
"I'd take years off my life, before life off my years"