SP370 Tracker ish build

kipper said:
Ok so I did some reading up on UK LED laws and as long as they do not exceed 2000 lumens in output and are angled at least 5 degrees belo horizontal on dipped and just below horizontal on full and they are a complete unit and NOT bulb replacement, then the lights will pass MOT.
Since mine are new units and 20w (1200 lumens) I should be clear to run then no probs.
Thanks for highlighting this Strang.
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I was curious myself as I may end up running something similar. Good to know, thanks.
 
Green199 said:
I don't believe your shock setup will work. The swingarm, possibly the weakest part of the chassis needs to be strengthened considerably if you're converting to a monoshock. Ideally, you need to change the way the shock mount attaches to the swingarm.

You need extra bracing on the swinger itself, and ideally to relocate the shock so it isn't so perpendicular to the ground plane. The image below shows what i'm talking about. Red represents the swingarm (plus additional bracing), and yellow the shock itself.

Please dont take the angles of the shock or bracing itself on my drawing as exacts, as it was a quick sketch in paint to convey the point!

If you look at any twin to monoshock conversions, must of them follow this pattern.

funny you should use that CB as an example as that is what inspired me to do the monoconversion initially, however

the road less traveled and all that - i decided on my current set up, i do intend to add more beef to the swingarm in the near future.
other have made similar conversions - so figured i would give it a go
 

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kipper said:
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but green 99 you cant compare those with each other his shock moved far forward and at a slight forward tilt is ok strictly from a correct geometry perspective,the path of movement is inline with the shock
but it is a terribly extreme leverage ratio ,what is it 5 to 1 ? thats insane and is going to give 15+ of rear wheel travel, with a 3" stroke shock,more than an mx bike ,and it puts a huge load into the swingarm and pivot and the tubes running up to the backbone,they were never designed for even a small fraction of the loads it would be getting right there,it is compltetely unworkable ,sorry
ok then well it needs to have the shock moved back on the swingarm to lesson the leverage ratio but the tire is there :( :( the only way would be the shock off to one side of the swinger back to a workable ratio like 2 to 1
] the honda you give as an example with the monoshock ,is another way to do a mono it is totally different geometry to a shock that is mounted down low close to the horizontal plane of the pivot and rear axle thjey are a different animal apples and oranges
and it looks a like it misses the sweet spot as well, i could be wrong i would need to model it,but i am going to say the shock needs to be layed down more .reason is with the high bottom mount(in relation to the pivot)and the closer it, the bottom mount gets to the vertical plane of the pivot ,the more the actual path of movement of the bottom shock mount is happening in the horizontal plane
if the bottom shock mount is straight up from the pivot all of the motion is horizontal ,not flat of course the path is always an arc no matter where the mount is but hopefully the concept is graspable
and generally speaking there is a sweet spot that the shock angle needs to occupy to give any kind of practical rate of progression, if any ...the rate can be flat that is acceptable not optimal at all it is better if it is a rising rate even a small amount which is all you can really get without linkage,rising rate means that the swingarm is loosing leverage as travel progesses,as travel progressis the rate of shock movement RISES the leverage ratio is decreasing numerically like from 3 to 1 to 3 to 1.5 a simple example wheel to shock travel ratio this allows for a spring that works soft enough for small bumps but the rate change allows it to take big hits as well
a falling rate is bad, that is gaining leverage on the shock thru travel progression, shock travel FALLING as wheel travel progreses ...falling rate you need a stiff as fuck spring rate to handle the big bumps but then it rides harsh as fuck on small bumps, 3 to 1 to 3.5 to 1 a simple example
all this stuff needs to be planned out and modeled ,that is the way the mfg's do it some of the geometries used are still a poor design to this day,but its not that hard to plan out ,but it seems aint nobody got time fo dat :D
you may not approve of this message, but i didnt make any off this shit up neither ,nor did i school learn it,no matter ,its all indisputable fact
 
kipper said:
funny you should use that CB as an example as that is what inspired me to do the monoconversion initially, however

the road less traveled and all that - i decided on my current set up, i do intend to add more beef to the swingarm in the near future.
other have made similar conversions - so figured i would give it a go
you have a very extreme leverage ratio as well what is it ? it looks over 4 to 1 the shock is so far forward
do you know how to calculate the ratio ? what is your wheel travel ? shock full stroke ?
 
No idea how to calculate the ratio - let me have it and I will do the maths

This is all done by trial and error with no real vision of the end result other than getting something that works so all advice is appreciated


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the basic starting ratio is easy, if the shock is halfway between the axle and pivot it is 2 to 1 just some basic math to figure
the angle of the shiock has an effect as well but to start the basic numbers is very useful to give an idea

the next thing you really must know is the full stroke of the shock take off the shock spring
you can actually calculate the lev ratio or rather physically just measure the wheel axle travel with the spring off the shock then bottom out the shock very hard to bumper by comparing the actual wheel travel to the actual shock travel when mounted this gets the ratio that should be same or very close to the math and measure
the best way to understand is to model and plot what is going on..not a cad computer needed just use some sticks and nails on a large bored or table and make a full scale model of the swingarm and pivot and shock mounts in plain side view
with the stick model you can move in 1/2'' increments of wheel travel starting from where it would be with shock extended and observe and write down exactly the travel from the layed out shock mounting locations at each 1/2'' inch of wheel/axle travel this way a simple graph can be plotted and you can see the rate of progression if any rising or falling
i would say you only want about max 5'' total rear wheel travel similar to fork travel not more not a lot less but you get more than 5 and it will be wallowy
 
Great thanks for that, I googled it and got a vid explaining the bit about using the wheel and shock with Spring off, but think the board and sticks method may be easier to gauge with the tools I have, so come the weekend I will know if I am safe or just plain soggy. I have a sneaking suspicion it will be the latter


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Whipped the spring off the shock and set it up ready for the travel measurements, I've got 7 inch travel in total and the overall ratio seems to be 1:4.6 below shows the progression on the shock
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What this all means I would be interested in your thoughts

Cheers



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actually would be easier ti dediced for me if you converted the left colum to decimals
the shock has less than 2" of travel ? you will need to include about half the bump stop as well because it aint jus stoppin as soon as it touches
the 2/16 and the like we never do that up here it looks so strange that i wasnt even sure if is also =1/8"
 
Converted to decimals, yes the 2/16 is essentially a 1/8 apologies for that I am not used to measuring in inches. And yes the shock only has 1 and a half inch of travel with Spring off.
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Thanks for this, proper learning curve here and enjoying every minute


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whoa no im sorry
i see what you did there where is that taught ? or are you pullin my leg ?it is different i have not seen anything like that before
decimal is when you break down 1 inch into a thousand equal bits
and 2/16 is never used it is ....1/8 of an inch ...;which equals 125 thousanths of an inch or in decimals .125"
a 1/16 is .0625" an onch
and so on and so forth,but no matter never mind
i learned something just now you guys have been on the metric system for a long while.it is a much better system than a kings foot or a nobleman's stride the arbitrary s==tandards that the whole kay otic sdystem goes by
why dont you plot it on a simple graph? it will help you understand.
what bike the shock and spring come from ?
we could have just used metric it doesnt matter the graph will l oook the same i can deal qwith metric i just still use imperial because it relaxes in my brain easier
what kind of riding will you be doing?
do you any idea where the 370 was when sTock ?
looking at the numbser that must be an error by you near the top
it does look like it is a slight rising rate
anyway i can see from here and new alll along that the shock is unsui table it is for a pretty heavy bike and a much less severe ratio.3 TO 1 or so i would guess "how much wheel travel was the 370 stock,where are its shocks prtetty sure they are gassers
 
Ha, it's not taught anywhere as its been metric all my years and honestly had no idea imperial decimal breakdown was such sorcery.
The shock is from a triumph speed triple and the original are long gone (rusted beyond recognition) hence the mono conversion escapade (along with rusted and bent up seat section) I will run the test again in metric and see if I can understand the maths. Riding will be street commuter / fun ride nothing too strenuous


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The stock SP370 did indeed have gas / oil dampened rear shocks according to the manual. It does not state the rear travel tho so I'm guessing somewhere between 4 and 5 inches


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yeah the shock is unsuitable you would needx to start swapping springs ,much lighter springs an totally remount the ashock,bgrace the swingarm and frame,it is literally a huge waste of time and effort
if a fella wants a motorcycle to tinker,and wants a mono shock cafe,brat ,tracker or whatever THEN YA GOTTA start out with one make IT better lighter faster, starting out with basics
 
Hi there dude, nice looking bike you have there!

As xb33bsa says you have a too high of a compression ratio. You must reduce that number in order to make the bike work correctly.

Compression ratio means the relation wheel travel has to shock travel. Your rear wheel (as of it now) needs to travel 6 inches in order to only compress the shock a little more than an inch. This is not good. First, the frame and swingarm are receiving loads in directions and places it was not designed to.

Second, if the shock you chose is too high in spring rate, your bike will need too much wheel travel in order to compress the shock appropriately. As the wheel is moving too much in relation to the shock travel, the shock will be unable to dampen the suspension, as it now has "too much leverage", added to the fact that the shock has a short stroke (and looks to be too stiff in spring rate) for the job it's going to be put up to.

Third, the setup you have there is weak in the swingarm area. You need to brace the swingarm and the shock mounting point in order to make the monoshock conversion work. This only after achieving a correct compression ratio.



So,

What bike did the shock come from?

Dow you have a clue of the spring rate of the shock spring? Any markings on the spring?

I would suggest ditching the monoshock conversion/suspension theory/frame design project for a future project, when you have a little more insight in those areas, get a simpler project and gain experience. If you choose to keep on this track, I'm open to help. You'll need to begin with getting a proper shock for your conversion, then suiting the shock to work properly on the suspension. Then you can finish your frame, and keep with the rest of the build.

Cheers bro!
 
Thanks dualero

The shock I was using came off a triumph street triple
As far as ditching the monoshock I am considering it but as the original twin shock frame backend was in bad shape I opted for this current plan. So am sit g back and having a good old think. I am still trying to get my head round the compression rates and stuff and when I get a bit of spare time going to take x's advice and do some modelling. Failing that I'm gonna need a new frame and go back to the twin shock which I'm fine with as this whole project is about learning and making mistakes, admitably this wil probably the biggest. I have a Ducati monster shock offered from a friend which uses a similar set up eg no linkage so might give that a go before giving up on the mono conversion


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All right, that's more like a plan!

Regarding the rear suspension, in terms of design you don't necessarily need to use linkages. Linkages complicate the way the swingarm transfers the load to the shock but allow designers to make for certain behavior of the suspension.

You can achieve the monoshock conversion still without using a linkage. What you need to figure out is the dimensions the swingarm shock bracket has to have in order to achieve the desired compression ratio, for the shock you happen to use
 
Thanks for the info, I have Had a rethink and am now looking at putting the mono on the right side so that I can do the maths right.
The new shock is a Sachs from a Ducati monster and has a 170mm spring at 130 n/mm. All I need to do now is work out the brackets and angles, which to be honest is scrambling the fuck out of my brains so any pointers here would be most welcome.


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Oh here's a photo of my plan which offers a good 6 inches of travel
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that is much better as far as slr
but you will need some extensive swinger and frame bracing and what kind of bushings are in the swinger ? they really need to be precision ball ,needle or roller bearings because of the extreme loading...
did you figure the rear wheel travel compressing the foam bumper ? you have to figure 1/2 the bumpers length at a minimum to get the true travel and slr
but carry on and above all ,take the spring off the shock now, before you do anything more
and tack up some mounts so you can visulize and measure travel into the bumper ;)
make the mounts with a cupl holes each end maybe ...i really seat of the pants feel it needs to be a little bit more upright just by jiggering the top mount tho
you still will need a shock that is valved lighter and a lighter spring the duc weighs close to 200lbs more and with similar wheel travel you are going to miss...by a lot... but you are learning and having alot of fun right ?
 
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