Bamby 4

[porsche930dude]

like this. purple mounts solid to the frame the rest pivots

 

[Hendersonjk]

that would probably work. i would tack it bounce on it and repeat till it feels right

 

[BAMBY]

porsche930dude, i see we think alike! that is EXACTLY how i had wanted to do it originally... BUT... Always a catch, am i right?!

Ok, i dont mean to get all mathematical and $#@% on ya, but here goes- its the only way i can explain my problem with this setup.

The shock i will be using is about 110LBS/IN.
The sprung weight of the bike with me on it will be around 325LBS.
the weight distrib is about 45/55, so lets say 175lbs on rear wheel.
the wheel is mounted on the swing arm at 18.75 from pivot, and the mount on the swing arm is at 5.75 from pivot.

175*18.75=3281.25
3281.25/5.75=570.65
570.65 Round to 570 for easyness

570 is how much pressure is pushing up from the swing arm at that mounting point (5.75).
Somehow, i need to get that shock 110LBS/IN to hold up a weight of 570LBS and still be able to have more room to travel for bumps etc.

For this reason, the bellcrank that you drew in yellow, cannot have a 1:1 ratio. it needs to be more like 4:1 so that the shock will act like a 110LBS/IN * 4 or a 440LBS/IN shock.
This way, the shock will only compress about 1.3" when all of this weight is on it...

However, this is only the basics, this doesnt incorporate the fact that none of these forces are acting at right angles, and due to that, all these numbers change drastically, not to mention, the angles all change depending on how compressed the suspension is.

The major problem is that i cant have a bellcrank with a 4:1 ratio because there isnt room for it to pivot, and the movement of the swing arm is difficult to move in unison with the small side of the bellcrank.

Uhh so if i didnt completely mind boggle you... lemme know what ya think about all this.

 

[BAMBY]

if some of this makes no sense at all, i can draw up a quick picture, to explain it more "normally" heck, sometimes i confuse myself hahaha

 

[porsche930dude]

yup i know what you mean. what you need is much stiffer shock when it comes down to it. the shock your using would be ok if the swingarm mount was way out by the wheel but because of the added leaverage by mounting it inward you have way too much force and way too little throw. So yea its going to be a real trial and error process to get it to work right. but i think the 4:1 ratio seems about right and it wouldnt be too hard to get that as youd just haveto move the front pivot point on the triangle back twords the middle pivot at 1/4 of the distance to the shock pivot point. makes perfect sence lol

 

[BAMBY]

well, i felt desperate to get shocks when i got these off craigslist for 10$... now i know i should have gotten a MUCH stiffer spring.

oh well, it will be interesting to see if it works... if not, well i guess i just know one more way it wont work. lol.

glad to know that whole essay i wrote made sense... sortof

prlly gna start work on the suspension guts on monday... so ill take pics then.

thx for the help and idears guys!!

 

[redsox985]

How soft is this thing? Can you compress it between your hands? I think my mountain bike came with like 700 or 750lbs to bottom it out. It's soft enough to bounce on and its a good ride. (That's what she said?)

 

[BAMBY]

hahaha, and soft is a relative term, but if this gives you an idea...
i weigh about 175 and by carefully trying to put all my weight on the shock can compress it between 1 and 2 inches.

 

[BAMBY]

After some consideration... i chose to remount the shock to make it easier to put a suspension together for the bike... heres a pic of the remount and the parts i have to make now. im sure it will require some fine tuning, and i plan to make the light blue link adjustable in length to change ride height and geometry more easily if necessary... the green pivot point will also have other bolt holes for adjustment of leverage aswell.
Thoughts?

 

[redsox985]

I'm not sure what you are getting at? (What great English, but I'm too tired to edit that.) That support confuses me. I don't see how it's going to compress. Could you just put a cross piece between the rear forks and a small c-bracket to mount the shock?

 

[BAMBY]

this should help explain how the suspension will function...

http://images.sportrider.com/bikes/2004/2004-honda-cb1000-upl.gif

 

[Hendersonjk]

just go after a small dirt bike to rob that off of . I had a 125 honda with that same setup

 

[BAMBY]

well as always... i want to do it on the cheap. and i already have metal to make it out of. plus, then ya get the satisfaction of making it anyway...

also, the ones off real bikes are made for alot stiffer shocks and wouldnt work anyhow because of how soft my springshock is...

 

[BAMBY]

Time for another update? I think so!

Yesterday(monday) I got the plans to full scale in mspaint, then copied that to some paper so i had a template that i could use to trace on the steel plate. Then, I got the side plates cut out, tacked together (so the holes will be exactly the same when drilled) and center punched for drilling.
I used the wire wheel on my bench grinder to clean up the plates real nice, so they actually look a bit shiny!
Also went to the HW store and picked up some 1/2 x 5/8 x 1-1/2 Steel spacers as well as some 3/8 x 1/2 steel tube.

Tomorrow all i have to do is:
Drill the holes in the plates
Get hardware to bolt this chunk of steel together
Make a mount for it to pivot on the frame
Make connecting pieces that hook the swingarm to the part im making
Then, if all goes to plan, i can bounce up and down on it all day. LOL

I think it might look nicer if i rounded the edges of the plates, like how the template was... but ill do that after i know all this actually is functional and working... who knows, maybe throw a nice coat of paint on the plates while im at it.

 

[BAMBY]

As luck would have it, i didnt finish the suspension today, decided to chill with friends for the rest of the afternoon... but i did get the plates finished and bolted together... Heres some pics>
GRADE8 BABY!!!

 

[migel101]

cool bike! the seat looks comfortable

 

[redsox985]

Lookin' good! What kinda plasma cutter do you have?

 

[BAMBY]

migel101- thanks! hahaha, yea, its not as bad as it looks, but i will end up making a seat out of wood and carpet for a little bit of comfort.
redsox985-thanks! i dont have a plasma cutter, i just used my angle grinder with cutoff wheels to cut the plate. last year, when i was a senior in HS, we had a thermaldynamics plasma cutter though. that is what made the jagged edges on the large triangle plate that i cut the pieces out of...

 

[BAMBY]

Okay... i was supposed to get the suspension done a couple weeks ago... but as you guys probably noticed, that didnt happen...

Wellll, its time for another BIGGG update!

Me and my buddy got to work yesterday and got some stuff done!
Long story short: suspension is done!

No pics this time, but i did manage to get 2 videos
http://www.youtube.com/watch?v=jwp-bGzAU8U
http://www.youtube.com/watch?v=ioiBzKXbcM8

So as you see, it rebounds or springs back REALLY slowly, so im thinking, if i drill a small hole in the shock body to allow the oil to drain out, that this will remove the damping effect, and then it will be perfect! Also, im planning on lengthening the bars that connect the triangle thing to the swingarm to lower the ride height a bit, because right now, i feel its a bit too high...

but ohhh am i sooo glad that it worked out as great as it did! I guess thats what good planning will get you.
Lemme kno what you guys think!!!

 

[sebastiaan]

Nice work, I realy like the design of this bike! I'm planning to build a minibike like yours but with a 150 cc 4 speed 4stroke rsr pitbike engine (+- 14 hp)