Search all of my sites with Google

Friday, December 4, 2009

Adding Rear Suspension to CrazyBike2

I've got several options for doing this, and this is just the first attempt, doing it the "easy way", that lets me revert back to the original at any point, without cutting into the original rear triangle. There are certainly better ways of doing it, but because of the cargo pods, they all require severe modification of the whole rear end that would preclude going back if it doesn't work out.

*This* method allows me to even undo it when I'm on the road, if something breaks unfixably on a longer test trip, though that would be pretty annoying and take at least an hour to do.

It starts with the 24" rear triangle originally slated for the unfinished ReCycle, and bolts it's crankshaft/BB to the dropouts of the CrazyBike2's rear triangle, in place of the wheel. The wheel now goes into the new triangle.

Then a shock with coil-over spring, origin unknown but possibly motorcross bike, bolts to the top of the new rear triangle, pointing forward to align with the top of the original rear triangle/toptube to keep all the pushing forces along a strong line. Below is a simple example of how it might work, bearing in mind that CB2's rear triangle is much less tall than the bike frame on the right.

This adds about a foot to the length of the bike, and about 15 pounds (so much for making it lighter) but it appears to be the easiest first try.

First up is finding a way to compress the spring enough to get a large plate on the shock to hold it in. Neither of these thrift store finds had that plate; it was probably part of the vehicle they came from.

I didn't have a large enough washer or other plate just laying around, but I did have a strong steel tile-cutting blade, too worn to cut anymore but still strong, so that has become the first temporary spring-retention plate. It's not very thick, so I suspect it won't survive all that long in real use, but it will allow me to perform some tests without manufacturing something for a shock I might not even end up using.

So I zip-tied the spring on each side and carefully compressed it just enough to be able to assemble the shock, always keeping everything parallel to my body so nothing could hit me if the zip ties broke. I also made sure none of the dogs could get into the room I was in, so they wouldn't be in danger--they tend to show up at the worst possible time if something is going wrong (before I even start cursing ;-) )

Once compressed suitably, it was easy to screw it all together, then tighten the jam nut against the pivot ring's nut so it can't work it's way loose later.

Next up was a simple way to mount it to the rear triangle for a test:

...a couple of pieces of 1/4" plate steel from a big old desk chair, from the L bracket that holds the seat back to the base, spot welded to the rear triangle at it's strongest point where the stays meet the seat tube. I can easily cut them off and move them elsewhere should the need arise (and it turns out it does, later in the post).

First bolted to the shock to keep them lined up, then clamped in place to the tube.

Then spot welded at each corner.

That's the basic alignment, and approximate position under load, at a guess.

It sticks up a lot more than I would like, so even with the wheel out of the way, it'll be too far up to lay cargo across the pods. :-(

It would be perfect if it were like this:

But that's only prior to loading, with the handy-dandy adjustable-ride-height mechanism.
The front mounting point for the shock is adjustable, to give me about two inches of adjust range, for altering rear end height when loaded if it is necessary. I did this by cutting a strong hard steel plate off the bottom of that same desk chair as above to weld to the square-tubing that supports the cargo pods and seat, as that is the strongest point on this section of the bike.

The silver part is the steering tube from a discarded Razor kick scooter, along with it's "fork". I bored a hole in the plate large enough for the tube to fit thru, with the "fork" end pointing rearwards. I threaded the top nut for the tube on it's "rear" end, behind the plate, and the locknut on the "front" end in front of the plate. Those are what let me adjust it's length, and secure it to the bike. It will not be easily adjustable on the road, as it is under the seat; it is only there so I can change the ride height if I anticipate needing to do so before a particular trip.

The preloading will be done by a tub-balancing-support cable off an old washing machine which will restrict how far the bike can pop back up after I get off of it, which will normally be sagging loose during the ride, so it will only ever have a load on it when it's not being ridden. It's main purpose is to keep the chain from ever having to rub on the frame, and so I can see what the bike is like when I am not on it, plus not having to climb up so high onto the seat. :) The shock itself doesn't have a preload method built in.

This is what the whole bike looks like now, without the preload yet, so it's a lot higher off the ground that it will be.

The spring compresses about 2/3 of it's length with me on it, leaving a bit for bumps and whatnot. If I have much cargo in the pods, it will compress most of the rest of the way, unfortunately, requiring a different leverage method for the spring to use it properly. I'm mostly doing it this way now so I can easily undo it, and because this method is easy to build and test.

This is the rear view, again no preload yet.

A closer view partly from the top/rear, where you can see the extra derailer used only for it's tensioner, at the original dropouts. It's there solely to take up the slack as the triangle moves back down, and provide enough extra chain that I don't need to worry about the actual rear derailer jamming due to chain overtension when I'm on big ring front plus big ring rear on the occasions that must happen.

Now, chainlines I'm still working out, so stuff may change as I do this. In theory with the spring preloaded by cable, the chain should pass normally thru both triangles to the rear wheel. That might not work perfectly thru the gear range at the back, so I might need to add a guide wheel to get around the stays. Hope not, but....

A close up shot of the ride height adjuster.

In first testing, I already had a problem (of course). The steel plate the whole thing goes thru under the seat is great when pushing against it, but apparently not in pulling.

I didn't take a pic of it, but when I sat on the seat and started bouncing up and down to see what would happen (before I put this thing on the road), at full compression the angle of the shock is just above and down onto the front pivot point. That forced the pivot to pull on the plate rather than push against it, and the whole assembly sort of folded down into the empty original rear triangle. Easy enough to bend back up once I got enough leverage, but it certainly suprised me (and shouldn't have).

So I'll need to do some rethinking on this. I have a 1/4" steel bar that I put under the pivot point across the square tubing for the cargo rails, and this prevented it from happening again, but I don't want to add essentially another pound just for that--plus eventually that might bend, too. A thicker plate for the pivot to bolt into would help, too, but again, it's just more weight. There should be a better solution.

I could move the rear pivot point downward closer to the BB, so it is always pushing upward against that plate, or even better against a pivot point that is part of the cargo pod rails. That would actually counter the weight of cargo in the pods pushing down on them, and if the pivot point joins the rails horizontally it will also stiffen that part of the frame.

Chainline currently does not come out right, so having to rework this is actually helpful. I might wind up putting a large sprocket (to reduce noise) on a jackshaft in front of the rear wheel, between the cargo pods, to keep the top of the chainline clear of both sets of stays even during pivoting.

Another idea DrunkSkunk from the Endless Sphere forums had is to use a seatpost in the seat tube, and attach the rear pivot point to *that* instead of my contraption from the Razor/chair up front. Then I could move the seat post up and down for ride height adjustment, and if I use a quick-release seatpost clamp, that'd be very easy to adjust.

The same problem I have with the first arrangement comes in here, though, where the shock is in the way of laying cargo across the pods--something I would like to be able to do, and could not do with the tire there.

If I move the front pivot point to somewhere within the dropout area of the original rear triangle, then I can have both the new adjustment method and be able to use the top of the pods for cargo, too. I just have to put the bottom connection point on in a way that won't interfere with putting the bike back together the way it was, if none of this works out.

It'd look something like this, I think:

where purple is the original frame, brown is the load-bearing cargo rails, orange is the new triangle, gray is the seatpost, and red/black is the shock and spring.

No comments:

Post a Comment

Alternate suggestions or improvements to anything that's been posted is very welcome, and extreme detail is preferred to brevity.

Keep in mind that unless you leave an email address in your comment, I haven't any way to reply to you except to reply to your comment here. That means if you want a reply, you'll have to come back to *this* blog entry and it's comments to see my reply to you, unless you leave some method of contact within your comment.