CB desert sled

String Theory?

Worse than napkin sketches. I'm learning to use Sketchup.



Frame on the left, not drawn. The shock is in gold, the popsicle stick is the link. Almost all the wheel travel is in the downward direction.



The linkages I've seen use an 'opening scissors' type of action to get the progressive movement. They give the shock an extra push at the end of the travel. This does the same, but indirectly. The rotation of the triangle moves the shock out to where it has more leverage on the wheel.

By the numbers, it should have good action. Fabrication looks like trouble, though. Keeping the shock/triangle joint from buckling sideways would be the big trick.

There are a few other ways, but that's one.

A dual-shock, dual-linkage setup? That's cool and very different; I like it!

As far as keeping the shock/triangle joint from buckling... how about an inch-or-so thick piece of 7075-T6 for the triangle, with some structural webbing created by milling out some of the material to keep the unsprung weight down and a supported roller bearing on the end closest to the axle? That bearing could ride on a stub welded into a gusset in the swingarm, with the bearing half supported by the swing-arm structure itself...

I need a napkin!

I think in the spirit of vintage motorcycle racing, that no computers should be used in the fabrication of this bike. You should only use a vintage-correct Texas Instruments calculator (the kind with the red numbers).

NORRA sanctions the use of any of these during the design and fabrication of this bike: http://www.vintagecalculators.com/html/texas_instruments_calculators.html

Any use of a computer (Commodore Vic-20 through present) is in clear violation of regulations.

The TI Datamath... According to my Dad, the best $136 you could spend in September of 1972

That's a really neat idea Luke, but I agree with you that supporting that kind of linkage hanging off the outside of the swingarm is going to be a nightmare. It's going to put some crazy torsional loads on the swingarm, and mounting that thing in single-shear is going to be difficult to make strong and light. Maybe see if you can get the linkage of up top of the swingarm. Would eliminate the twisting and make it super-simple to mount in double-shear.

...shocks with progressive springs instead?

So you guys are volunteering to do the geometry calculations by hand? That's awesome. Too bad you both have terrible taste in calculators. HP FTW.


Making the linkage strong should be easy. Starting with bracing the triangles around the front of the tire. The hard part is doing it without adding 20 pounds of unsprung weight.

Progressive springs are the usual way for traditional twin-shock geometry, but my understanding is that a progressive geometry is better. Especially since the progressive action works on the damping as well. And yes, I've though of putting a pair of PDS shocks on this thing. If I knew how to tune them properly I'd go for it.

Version 1 will definitely be no-linkage, straight-rate, twin shock. For simplicity and expense. Depending on how well it works I'll try something else, but it's fun to think about the possibilities.

Hey now, I'm no rocket surgeon! Kidneys man, kidneys!

Seriously though, I'd love to see a dual-shock, dual-linkage setup in action...

It's long past time for an update.


Not much happened for a while- there was planning, but no doing.

The centerstand was extended. It's not going to work as-is; it hits the chain when retracted. It's nice to have for moving the bike around, fitting things that go under the frame (like the exhaust), and checking the ergonomics.



On the ergo front, I made a seat and footpegs out of wood. They're in about the right spot relative to each other.




The shock isn't supporting anything. On the other side of the bike is a strut that is drilled out for four different lengths. Compressed, swingarm straight, race sag, and extended. That makes it really easy to check chain length, shock position, etc. Here it is at the 'swingarm straight' position.






So yesterday Cory (Twinduro) came down from WA. It was a long day. Six hours of talking followed by six hours of work. As Cory said, "Bullshitting can't be rushed." Another friend David stopped by around noon to help with the talking.

Eventually we got to work. The plan is for a hybrid 550/650 motor, so we did a dry fit of the bottom end parts. We started with the transmission, which is getting 550 shafts and 1st gear, and 650 2nd-5th. This should give almost the same wider ratio of the 650 but without the big 1-2 gap that Honda seems to like.

The start. 550 on the left, 650 on the right.


Definitely no 'during' pictures. It took all four hands and a lot of concentration to do the swap.

Cases ready for the transmission:


And with the transmission in place:


It all fit except for one of the shift dogs. It had to be ground down a little to clear second gear.


Checking the 650 oil pan and pickup fit.





After flipping the motor over (the bottom end is assembled upside down) and installing the rest of the parts we had an almost complete bottom end.


Other side, with a list of needed parts.



Halfway through the motor and a page of parts to buy? Sounds about right.

Hey Everybody,

It was a ton of fun wrenching with Luke on the CB desert sled the other day, as well as having the chance to hang out with Rick, his wonderful wife Jan and Luke's friend David; all fantastic folks! As was mentioned, a brief "bench racing" session may have occurred before we got to work....

The first order of business was removing all of the cylinder studs from one of the spare CB550 cases. Man 'o man those things were in there! With Luke manhandling a 2 foot, half-inch drive breaker bar attached to a stud puller, myself armed with a heat gun set to burn, and with both of us holding the case to keep it from flying off the stand, we eventually got them all out. No photos, but I'll let you use your imagaination...

Here's a photo of the offending shift dog rubbing against 2nd gear after the 2nd-5th gear CB650 transmission swap:



Comparing the CB550 shift dog to the CB650 dog showed that the 650's dog had about 1mm less material on it then the 550's, where it sits near 2nd gear...

...which resulted in "the daily grind."



It was a blast to help Luke do the dry-fit on the bottom end; to say I learned a ton throughout the day would be an understatement! I've never been this deep into a SOHC Honda Four, so it was an enlightening afternoon.

Can't wait until the next installment...

Went out to the shop Friday night after work, everything was all cleaned up and the dry fit bottom end sitting on the engine crate (note Danny sniffing around Luke's work bench, apparently uncle Luke has been feeding him sandwiches and beer when I'm at work!);



Close up;



Luke and Cory had a great time working together, being the old man, I went in the house at 9:00 PM Thursday night and they were still in the shop working and laughing. Good stuff!

i have an old cb750 gathering dust and not going anywhere for awhile so i'm dying to see where this ends up cuz now i'm thinking of turning it into an enduro.......

Wasn't that crate screwed shut?


I'll definitely do the D100. The only question is which class. ADV? Vintage? Full distance?

It's happy hour in Kansas, I take it. I didn't want to do a 750 because they're heavier, but it'd still be lighter than a BMW. And if you don't mess with the motor it'd be much simpler than what I'm doing. Hey, why not?

That's a great story about the crate Ladybug, thanks for sharing! I would have never figured...

To Bluebone: As long as the tires aren't super-slick, try cow-trailing your CB750, it's a lot more fun then it sounds... I used to do that on my CB500 Four and the upright ergos and soft suspension made it pretty fun on smooth trails and gravel logging roads. Just go easy on the throttle at first, as off-roading the stock CB takes finesse and is horrifying, I mean, is a fantastic learning experience

When I got home from work last night, Luke and Doc Sprocket (and Larry & Danny) were busily working on the CBZR-650 frame. Pretty soon Ken Fisher stopped by to pick up a truck we had worked on for him and dropped off some of his delicious beer. Ken Fisher is the mastermind behind Grateful Deaf Brewing and an old friend and classmate.

Luke seemed to like Ken's beer;



Later in the evening, we dug out the 110V wire feed welder (it hadn't been used in at least five years) and after several adjustments and test welds, used it to weld in two additional footpeg mounting bosses. The initial welds weren't pretty, but as soon as a dedicated power supply for the welder with the proper amps is installed, we should be good to go for smaller gauge welding projects.

Yup, yesterday was a welding day.

First off was to replace the part that was cut out of the frame before.



It went back pretty easily, including a little round plate we added in to make up for the kerf.

The stock footpegs go on with a single bolt. That doesn't have a history of working very well on dirt bikes, so I added lugs for a second bolt. Well, tried to add them. The first weld was a disaster, so I ground it out and Rick saved the day with the MIG.


Anyone have any tips on doing fillet welds on thin steel with an AC stick machine?