CB desert sled

Your a Brave man. :eek1
CB550's and 650's are heavy stock and yours is heavier and taller.
And probably alot more powerfull with those CR26's.

Good Luck.

I say if your going to make it, go all out.....
Open Megaphones. There is no other way.

You want the competition to do a double take and say
"What the f*ck was that" as you blow by them with the bike wound up to 9500rpm.

Only just found this thread. Spent the last few hours refusing to leave our hostel (despite my Gf telling me we should do some sight seeing) reading over it and finding myself getting so excited that a little bit of wee dribbled out. Hats off guys, this is awesome.



Can't wait to see the finished product.. You tube video link of you giving it some stick please...

Giving it some stick??????


Oh, like this?



I was hoping to avoid that sort of thing, but if there's video I'll post it.

Tonight Luke wanted to get the bike up on it's wheels and take several measurements and also weigh the bike. It appears Luke's amazing mojo on this project is still intact!

With front and rear suspenders at full extension, measuring fork rake, seat height, ground clearance, etc. Will let Luke fill in all the details.



Next at "sag" height;



And then at full compression;



Then we stuffed in the engine and hung on as many components as possible, time to weigh in with the Last Chance Garage's super scales, accurate to within + or - one stone;



Front wheel @ 150lbs.



Rear wheel @ 175 lbs.



Total weight at this time; 325 lbs. To say we were happy with those numbers (and the rear weight bias) is putting it mildly!

Today was short but important. I finished up the lower shock mount that was started on Saturday, then clamped it to the swingarm in about the right spot. I mounted up a shock and measured the bike's geometry. Unfortunately it didn't turn out right- the rake is a couple of degrees more than it should be.

Rick dropped in about this time and we re-measured everything, which confirmed it. I'm not sure why it's off, best guess is that rake was off when I measured the original frame. Regardless, the choice now is to move either the top or bottom shock mounts. Moving the lower mounts will be easier, moving the top mounts should give better results.

Bad news, but not the end of the world. The results of the weigh in make up for it. Rick was predicting 400#. I was hoping for not much more than 350#. We were both surprised.

Here are some interesting and possibly useful specs for a big desert sled (note rake/trail & weight, wheelbase is another factor);

BMW F650GS Dakar

Manufacturer BMW Motorrad
Production 2000–2008
Predecessor F650 Funduro
Successor F650GS (twin) & F800GS
G650GS
Class Dual-sport
Engine 652 cc, Single-cylinder, Water-cooled, Four-stroke, DOHC, 4-valves
Power 50 hp (37 kW) @ 6500 rpm
Torque 44 lb·ft (60 N·m) @ 5000 rpm
Transmission 5-speed, O-Ring Chain
Brakes Front: 1 disc, 2 piston caliper; Rear: 1 disc, 1 piston caliper; ABS optional
Tires 21 inch front, 17 inch rear
Rake, Trail 29.2°, 4.9 in (124 mm)
Wheelbase 58.6 in (1,488 mm)
Dimensions L 86.2 in (2,189 mm)
W 35.8 in (909 mm)
H 49.8 in (1,265 mm)
Seat height 34.3 in (871 mm)
Weight 390.7 lb (177.2 kg) (dry)
425.5 lb (193.0 kg) (wet)

We all know those are terrible bikes and shouldn't be emulated in any way

Instead of changing the rear suspension how about making an off set stem to decrease the trail?

How do you make those?


Just making a regular stem would take me a lot longer than moving the shock mounts. There are a couple of other advantages to lifting the back end: I get a little more progression in the suspension action and about the right amount of ground clearance.

It was just an idea for if you had the swing arm angle where you wanted it but if you want a more progressive shock action and and more anti squat then moving the shock mounts will certainly solve all those at once.

Have you had a look at tony foales book and suspension program? Its a great read and the program really helps with working out suspension geometry.
There is also a program for push bikes that helped me a lot to work out the rear suspension on my project bike, I think it was called linkage but I'm not 100% sure.

I've been borrowing Foales' book for almost a year now, should probably just go and buy my own copy. I wrote some of my own software to figure out linkages and levers and mounting points, but it's always going to be garbage in garbage out.


What did you end up setting the rake to on your drz fork build? I like the idea of just copying the drz rake since the offset (and trail) are already set on the forks but I can only find one second hand source for the rake (27.6 degrees). I can slide the forks to give +/- half a degree on the rake, but would like to start out as close to right as possible.

I like you tried to copy the drz rake but ended up setting it less than the drz because it the bike just handled better.
I'm not sure what I ended up with, I'll measure if later and get back to you.

With my dr the biggest factor for improving the handling was increasing the swing arm angle and getting rid of some squat, The dr has very flat swing arm compared to other dirt bikes.

Have you seen the article tony did about rake v trail
He experimented with the rake in a bike by reducing it in stages all the way back to 0 degrees the whole time he kept the trail the same.
The result was quite interesting. He found stability was not changed as long as trail was kept the similar.
There where other effects like less front end dive and the steering was not effected by terrain so much.

Yes, I just re-read it last night :)

For those reading along, this one:
http://www.tonyfoale.com/Articles/RakeEx/RakeEx.htm



That'd be great if you could measure what you ended up with. That'd make a great starting point. Especially since the more I research the DRZ geometry, the more numbers I get. On the upside, I've been measuring other bikes and it seems that the rake is always* measured with the suspension fully extended. That makes things simpler. If it's true.


*well, on 3 of 3.

I'll measure it tomorrow.

To be honest I think I fluked the geometry on the dr.
It was pretty good at the first attempt but I felt it needed less rake but I couldn't get any less without major operation.
After a while I thought just for an experiment I would drop the forks back down the triples and increase the rake slightly.
Low and behold it handled even better, its more stable and turns quicker.
After that I decided that the next bike I would make any thing to do with the bikes suspension and steering geometry adjustable because the chances of getting it spot on first time is slim even if you do copy another bike.

I think power, weight, centre of gravity, gearing and riding position has got more to do with a bikes handling than I first thought.

I get what you're saying about adjustability. I just may end up making the rear frame totally adjustable. As it is, I should have at least made the top shock mounts eccentric, as that would give an easy inch or so of adjustability. I might still do that.



I went over to a friend's place last night and measured the rake on his DRZ, and got a different number than any of the published ones. Oops. Then I noticed he had street tires on it, so I measured the axle heights. It turns out that the different tires change the rake by almost a full degree, so maybe I'm just obsessing at this point.

Uh maybe just slightly.

Next you'll be worrying about how many tacos you eat before the start of the race, you wouldn't want to move the centre of gravity buy over eating.

Ya think??

Luke, you have 105 days until the starting line. 105 days. Just build a damn dirt bike...stop engineering a freaking masterpiece!! We still need to test the shit out of that thing!! You have less than 3 weeks left to have the bike running and rideable;; according to the schedule you told me

I need a drink....maybe you can build us a still so we can make our own whiskey while you're in the fabricating mood....




Just kidding...looks great! Now how about turning that inline four into a rotary engine for the race?? I mean, you have the lathe all warmed up...shouldn't be too tough....

Hmm... A rotary powered desert sled? That sounds like fun!

Except for the blown rotor seals just south of El Arco.

A quick progress report and pics from today, Luke and Al will add more later, I'm sure!

Stator cover is off to make an adapter or shim, in order to mate the two dissimilar gasket surfaces;


Shim is progressing;


Shim finished, now to shim brush pack;


Brush pack is held in by two small bolts, removed those (after this pic was taken) and it appears no special shim will be required, just two small matching washers;


Custom footpeg mounts in progress;


Measure once, drill twice??


Doesn't the kick starter go in here somewhere?


Progress seems to create debris fields;


Progress!!!