CB900 Scrambler - Rally Approved!!

[LBC]

Re: CB900 Scrambler - Stage One Complete

I got the new charging rotor in the mail. Put it on and put the cover back on. 13.1V at idle. 14.5V at 5000 rpms. Money!

So the stock Reg/Rec is good and will serve as a nice backup when I install the CBR R/R upgrade.

One thing I did notice is that it seems the old rotor was rubbing the inside of the coil at some point. I'm not sure if that is a new thing or an old scar and I'm not sure if the friction caused the failure or just old age. But it's all good now....

 

[LBC]

Re: CB900 Scrambler - Stage One Complete

Alright, the forum doesn't like it when I post pictures.

The bike looks awesome. I definitely get the dRift Bike concept. Progress and execution thus far has been inspirational.

So to keep it going, I'm offering up some old XR650R junk. For your consideration: triples, forks, stem, axle, wheel, caliper. Let me know if you need specs, or anything. You pay brown and I'll send it your way.

Crowd-sourced custom budget ralley race bike would be awesome. Swingarm anyone?

Hey Rally Sport fans! Here's a cool update from the ADVrider.com version of this thread. Looks like I'll have a fork upgrade in my future. Stockers are 35mm tubes. The XR650R has 46mm tubes and more travel.

My measurements lead me to believe I can modify the stem and this whole front end will bolt on.

"Those are way too long you idiot," says you
"I'll shorten the travel with internal spacers," says me.
http://www.therevcounter.co.uk/trc-shed/64382-shortening-xr650r-forks.html

I'll end up shortening the springs as well which will in turn give a stiffer spring rate. (something I'll probably need anyway.

Here's the math that explains the spring rate change. Found on Thumpertalk.

Wire diameter =W
Center to center coil diameter =C
Number of active coils =N

Spring rate = W⁴x 80400/ (8xC³xN)

Answer given in N/mm. Convert to Kg/mm by dividing by 9.81

A coil spring is, after all, just a torsion bar. The wire is twisted as the spring is compressed. Shortening a torsion bar without changing any other dimension will obviously make it stiffer. As a previous poster said you need to check coil free space to make sure the spring doesn't coil bind with the new fork travel.

 

[DV2000NJ]

CB900 Scrambler - Stage Two Begins!!

I like that plan a lot, actually! I may have missed it, but did you figure out what you're doing for the rear yet?

 

[notlob]

Re: CB900 Scrambler - Stage One Complete

Hey Rally Sport fans! Here's a cool update from the ADVrider.com version of this thread. Looks like I'll have a fork upgrade in my future. Stockers are 35mm tubes. The XR650R has 46mm tubes and more travel.

I put 2001 ZX9r forks on my CB900 and they are 46mm. Just needed a bush making to fit the CB stem to ZX Yokes.

 

[LBC]

Re: CB900 Scrambler - Stage Two Begins!!

I like that plan a lot, actually! I may have missed it, but did you figure out what you're doing for the rear yet?

The stock forks are 31" from top of the fork to the axle. If I add 3 inches up front and 2 inched in the rear. I'm at 27o of rake and 6.5" of trail which is in the neighborhood of where most dirtbikes fall.

Completely uncompressed length of the stock shocks is 360mm (~14"). I think I can add that 2 inches to the rear and still be fine in terms of chain rub on the swingarm. If I do have a clearance issue I'll machine a teflon slider to mount on the swingarm. It's good enough for a 100+mph dirtbike so it will work here. With the shocks off I run out of clearance between the seat and tire at 285mm(~11") so that's my limit of the compressed length of whatever I get.

I'm still searching for a set of shocks in the 16" range. Aftermarket shocks that long seem to generally be spung soft for lighter vintage off road bikes or sprung heavier for quads/side-by-sides.
I'm open to ideas.

Hagon makes some 420mm trail shocks with adjustable pre-load but I haven't heard back yet whether they have springs that are heavy enough. Progressive suspension has some in that neighborhood as well.

I haven't decided if I want to extend the axle carrier to gain some travel. I need to do some more research on how that might effect overall handling.

The thought of that KTM swingarm with the linkless PDS suspension has crossed my mind more than once as well. :evil I don't think I can tackle that before race day though. Also not sure how it would handle the extra 200 lbs of bike even with a stiffer spring.

 

[clem]

Re: CB900 Scrambler - Stage Two Begins!!

KTM linkageless was a stinkbug. Pretty much retired the king of supercross Jeremy McGrath. Oh but this is a street forum LOL!

 

[LBC]

Re: CB900 Scrambler - Stage Two Begins!!

KTM linkageless was a stinkbug. Pretty much retired the king of supercross Jeremy McGrath. Oh but this is a street forum LOL!

street forum? Who says you can't do the ton off-road?

I guess the MX guys never came around the the PDS system but it seems to work in the woods. (shrug)

 

[LBC]

Re: CB900 Scrambler - Stage Two Begins!!

I put the order in for new shocks today.

Going with Progressive Suspension Series 13 in a 16.5" length. The part number is 13-1241B for anybody keeping track. These have 5.9 inches of travel without the bump stop so I should get a full 5" of travel out of them. ;D The dampers and springs will be $200 for the pair. To go with something that offers some adjustable damping is closer to $600 and I'm too cheap for that right now.

The part number for the springs is 03-1323B and each one uses two springs with rates of 80 and 175 lbs/in.

This shock isn't listed as a fit for the '79 CB750k but the 15" version listed for the '84 CB700SC has this spring rate as the one to use so that's what I'm going with.

Some of the progressive suspension instructions list a set of springs with part number 03-1324B that has 90 and 195 lbs/in springs but they seem to be discontinued. If the springs I ordered end up too soft maybe I can find a set of these stiffer springs.

Here's the propoganda:
Double-wall steel shock body for durability.
Low pressure foam cell for superior, fade-free damping.
Velocity-sensitive valving automatically adjusts for optimum ride and handling.
5-position spring pre-load is adjusted by a cam.
Precision ground, hard chromed 1/2" diameter steel shaft.
16.5" long, mounts eyelet-to-eyelet.

 

[notlob]

Re: CB900 Scrambler - Stage Two Begins!!

I think the sc is a heavier than the K and you have shed quite a bit of weight anyhow, so I'd be tempted by the lower spring rate.
I put stock hagons for a CB750 on mine and they were a bit harsh after the weight loss program.

 

[crazypj]

Re: CB900 Scrambler - Stage Two Begins!!

High ratio linkless shock is never a good idea, someone at KTM knew that so it was probably deliberate sabotage (Maico tried 16:1 ~18:1 ratio in the 80's, problem is you need super heavy spring then super heavy damping to control it)
Yamaha were on;y using 1.7:1 on the 'Monoshock', anything over about 4:1 gets very difficult to keep under control, less than 2:1 is better so shock doesn't overheat too quick

 

[LBC]

Re: CB900 Scrambler - Stage Two Begins!!

I think the sc is a heavier than the K and you have shed quite a bit of weight anyhow, so I'd be tempted by the lower spring rate.
I put stock hagons for a CB750 on mine and they were a bit harsh after the weight loss program.

I went back and forth a bunch of times. Especially since the shorter shocks that would be on the dohc cb750f/cb900f have a different rate listed as standard(that spring is shorter though)

Specs on the 84-86 nighthawk said the wet weight is 497lbs(255kg). My bike is weighing in at 520lbs full of fuel. (Plus a 215lbs of rider. Plus 20lbs of gear)

There are heavier and lighter spring options from what I've ordered and the springs are only $60. I'll let y'all know how it goes,

 

[LBC]

Re: CB900 Scrambler - Stage Two Begins!!

High ratio linkless shock is never a good idea, someone at KTM knew that so it was probably deliberate sabotage (Maico tried 16:1 ~18:1 ratio in the 80's, problem is you need super heavy spring then super heavy damping to control it)
Yamaha were on;y using 1.7:1 on the 'Monoshock', anything over about 4:1 gets very difficult to keep under control, less than 2:1 is better so shock doesn't overheat too quick

Sounds right. I've seen over sprung shocks come apart from the increased work load. I think I have to stay dual shock for the look and feel of this one.

 

[crazypj]

Re: CB900 Scrambler - Stage Two Begins!!

Good shocks and braced swing arm will be the best option for a 900 that's going to be traveling way too fast on gravel, etc
Pretty sure the extra 4~5lbs isn't going to be noticeable tossing it sideways around 130mph 8)
I'm still having a problem getting my head around the concept of CB900 'off roader' (although I'm having trouble thinking about most things at present)

 

[Cuba Libre]

Re: CB900 Scrambler - Stage Two Begins!!

Specs on the 84-86 nighthawk said the wet weight is 497lbs(255kg). My bike is weighing in at 520lbs full of fuel. (Plus a 215lbs of rider. Plus 20lbs of gear)

Not trying to be a smart ass here, but 497lbs are 226 kg (which is conservatively heavy - 255 kg is mamoth-like!).


I don't mean to highjack this thread, but speaking of shocks, is there some sort of information on how to choose spring rates based on the weight of the bike, or its front/rear distribution, or the rider, or whatever?

Thanks
M.

 

[johnu]

Re: CB900 Scrambler - Stage Two Begins!!

High ratio linkless shock is never a good idea, someone at KTM knew that so it was probably deliberate sabotage (Maico tried 16:1 ~18:1 ratio in the 80's, problem is you need super heavy spring then super heavy damping to control it)
Yamaha were on;y using 1.7:1 on the 'Monoshock', anything over about 4:1 gets very difficult to keep under control, less than 2:1 is better so shock doesn't overheat too quick

No problem with the linkless shock in the dessert or for supermoto as far as I can tell!

 

[crazypj]

Re: CB900 Scrambler - Stage Two Begins!!

Of course it works, just no where near as well as it could work
They took weight reduction to an extreme at the expense of reliability
All high ratio shocks can be made to work properly, it's just real expensive plus high maintainance (even on the little things like mounting bolts/bushes)
Majority of people will just ride until it breaks then criticise the POS, routine maintainance at 'playbike' level mainly consists of putting gas in it and maybe some air in the tyres

 

[LBC]

Re: CB900 Scrambler - Stage Two Begins!!

Not trying to be a smart ass here, but 497lbs are 226 kg (which is conservatively heavy - 255 kg is mamoth-like!).


I don't mean to highjack this thread, but speaking of shocks, is there some sort of information on how to choose spring rates based on the weight of the bike, or its front/rear distribution, or the rider, or whatever?

Thanks
M.

Good catch. I broke my "never do english to metric conversion in public" rule and have paid the price. ;D

The best way to get the right spring rates is to call an expert. Every continent seems to have a go-to guy that sells the good stuff (Hagon,YSS, etc)and knows it in and out. They take into account your bike weight, your weight, riding style/terrain, wallet weight, and they build the combination you need.

I chose to base my number on the recommended spring for a similarly heavy bike (in lbs not kg) and cross my fingers. It's the guess and check method. 8)

Here's where I got my info for this particular brand.
http://www.wps-inc.com/catalogs/street#PRVCID=ST&CICID=ST&PRVTID=A105++++++&TITID=A255&PRVSID=A110++++++&SISID=A110&PRVPAGE=85&PAGEGO=Page%23

https://www.google.com/url?q=http://www.progressivesuspension.com/pdfs/7100-105.pdf

 

[LBC]

Re: CB900 Scrambler - Stage Two Begins!!

No problem with the linkless shock in the desert or for supermoto as far as I can tell!

HA! I knew the linkless discussion would polarize the audience. :-X

Nice avatar johnu!

I'm a mere mortal rider so I've probably never been held back by the capabilities of my bikes suspension, or tires, or motors, or.....

 

[crazypj]

Re: CB900 Scrambler - Stage Two Begins!!

Get a wood block the thickness of bathroom scale so you can keep bike level and just weigh front/rear, it may be a little lighter than you expect (maybe not )
You'll get an idea of bias as well then can move footrest/handlebar position
Gotta love someone who's going to motocross Godzilla, your madder than me ;D

 

[LBC]

Re: CB900 Scrambler - Stage Two Begins!!

Get a wood block the thickness of bathroom scale so you can keep bike level and just weigh front/rear, it may be a little lighter than you expect (maybe not )
You'll get an idea of bias as well then can move footrest/handlebar position
Gotta love someone who's going to motocross Godzilla, your madder than me ;D

How far off is the measurement if you don't use the wood block? Another way to ask it would be how far forward or back have I moved the cg by lifting the tire 1.5 inches to put it on the scale? I'm going to say that's within the uncertainty of my bathroom scale accuracy.

I like free-body diagrams. Where's that geek emoticon? :

I got 250lbs at the front wheel and 270lbs at the rear wheel. 4.? US gal of gas and 3.? quarts of oil.


I'm just going to be running some gravel roads. No moto-x jumps...well...only a few for a the highlight reels.