The International CBX Owners Association

I agree with you from an aesthetic point of view EMS but the weight saving and convenience of a 6 into 1 are compelling on a race bike. I have kept the 6 into 6 for sentimental reasons despite the extra weight and having to spend far too long undoing lots of nuts and bolts just to adjust the chain or pull the rear wheel off, something that has to happen far more frequently on a racer than a street bike.

On most race bikes it is all springs and dzus clips and as a result it takes only seconds to pull off the bodywork and exhaust. I must say the work evident in those Schuele pipes is impressive. It has prompted me to dig around in my archives for what I think is the sine qua non of the art - the Britten. I had the good fortune to help out with this particularly special Waterworks Britten at the centenary IOM TT in '05 when Hugh Anderson road it in the Legends' parade (not) - it was a race once they cleared the Gooseneck! Mick Grant Hughie and Ago all on very different machines, they finished in that order under the proverbial blanket.
R.

They had all but one Britten ever built at Barber Vintage Festival a couple of years ago. Intriguing bike. Britten passed away way too early. I wonder what else he could have come up with.

John Britten inherited his old man's property development business in Christchurch, NZ. It financed his 2 wheeled and other adventures. I suspect he may have ultimately given the bikes away after the tragedy at the TT and had he lived I'm sure he would have contributed a lot to the re-building of Christchurch after the devastating earthquake. His designs and buildings were all interesting and he was something of a polymath.

He put together a talented team to develop the bike (and service them still) but it was his drive, determination and money that really made it into a world beater. At considerable cost to some of the team, it has to be said. He was also a pioneer in developing carbon fibre as a material for motorcycle wheels, chassis and bodywork. Most of it done by trial and error and painstakingly hand made. The Kiwis continue to impress.

I have bitten the bullet and now have a custom rear shock for the lump made for me by Wilbers. It has a 10mm longer shaft in addition to an extended 15mm threaded ride height adjuster as opposed to 6mm b4. I use that military expression because Roly Skate has assured me many times that this will not work and will make the rear end pogo. The Oz CBX guru gave me to pause for a long time.

However, some time ago I had the CBX Computracked. That is, measured very accurately with a machine like a bench top theodolite that triangulates all the critical points on the bike. The Computrack for those not familiar with this tool was developed by Australian Greg Roberts and is licensed to manufacturers, race teams and specialist chassis builders all over the world. Many motorcyclists are familiar with Tony Foale's book on m/c geometry but few have the mathematical facility to follow the theoretical intricacies of a dynamic 2 wheeled machine. The Computrack does it for you because as the data is painstakingly collected it is stored in a program that at the end enables you to bench top your chassis and suspension. This predictive ability will tell you what effect a modification to any single component of the chassis will have on castor, camber, swing arm angle, sprocket positions, rake, trail, ground clearance, etc. This tool takes the guess work out of chassis set up and is extremely accurate.

So when I looked at my Computrack results and considered extending the rear ride height as a means of over coming one of the CBX's biggest drawbacks as a racer - lack of ground clearance; all the indicators were positive. In particular, the current fairly flat swing arm angle of around 6 degrees had plenty of scope for improvement to around the ideal of 12-13 degrees. Rake and trail can be balanced out by dropping the forks in the clamps and lifting the rear ride height. I say "indicators" because I could not rely entirely on the figures because I had de-raked the front end by around 3.7 degrees and I really should re-measure the machine again after this modification. Nevertheless, that is no longer convenient to do and the 'indicators' are still pointing in the right direction.

I should add here that the other most active CBX racers, Jame Fisher in the UK and Bill Brint in the USA are both running on 18" wheels whereas here we use 17" wheels that provide a wider choice of rubber but involve a 2" loss of ground clearance from stock at the front. So lack of clearance is less of an issue for stock CBX road bikes but a real problem for my race bike. So much so that if Roly is right and the theory is all wrong, an option of going back to 18" wheels will be under active consideration.

Now all I have to do is re-build the bike and try it out. My next planned outing on the big 6 will be in May at the Mac Park Seniors. Hopefully I will get some practice in b4 then to get the settings optimised but I also have some other mods in the pipeline so I'll be in the workshop if any body calls...
R.

We are working on the possibility of a series of GMD Computrack workshops at the track. Measure up some bikes, discuss race set up and combine it with coaching sessions with some of our international pro racers, who are also members of the Mount Gambier Motorcycle Club. There is a possibility that Greg MacDonald himself may attend. If we can pull it off the CBX wll be put back under the microscope after the various chassis and suspension mods.

Have I mentioned that this club is the only m/c club in Oz and possibly in the world to own its very own road race circuit? And it is not a dinky go kart track but a proper grown up race track that meets international FIM superbike standards, with a long history.

Here are some pics of the new 6 into 1 exhaust we are fitting to the lump. Bill Brint has fitted the same system to his racer and thinks its a bit of alright. We shall see...
R.

These are all the bits that make up the exhaust. The only mod I would make at this stage is to replace the exhaust collets and nuts at the headers with springs because they and the 12 nuts are too fiddly for the track.

That looks like the new Delkevic system. I like the looks and it is very reasonably priced.
Best,
Phil

Yes and yes.
R.

OK, I'm finally about to install the extended rear shock; something that I started canvassing here a long time ago. For those of you interested in the finer points, the rear 640 Wilbers shock that Trevor Manley initially installed and set up was 390 mm long with the ride height extended almost fully to a final fitted length of 395 mm.

My now newly modified shock has an extended internal shaft by 5 mm for a starting length of 395 mm and if I wind out the ride height fully it will have a new length of 400 mm. In addition, I have a second screw in adjustable eyelet & bush (alternative adjustable ride height fitting) that has a 10 mm longer thread meaning I can go as long as 410 mm, in theory.

Some time ago SWarren suggested on this forum that the prolink rear linkage ratio was something like 4:1. I said I thought it was more linear and as best as I can determine (from an early Oz review of the B model) the ratio varies from 2.78:1 to 1.92:1 when fully compressed. If this is correct then every 2 mm I add to the length of the shock should result in roughly 4 mm increased ride height at the rear. Roughly.

This means that to match the front and retain current geometric neutrality I need only extend the rear ride height by about 10 mm.
I invite the reader to challenge my figures on this because it will have a direct and critical impact upon trail at the front. I have already lost 20 mm of trail with the de-rake from 27.7 to 24 degrees going from 120 mm to 100 mm and I don't have that much left to play with.

Accordingly, I plan to start cautiously by dropping the front forks down fully in the clamps giving me an extra 20 mm at the front and more importantly, increasing trail. Then go up in increments of a few mm's at the rear until I start to find a compromise between lighter, quicker steering and adequate trail, so it retains some stability. This could be a bit tricky and I am in dangerous territory as there is also swing arm angle and the run of the chain over the arm to consider. If SWarren is correct the affect on the front end will be considerably more dramatic. Any suggestions or advice from the CBX Brains Trust at this juncture would be appreciated.

Now Roly reckons it won't work and I'm wasting my time but we will see. Most of Roly's development followed on from his long experience with the twin shock and I know he simply carried over a lot of that to the prolink. I'm not sure how much he experimented with the single shock. And as far as I'm aware he was the only person to seriously race a prolink. So, I'm in virgin territory with this crazy project. But nothing ventured, nothing gained.
R.

I've now measured everything with the 2 ride height adjusters set to their extremes. The minimum length of the shock with the longest adjuster screwed all the way in is 302 mm, exactly the same length of the shortest one screwed all the way out. That is a 7 mm increase over the length of the shock as Trevor set it up and when it came out.

If the linkage ratio given above is correct for my C model prolink, the maximum ratio is 2.78:1 meaning that 7mm at the rear will equate to a ride height increase of 19.46 mm, very close to my 20 mm increase at the front. Perhaps too close and therefore I will start with he shorter ride height adjuster. This is all in theory, of course.
R.

And if you are more awake than I was last night you will notice that my shock length figures were a bit awry. 295 not 395. Only by 100 mm! Duh!

If that was me you are referring to Warwick it was probably just an estimate on my part based on the longer shock I had made to make my bike easier to get on the centre stand and the side stand more secure. It should be relatively easy to actually plot the geometry on a CAD package - I might try and have a go sometime for you but it will mean re-learning how to use CAD so if anyone is already adept it might be better they do it!!
Simon

Thanks Simon, it was. and at nearly 3:1 you were probably closer to it than I was. That is, if it really is 2.78:1

Somebody in ICOA must know. C'mon guys, the linkage ratio for a C model prolink???????????????????????

The shock is now back on the bike after a lot of fiddling. Its not the most accessible thing to fit and the linkage has a very limited arc to work with but lots of cable ties and a rubber mallet convinced it to return to its proper place. And I used the shorter adjuster.

Now I have to fit the new exhaust, make up some new throttle cables (a truly horrible job) and re-jet the carbs for the new pipes and have it ready for a practice day in a few weeks when all the BEARS will be out en mass for their Easter Cup.

swarrans wrote:If that was me you are referring to Warwick it was probably just an estimate on my part based on the longer shock I had made to make my bike easier to get on the centre stand and the side stand more secure. It should be relatively easy to actually plot the geometry on a CAD package - I might try and have a go sometime for you but it will mean re-learning how to use CAD so if anyone is already adept it might be better they do it!!
Simon

It's simple geometry. You don't need a CAD program. Good old Pythagoras will yield most of the results you are looking for.

That alloy dog bone is aptly named and an irregular shape. It is not as simple as measuring the distance between the pivot points. If you just look at the struts they appear to be of similar length but it has a progressive action. Somewhere, Honda must have published the linkage ratio (it would be on the patent application) and somebody somewhere must know...???