CBX Racing

[Warwick Biggs]

The 17" wheels I've acquired are from a VTR Firestorm 3.5/17 front and a Kwacka ZZR600e 4.5/17 rear. I think these will fit with some special brackets and spacers because it is what Roly runs in his prolink Beastess with an only slightly modified prolink swing arm.

I will run the bike with the 'pretty' 18's at the PCRA 1st round at Wakefield Park in 10 days time and then attempt the conversion to 17's in readiness for the Classic Master of Mac Park in South Australia in early March. WP will also be a test for the Dyna ignition with the new 20 cell Antigravity LiFePo battery. If that goes well I will reconnect the Ignitek digital ignition that gives me 5-6 extra ponies across the range but consumes more amps and is more sensitive to voltage drops. In many ways a total loss bike becomes more of an electric bike as voltage consumption is more critical than fuel consumption.

To answer your question Syscrush, confidence is important of course, especially for an old fart attempting to reprise his 'glorious' youth after 40 years away from the race track. However, after being lapped and following champion pro racer Chas Hern on his mega hp Harris Honda at Broadford (he won the Oz Historic Titles on it) I could easily see how much more traction full width slicks give over my skinny half width 18's. I was sliding to the edge of the track coming out of the corners at a much slower speed and he was coming out fast & tight and had the throttle open long b4 I could pick my thing up. So yes, it is very obvious that it is an essential step in becoming competitive. Nobody in the paddock can believe I'm attempting to race such a big heavy bike on such skinny wheels.

I am hoping this will be the biggest single performance enhancement I can make without spending big bucks. I also expect that it will create ground clearance problems and I'm trying to figure out the simplest (ie: cheapest) way to pull back the 'easy rider' fork rake from 29.5 degrees to somewhere around 25 degrees. The stock 39mm forks with the TM modified dampers work OK but when pulled back should help with ground clearance and turn in. The 17's should also help the latter but create problems with the former. The stock TC offset is OK too as Roly doesn't need to run a steering damper, unlike Bill Brint's complex laser jig finangled front end on his racer (which does).

To that end I'm considering:- 1.cutting and shutting the steering head, or, 2. welding the head centre tube and re-drilling it at a 5 degree reduced angle or, 3. slightly modified triple clamps with the fork holes drilled at 85 degrees rather than 90. Any other suggestions gratefully received (and b4 anybody suggests it, yes, I have considered riding it into a brick wall - many times!).

Rick.

[Syscrush]

That all makes sense - modern sticky rubber is an absolute miracle. I don't miss it on the street but I also wouldn't race with the wheel/tire combo I have now. Keep in mind that with more grip, the forces that the suspension & chassis have to handle will also see a commensurate increase.

As for the rake changes - you could get 2° by jacking up the rear end 2", which would also give you back whatever cornering clearance you'd lose with the move to 17's - and would be much simpler than modifying the headstock and/or triples. This would also take out quite a bit of trail. However, it would mess with the anti-squat and could cause chain interference with the swinger, so it might be a no-go.

As for wheels - I learned this week that Dymag can make almost any size for almost any bike. Their normal manufacturing process is to forge a wheel of the specified size with a blank hub, then machine the hub to the spec for the intended bike - so they can do almost any combination at no extra cost. I know that the wheels would be a lot more expensive than the used OEM stuff you've lined up, but if this route would eliminate the need for custom fitting of stuff like spacers, brakes, axles, etc. the net increase might not be too bad. Also you'd get the benefits of lower rotating mass and lower unsprung mass in addition to being able to run the tire sizes you want.

Good luck!

[Warwick Biggs]

Since the last post we have completed the 1st Round of the PCRA (Post Classic Racing Assocn) championship and we are lying fifth in the unlimited Period 5 despite running skinny 2" rims. Then we travelled approximately 3,000klms to far southern South Australia for the Classic Master of Mac Park in Mount Gambier. On a tight rider's circuit we could only manage 22nd out of 27 starters in the main race which was won by Norton's top pro racer Dave Johnson (this time on an immaculate 900SS Ducati) who did a 132mph lap at last years Isle of Man TT. I raced against Dave's dad in the 70's and Dave held the 125 lap record at Mac Park for a decade b4 going off to the UK so I was happy just to finish on the same lap (again on skinny tyres). (pics coming)

So I am half way through the conversion to 17" slicks. I still have to find somebody to machine up the spacers for the axles and the front brake pistons (to accomodate much thinner VTR disks) after Dennis became indisposed. I need my own lathe.

I'm hoping to get this finished asap in readiness for another trip to Mount Gambier for the Old Farts meeting in mid May. I will have to miss the PCRA Barry Sheene mtg which is one of the biggest classic race bike events in the world and lose points but the atmosphere and racing at Mac Park is worth it. The Barry Sheene is held at Eastern Creek. It was built in the early 80's as a top international GP circuit but is now surrounded by the endless suburbia of greater Sydney. Sadly New South Wales has lost most of its great race circuits to a mindless enthusiasm for growth with the quality of life declining commensurately with the shrinking size of the houses and the increasing social problems. The 'global village' is a slum.

I will post more pics and details of the conversion as we go.












Rick.

[steve murdoch icoa #5322]

Well done, Rick.
Any race that you and the bike finish in one piece has to be considered a good thing.
And again, kudos for representing the CBX in such a positive manner.

[Mouse]

Plus..... Best sounding race bike at any race you enter!

[Syscrush]

It sounds like you're having a lot of fun hustling that big old machine around! I love seeing pics of the CBX at full lean.

I'm really looking forward to hearing about your experience converting to (and then racing on) the 17" slicks.

[Warwick Biggs]

So the 17's are fitted and everything is straight. I was incredibly lucky to find Nige Pilsch, a machinist who was a member of the Coronas GP team and worked in the US for some time in racing. How lucky can you get living in the Bush as I do but he is just down the road in the nearest town about 30klms away and altho retired he does do a bit of race bike work as a hobby. As you can imagine, he is now my new 'best friend' (altho' I haven't forgotten any of my other 'best friends').

He has a very expensive and special machine called a Computrack that surveys the bike using a very stable frame with a sliding laser theodolite-like device that triangulates and very accurately measures all the major chassis points on the bike. The results enabled him to turn up some very accurate spacers and mill down the calipers to ensure the wheels are exactly in line and the chain is straight. The end result was confirmation of what my seat of the pants suggested was a very good chassis with no twists anywhere and everything within a thou of specs. Something of a miracle for a heavy 36 year old bike of uncertain heritage. And now fitted with full width racing slicks. Hopefully, no more skinny cross plys.

Not Only That But Also...the computer program has a predictive function that enables the operator to adjust various components to see how it affects steering geometry. No guess work here. So in theory, pushing various components around, we came up with a more race bike like 24 degree rake with 100mm trail (as opposed to 27.7 degrees and 119 stock) and an additional 50mm of ground clearance at the swingarm pivot simply by fitting triple clamps with a 30 degree offset (as opposed to 45 stock) and a 1" extension to my Wilbers shock. In addition it sharpens the swingarm angle to improve drive out of the bends. These are the magical figures for anybody contemplating getting their Prolink to handle more like a modern sports bike and in theory it can be done without major surgery. You heard it here, first.

I certainly need the ground clearance as I reckon we lost 40mm's at the rear and over 20mm's at the front in the conversion. So my 'pretty' Bol dor wheels and skinny tyres are now retired to the bottom shed with the frogs and the Pipemasters 6 into 6 exhaust is destined for the same pile because I now need a high level race exhaust, at least until we can start experimenting with triple clamps and shock settings. I'm hoping Roly can help me with that if he can make it over to Mac Park for practice. For now I need to do a lot to get the bike prepared for the Mac Park Seniors races at Mount Gambier in mid May. I discovered when I returned from last month's meeting that I had been racing with a rigid front end as a result of over filling the front forks when I changed the oil. it resulted in hydraulic lock causing the front end to push when I hit any ripples, forcing me wide on some bends where I had to back off a bit. Now why didn't I realise that at the track? I do recall commenting to other riders that "the ripples seem to be getting worse" but they looked at me strangely. Well, yes if you have a rigid front end they would!

So I'm hoping for much better times with a functioning front end and a heap more traction. Finally, I feel like we are heading in the right direction to turn the CBX into a more competitive racer. Its still a crazy project but starting to look a little less crazy. Its not short on power but just needs more corner speed and something approaching agility so that I can change line mid corner without losing momentum.

More pics to come....

Rick.

[Syscrush]

So the 17's are fitted and everything is straight.

Great news!

So in theory, pushing various components around, we came up with a more race bike like 24 degree rake with 100mm trail (as opposed to 27.7 degrees and 119 stock) and an additional 50mm of ground clearance at the swingarm pivot simply by fitting triple clamps with a 30 degree offset (as opposed to 45 stock) and a 1" extension to my Wilbers shock. In addition it sharpens the swingarm angle to improve drive out of the bends. These are the magical figures for anybody contemplating getting their Prolink to handle more like a modern sports bike and in theory it can be done without major surgery. You heard it here, first.

Those rake & trail numbers sound nice for the track. My bike is about halfway between stock and where you are, and it made a huge difference and is very well-suited to the street.

But I don't understand the numbers for the triple clamps. Nothing on a stock triple clamp is anywhere near 45 degrees. Also, I'm sure that your newest best friend reviewed this, but are you going to have problems with too much anti-squat and/or chain fouling on the swinger with that 1" change in shock length?

More pics to come....

Can't wait!

[Rick Pope]

Are you sure that isn't a 30mm offset rather than 30 degrees? It would make more sense, but I haven't crunched the numbers. Where's EMS when you need him?

[Warwick Biggs]

Yes, you are correct, triple clamp offset of 30mm as opposed to 45mm stock. Hole in the head.Well spotted.

R.

[Warwick Biggs]

"....are you going to have problems with too much anti-squat and/or chain fouling on the swinger with that 1" change in shock length?"

Quite possibly, yes again but we won't know until we start fiddling. The figures are theoretical. I'm not sure whether I can get or make an extension to the shock or whether the linkages will accept the change. At this stage I'm just happy to have the wheels on and everything straight.

The disks I was obliged to use are slightly turned down VTR Firestorm floaters with the original floating CBX calipers. This creates another problem as our period rules ban floating disks unless they were original. Well the calipers were but the rotors do not look period. Roly riveted an alloy plate over the offending pins to disguise them (originally for use on a completely different bike) but we had to pull the covers off as there is less than 2mm clearance to the inner fork leg and short of shaving the legs, we will have to come up with some other solution. At the moment I'm thinking of filling the pins with silicon and painting them black. Or I might have to get another set of cast iron disks made up as stainless are not ideal on a racer anyway.

At the moment I'm trying to work out where I can take it locally just to test the basics. The nearest track is Eastern Creek in Sydney and hard to get onto. We used to have quiet rural roads here 30 years ago but these days there is traffic and fuzz everywhere. There is a spot called the Hunter (un)Economic Zone where a bunch of corrupt politicians put an industrial estate in the middle of a National Park based upon a proposed gas pipeline that never eventuated (all our abundant natural gas now goes to China and we have a domestic shortage) so the HEZ is mainly an empty blot on the landscape so I may be able to take the bike over there early one morning. Don't tell anybody.

R.

[Syscrush]

Yes, you are correct, triple clamp offset of 30mm as opposed to 45mm stock. Hole in the head.Well spotted.

You might need to have a talk with your suspension guru. Less offset means more trail, not less. Also, I think that the stock offset is more like 42mm, not 45. Measure for yourself to be sure, but if you're making mm adjustments, you want to start from the right baseline.

Explainer here.

This is something that a lot of people get stuck on. The machinist who fabricated my triples thought I was nuts when I told him I wanted to add offset to remove trail. I checked, double-checked, and triple-checked to make sure it was right - ultimately he built it to the dimensions I requested, but I'll bet if you asked him now he'd still swear I was wrong.

One thing I do get stuck on regarding this, though, is that if you look at 70's triple clamps, they have a pronounced V shape, while modern supersports with significantly less trail are more shallow. My best guess about this is that the smaller dia tires and much steeper rake angles of modern bikes both decrease trail so much that they don't need as much offset to get a nice trail number.

Also, perhaps trail hasn't changed as radically over the decades as other bike metrics. 1969 CB750 had a trail of 95mm, 2017 CBR600RR has 98mm. Interestingly, the CB750F had 19mm more trail than the K model, and a longer wheelbase and 1 degree more rake to make it more stable at high speeds - kind of the opposite of the modern trend for sport models.

[Warwick Biggs]

Yes Syscrush, you are right on both counts about the offset and trail, in theory. However, the 42mm offset at the top of the triple clamps is not exactly what the Computrack measures.

The offset it measured is from the axle to the steering axis. It is not a direct clamp measurement but it is the real measurement and this accommodates any fork flex, bush clearance and any wear or damage that may be present as well. The amount of trail required does vary with the rake angle. A 30 mm offset will increase the trail a large amount but changing the rake to a much smaller number by jacking up the back reduces the trail. With the CBX the rake is 27.7 degrees and in order to reduce this and recover the lost ground clearance we need to extend the rear shock by ~40mm + and the extra trail is then required from the shallower offset clamps. Increasing the swing arm angle by extending the shock should also give better drive and grip under acceleration out of the corners and it is trite to say that a motorcycle is always more stable under power in a corner. At least that is all in theory. How it works in practice is something we will have to determine.

I have just found out that Bill Brint at TIMS in Georgia used the same Computrack system to set up his CBX race bike so it seems I am not the pioneer that I imagined.

Rick.

[Ringnalda]

Bill tried to make the CBX identical in most angles to a GSXR750 Suzi... And used Computrack. But his is a twinshock.

[Warwick Biggs]

I wonder if anybody has any experience changing the stock linkage on a prolink CBX? I think that I recall reading somewhere that Honda's race kit for the SP2 or one of their proddy racers contained different length links for its prolink back end in order to adjust squat. This may be another area I have to consider.

In my thinking about extending the rear shock I am assuming the CBX linkage ratio is approximately 1:1 so that if I extend the length of the shock by 40mm it will translate to a 40mm increase in ground clearance. Does anybody know if this is a correct assumption or not?

This all boils down to pretty rough guesswork because the critical area for ground clearance on the CBX is the crank end caps that are located closer to the front wheel than the rear. The front has dropped about an inch and the rear nearly 2 inches. What this means with the wider tyres at full lean at the crank end caps is anybody's guess but I'm guessing I need around 2" extra height at the back.

Altho' I have an inch or so that I could extend the front forks in the clamps the Computrack tells me my trail will disappear if I do this so working from the rear seems to be the only way to go.

I was not scraping anything at the limits of adhesion on the old high profile 130/80x18" Conti's but I know that Trevor Manley on 'the Beast' with the same sized slicks that I am now running but with a raised motor (the Dr Tom mods) was wearing through his crank end caps and occasionally lifting the front wheel off the ground on them mid corner. This makes me a bit trepidatious about tipping it in too enthusiastically in my track testing in a month's time. I think I will need the full practice day b4 the races to work out how to ride it with the reduced ground clearance. I expect it will also have a more nervy front end with the reduction in trail from the smaller diameter front wheel. I certainly won't be going gangbusters, that's for sure.

Rick.