Part 10. Springs in the air. Wheels on the ground…
The Katana offers a bit of a challenge suspension-wise. It’s heavy, it’s fast, my build won’t use any original suspension components, we’ve made the frame much stiffer and to cap it all, I haven’t done anything like this before; what could possibly go wrong?
Let’s start with the obvious. I’m bothered about ground clearance. Very bothered. The big GSX lump in the Kat has a huge alternator case hanging off the side that’s big enough to smuggle 13 tightly packed kittens into Belgium when we go racing (no actual plans to – I just could).
Not paying attention to this unwelcome protuberance could mean I end up dragging the thing to a premature and messy end in the race.
Now, we have already removed the alternator/flywheel and slimmed the casing down, but we do still need help from the bouncy bits.
Let’s start at the front. The wish-list for forks was actually pretty simple; make race-ready items that use GSX-R750 Slabside fork bottoms.
Why? Well I want to run with original Suzuki forks, and ones that have anti-dive adjusters. Although the anti-dive units won’t be functional, they doff a reverential cap towards a long since passed piece of Suzuki high-tech. I’m imagining those quiet moments of just staring at the bike whilst sipping a brew, I’m also imagining they’ll be one of the things that make me crack a smile.
The Slabby forks I’m using needed making longer. Here’s why. The Katana in its original guise sat really high at the front and low at the back. It wore a 19” front wheel and 17” rear, whilst we’re on 18” at both ends.
Adding even more height at the front on top of the Penny Farthing wheels, are the incredibly long Katana forks, which measure a whopping 75mm longer at rest than our Slabby items.
Ideas on geometry and suspension have changed stacks since the Kat left the drawing board, and I’m set on dropping the front (measured at the headstock centre) by around 80mm, and raising the rear (measured at the rear shock mounts) by around 50mm. Radical.
I’ve looked at Steve Adams oft-referred to Lucky7 Katana for inspiration and I’m in the same ballpark with these. I’ve ridden that bike, and it’s good. Very good! That’s the proof I need on these starting numbers, directly from eating the pudding as it were.
Anyway I reckoned the Slabby forks were so short they could drop things just a bit too far, so I decided that adding 25 to 30mm more fork length would future-proof any geometry tweaks needed.
The spare length sits above the top yoke if unused, but it’s there to allow me to pull the yokes up the forks giving me extra ground clearance and more trail should I need it.
Like almost everything in life from diet to working hours, good geometry is a balance. Too high at the front and it won’t deck out the wide bits, it also won’t turn. Too low at the front and it’ll turn better, but it will ground out. With extra fork length in-built, I have more options, and that’s cool. Spa has man-sized corners, 20 of them; this is a job that needs to be done properly!
So, not for the first time in my life I’m asking where I’m going to get that elusive extra inch of length from… Well, with forks it should be easy! Fit some longer stanchions that work with the original bottoms. At least, that’s what I thought…
Once lengthened the plan was to rework them with better innards, as they’re fitted with an 80’s damper rod design as stock. Not great performance, not particularly tuneable. More scrap for the pile!
I made a trip with my forks to ABE in South London, and Russ Bartlett, the boss there, heard what I wanted and came up with a cunning plan. Mate my Slabby bottoms with the longer tubes (exactly the 30mm I wanted) and importantly cartridge innards from a later 1995 Kawasaki ZX6R-F fork.
It seemed for a while my wish list was totally fulfilled! The fruits of his clever idea came through the post a few days later when he’d managed to source a set of ZX6R forks from their extensive stock of used bits, and machine them to fit. (Thanks Russ).
Cue a visit to Suspension specialist Jon Slenzak of S1 suspension near Brackley in Northamptonshire. Jon’s CV in bike springy bits is extensive and ranges from Suzuki Factory MX to the GP paddock.
Jon has a shock dyno to work out what’s happening, which made him perfect for refining my largely unquantified mix and match suspension. It measures both spring and damping rates. Much like a good race engine, once you’ve built a suspension compnent, you want to know what it does; what’s good, what could be better.
The dyno is the perfect tool to get a baseline setting then make measurable changes not based on “thumb in the air” guesswork. Suspension is definitely a black art, the dyno at least makes it a black science!
Unfortunately, what Jon saw from the hybrid forks on the dyno wasn’t great, and once stripped, the fork internals told a tale of woe – they’d probably be ok on a road bike, but the early cartridge design, mass produced manufacturing tolerances plus 19 years of added wear and tear meant they weren’t giving the race standard performance we needed.
Adding an extra splash of pain into the equation, was the fact that getting forks to be 30mm longer isn’t just about the stanchion length, it’s about the amount of movement and spacing of the fork internals. My simple fix hadn’t added 30mm of length. The shock dyno also revealed my grand idea had also reduced the amount of usable travel in the fork as the longer tube “bottomed out” inside the slider. Oh the pain and expense of ignorance (again).
We weren’t quite back to square one, because we knew the fundamental idea of fitting Kawasaki stanchions and innards into the Slabby bottoms was sound, and they could be modified to bring back the missing stroke. After the dark hours of realising I was in a bit of a hole, Jon’s suggestion to fit brand new race ready ZX6R Andreani Cartridges and modify the stanchions into operating at the full stroke again was one I embraced with enthusiasm.
The Andreani cartridges provide the rebound adjustment on one of the forks and compression on the other. Several modern Superbikes do just this, so we’re being very 21st century for once.
I also have a major lusting for Titanium Nitride gold coating on forks, and decided new Tarozzi fork legs already coated in it would be ideal. The ZX6R legs were jaded and would have cost more to get retro-coated than buying them new.
Rounding off Jon’s work on the forks was a refurb of the lowers using Xylan Fluoropolymer coating (it’s beautiful), and a refresh of the anti-dive units, albeit now disabled for good.
The forks look amazing – the smooth satin Xylan coating replicates beautifully the look of 80’s factory fork hard anodising, and the anti-dive units look really fresh after being given some love as well. I’m totally smitten with my forks, it almost seems a shame to have to use them! I’ve already sent more bits to him for the Xylan coating, it’s that good I want it everywhere!
Jon’s also worked out what we need from the rear shocks, and I’ve bought a set from YSS. Thanks to having the right “contacts”, Jon had these made to order to his own spec. I’m sure he’d love to do the same for you if you asked nicely (and paid him the appropriate amount!) Although they’re around £700, for the spec these shocks are incredibly good value.
We’ve gone for a 360mm length out of the box, with a +/- 5mm adjustment being useful to get things just right. The stock Kat shocks are just 305mm to give you an idea of where we’ve pitched it.
The bike set up on blocks with the suspension at full droop happily suggests that one of our big concerns has been avoided; Just whacking length into rear shocks/ride height can mean that the chain run ends up sawing the swingarm in half. We’re going to run a nice big front sprocket and a nice big rear sprocket too, this all helps to keep the nasty abrasive properties of high speed chains well away from our (expensive) swingarm.
We’ve set the bike up sitting quite high, the forks are popping through the top yoke, and may even need to go more. We’re running .95Kg/mm springs in the front and at the time of writing a spring weight of something I’ve forgotten in the rear!
So, thats another key part of the build done and dusted, and yet another bit of the build that I feel is more than up for the challenge of Spa. The one key element at this point that may not be as good as planned for the challenge of Spa is the remaining budget… Gulp.
Still, at least the bike is good, I can always take comfort in that once the food stops appearing on the table…