Katana Owners Club U.K.

I'm putting a slingshot 17'' wheel on the back of my '82 Katana, with a rf900(ish) front end.

I'm just looking at rear twinshocks. Would you go for standard length, or something longer?

Any advice, including sources, very welcome.

Thanks

Liam

Longer shocks are not really necessary, as the stock 1100 rim is a 17" as well. I'd be a bit more concerned about the front end, to be honest. The RF 900 forks are a bit on the short side, so some extensions or stepped yokes would make sense.

Thanks for that.

I was expecting to have to do something with the forks. The tops will be covered by the fairing so extensions would probably do the job and are probably the cheapest aren't they?

Liam

Hi Liam,
as Pedda said mate, the RF900 is a bit shorter than the original so your steering angle changes by increasing the rake angle and reducing the trail which can be sorted with 'slugs' inserted, but the important thing in retaining the geometry is the ride height , unless you want different handling characteristics in some way? If you have to go out and buy a front end I'd go for the GSX400S Katana since it has 41mm forks, albeit non adjustable. If you already have the RF front end, measure the height under the frame from floor to rear of frame cradel, then again at the front of the frame cradle. Using a pair of axle stands and a steel bar through the swingarm mounting, rotate the bike with a trolley jack until the height is the same as before modifications, then offer your rear shock up and mark where the new top mountings need to be on the frame. It's easier than having lower mountings moved on the swing arm, unless you have a tig plant at home? Anyhow, with this set correctly, you can see exactly how much UNSPRUNG length you need added to your forks. Don't change the rear shock angle more than 15degrees from the original or you will increase the resistance too much on the springs and get a twisting force when throwing her into a tight bend as the opposing shock unloads it's energy! That'd upset the front end more than a poor trail relationship!
Sorry I can't advise more without knowing what you can do mate ability wise and forgive me if I'm teaching my granny to suck eggs as it were.
Just balance the opposing sprung forces more than anything or she'll make a twitchy bike.
Good luck mate,
Mark

Got a mate who uses RF 900 forks in his Kat. He made some yokes, stepped by 40mm. Worked well on the racetrack in the past and still works well on the street now.

back in the 80's when options were a bit more limited, the accepted logic was to fit a set of shocks from a cb900 cos they were about an inch or so longer than stock kat ones and so they raised the rear a little and thus quickened the steering a fraction.

Thanks for all your help everyone.

Mark, I can spanner ok, and I'm okish at problem solving. I don't weld, although I have a mate who can weld steel and aluminium pretty well, but I am ok at making brackets etc. In terms of knowledge and real ability, I'm a beginner compared to you guys, so please dont worry about tellings me things that you would consider basic. I'm expecting to take a while building this bike, and to learn quite a bit along the way.

A quick web check says that cb900 shocks are 365mm. Thats 30mm longer than stock and there seems a reasonable range of options.

With the smaller radius wheels combined with 365mm shocks, am I right in thinkiing there would be minimal change in shock angle? If so, do I need to change the shock mounting points? I am happy to raise the rear end a little (I think it works out at about 1/2 inch) and I will probably look at fitting a steering damper at least to start with.

I am thinking of fitting the shocks first then measuring for fork extensions (my mental maths tells me that it should be about 40mm). Am I being overly simple?

Then I'm assuming its a case of playing with spring rates (or asking a pro) to get the balance right?

Apologies if I am missing the point, please feel free to put me stright if I'm being an idiot.

thanks again

Liam

id fit the wheels first and see how it rides. when i did my kat it ended up a bit lower than stock, but it still rode ok so i left it alone!!

BigLT wrote:
A quick web check says that cb900 shocks are 365mm. Thats 30mm longer than stock and there seems a reasonable range of options.

With the smaller radius wheels combined with 365mm shocks, am I right in thinkiing there would be minimal change in shock angle? If so, do I need to change the shock mounting points? I am happy to raise the rear end a little (I think it works out at about 1/2 inch) and I will probably look at fitting a steering damper at least to start with.

Liam

i have cb900 shocks on my kat... raises the back nicely... koni 365mm. only issue is that with a 530 chain n sprocket i cant really use a small front sprocket with the std arm as the chain hits it.... no bother, i use a 17 tooth on the front and then can change the rear if i really wanted to

hth

Hi Liam,
its great to have so many ideas bounced about and I forgot about the fittings on the CB900 being the same. Awesome, as Johnr and Steve say, the extra length of these saves a load of work and expense too. You got the principals mate so everything else is just shortcuts from what others have tried and tested. As for the front end and spring rates, don't worry too much. Ive found personally that if you use a lighter fork oil like a 5 weight, the contact is kept more constant between road and tyre whilst putting spacers on top of the fork springs to increase the compression rate.This will help with static sag without compromising grip under braking.
There's more benefit than spending a lot of cash on progressively wound springs. Get your ride height, the rest is personal choice. You could end up with quicker steering and liking it though 40mm is quite a lot mate. Try it and see like Johnr said, we can all pitch in with advice to help change the characteristics you end up with if it's a bit too twitchy compared to what you're used to.
God wouldn't have let us invent the hacksaw if bikes were all supposed to be the same ;O)