500 K cafe racer conversion - build thread

[Ewan 500 K1]

Now not sure what effect the longer rear shocks will have will have on the bike's geometery and in particular on the front end rake and trail and thus handling. Folk on the US site have been raising concerns about effect on handling and particularly about the potential for rear end twithches and tank slappers.
I will now  calculate the effects on the rake &  trail once I have the bike togeather and see if I need to do any front end mods (should have done this before I took it to bits again ... but didnt realise the potential issue)
One thing in my favour is that the bent steering stem that has been on the bike for at least 17 years would have reduced rake and trail. I now have a new and straight sterring stem, so running with this and longer rear shocks might make me end up in the same place as when running with the bent triple T and standard rear shocks .. we'll see)
Alternative solutions posed if effect on trail is significant are:
a) rework the bike to remove the battery tray from the rear hoop to allow more clearance for the tyre . This would mean undoing the frame mods and electrical work already done ... not keen on that.
b) get longer frnt fork stanchions to counterbalance the lifted rear end
c) get a triple T / top yolk set with a greater offset than the standard to counteract the shortening effect of longer rear shocks on the rake an trail
d) get a steering damper

Views welcome !

Must get to bed ... been up all night reading about bike geometry, trail and rake which was all new to me 

[Ewan 500 K1]

Put everything back on the frame today to compare the original front end rake and trail with the rake and trail impacts of the new longer rear shocks. I was intending just to put the essentials back on to do this, but ended up putting everything back on .... as I'm swithering between black and silver for the frame. A few pics to follow

[Ewan 500 K1]

With the old shocks and the old bent steering stem. First in sliver frame , then in black frame.  Gone off the silver , think I'll stick with black

[Ewan 500 K1]

new longer rear shocks in black frame and silver frame

[Ewan 500 K1]

dont like the rear mudguard anymore ....

[Ewan 500 K1]

some random photos

[matthewmosse]

I'd get a few miles in and see if things need to be altered, a diffrent steering stem / tripple tree arrangement could then be looked at or longer forks. In the 70's there were extenders that  screwed into the top of the forks to make them longer but I think that would be pants as a long term solution, might be worth trying if a cheap set were to be found to see if they helped if the geometry proved problematical. I'd try it as is as often forseen problems fail to materialise - I have had all manner of advise about the way my 500/4 sidecar wouldn't handle at all well, but I like the thing as it is and havn't even re attatched the steering damper when it broke a mounting.

[Ewan 500 K1]

I'd get a few miles in and see if things need to be altered, a diffrent steering stem / tripple tree arrangement could then be looked at or longer forks. In the 70's there were extenders that  screwed into the top of the forks to make them longer but I think that would be pants as a long term solution, might be worth trying if a cheap set were to be found to see if they helped if the geometry proved problematical. I'd try it as is as often forseen problems fail to materialise - I have had all manner of advise about the way my 500/4 sidecar wouldn't handle at all well, but I like the thing as it is and havn't even re attatched the steering damper when it broke a mounting.

cheers for that , really heartening ... unlike the state to which I'm being pulled apart across the pond .  I quite fancy doing all the measurements for working out rake & trail stuff anyway... as a learning experience if nowt else . One day of holidays left to play before its back to grindstone for a while

[Ewan 500 K1]

so .. the speedo cable rubs against the new disc. High tech fencing wire speedo wire restraint fabiricated and installed

[Bryanj]

I wish you would stop remining me of all the problems i had mate

[Ewan 500 K1]

I wish you would stop remining me of all the problems i had mate

When your only tool is a hammer, every problem looks like a nail. 

[Bryanj]

I do know of a "fitter" in a big print works whose tool kit consisted of 4 different hammers and 6 different chisels

[Ewan 500 K1]

So today I played about with taking trail and rake measurements . I tried various combinations of rear shocks and steering stems:

a) standard length rear shocks (315mm between mounting hole centres) + bent steering stem
b) standard length rear shocks + good steering stem
c) new longer shocks (361mm between mounting hole centres) + good steering stem

Diagram of what I was measuring is attached, along with raw results

Some key points:

1. All measured rakes were substantially below the stock rake of 26 degrees that is quoted in the shop manual. I calculated mine with someone sitting on the bike and hanging a plumb from the top of the fork (line Y in the diagram) and then measuring the perpindicular distance from the plumb line to the centre of the axle (line X).  Length X divided by length Y then gives the tangent of the rake angle and the rake angle can be derived from that.  In looking closer at the frame today it has some slight curvature in the front rails .. bowing in by about 2mm half way down on both sides. This must be another artefact of its front ender, as with the bent steering stem. This bowing of the rails is presumably reducing the rake angle ?.

2. The longest trail was found using the bent steering stem despite this set up having the least rake. Presumably this bent stem has the effect of reducing the triple T offset , such that the trail is lengthened rather than shortened that would be expected from  the lower rake figure?

3. All measured trails appear to be within an accepatable range and exceed that of the CB350 (85mm according to Clymer). The lowest figure (with the new shocks on) is comparable to some of the CB750 models (94mm?)... all subject to possible marg ins of errorin my methodolgy of course.

I also had a play with RB racing's rake and trail calculator at:
http://www.rbracing-rsr.com/advchoppercalc.html

I couldn't get the calculator  to reproduce Hondas stock rake figure by entering standard dimensions  for the 500 four,,.. so maybe it doesn't work for Honda SOHCs ? ... but what did seem consistent is that for a change of one degree in rake a change of about 6mm in trail resulted. This is roughly in line with results for the 400/four at : http://www.denoonsp.com/user/image/v3-issue1.pdf. In this article 0.4 degree of rake change results in 2.2mm of change in trail... so 1 degree rake = 5.5mm trail.  This is consistent with what happened when I changed the shocks, I got 1 degree of change in rake and 5mm of change in trail. The 440/four article also notes that a 6.25mm increase in rear shocks procuded 0.4 degree reduction in rake. I've increased  my rear fork length by 46mm, so if the 400 four figures can be extrapolated this would result in a change of 3 degrees in rake and 15.5mm of trail. I didn't get this level of change in rake or trail when changing from standard to longer shocks , mine are more like 1 degree of rake and 5mm in trail.

If 1 degree of rake change = 5.5 mm of trail change , then my measured rake of 23 degrees with the new shocks should have procuded a trail reduction of 15.5mm against the stock figure of 102.5mm .. but I only get about half that.

So what does all that tell me ? .... that the tolerance in my measurements were too wide such that my figures are unrelaible , or just that that I should follow the advice in mathewmosse's post .. get the bloody thing on the road with the longer shocks and see how it feels ...:    

[Ewan 500 K1]

Heres how I went about measuring the trail with ref to attached photos

1. Got a broom handle and cut slots in it to accodate various bits of the bike...so that it would sit parallel to the headstock / forks . First photo shows the broom handle with cut outs

2. Second photo shows broom handle sitting on bike .. orientated to extend the line of the headstock down to the ground. The broom handle is lined up with the centre line of the headstock and parallel with the stanchions

3. Third photo shows the plumb line hanging from the centre of the axle and the bottom of the broom handle when orientated as described at point 2. Trail is then the distance between the bottom of the plumb line and the bottom of the broom handle.

The large washer hanging on a string in the third photo is used to measure the rake angle as shown in the diagram in the previous post.

[Johnwebley]

you are working hard Ewan,

 I suggest you get it on the road,run it for a while,get used to the feel.bed the tyres and brakes in ,
then explore the handling,

I guess you have some great bike roads around you,

go and have some fun