TOPIC:

wrote:

Thanks zed, I'll come back to this when the forks are apart but that means I can go to 20-25mm and have some room to move on the rake. I assume you haven't noticed any extra flex in the forks? I'll probably be putting a brace on so it shouldn't be an issue anyway.

EDIT: Removed brain fart

No extra flex or any other issues.
Have done this to both RWU and USD forks into Z frames.
I have used GSXR, RF600, ZXR ,ZX7R, 9R, ZX10R, FJR1300 forks to name a few with their matching yokes and all handle very well.
Despite the differing offsets etc the main factor is keeping the frame stance and consequently the rake close to stock.
Having the back end jacked too high or the front to short results in shopping trolley handling that will tank slap to death and try to kill you once you are approaching 100mph.

Any chance you want to make some extra cash making some ZRX damper rod extensions?

wrote:

The excerpt I posted is for Sanctuary's street bikes with 18 inch wheels, it says so in the excerpt, the pic was meant as a guide or reference, they would adjust that according to the track, I should have stated that.  97 mm is pretty good for trail using the measurements mentioned, if that's the case, I haven't run them through my calculator, I'd want exact measurements for that, or at least a full set of numbers.  The old Honda fours only had 94mm of trail and weren't known to be "twitchy, the Triumph triple i got my forks off had even less, Rake,  22.9°, trail 91.44 mm (3.600 in). Its not only trail that bike nervous, its the combination of all four, rake , trail,  wheel diameter and offset...  The ZRX numbers are for 17 inch wheels, Calum is using 18's which require slightly more rake than a bike using 17's, around 1 degree.  I absolutely agree with the fork length, Calum needs to factor in the differences, that shouldn't be too hard.
I'm setting my frame up with the wheels on at the moment to determine shock length and swingarm angle, my triples are 40mm offset, ideally they would be 45mm  but i'm changing to 17's in the future and 40mm offset is spot on, i'll have to raise the rear a touch to lose 1 degree of rake, or lower the front or a combination of both,  and it will be very close to perfect, I'll then fine tune it on the road....



 

You only need three values to calculate trail, and you are correct that the sum of the values is more important than any one value on its own. However, trail is the summation of the 3 other values you listed and is a good indicator of the general geometry which is why it is listed in manuals rather than tire diameter and offset. The main reason the rake and offset are changing when moving to a different wheel size is to get the trail number back to the magic number of around 100mm.

Since we are looking at bikes with low trail values look no further than the KZ1000A that had 26 degree rake and 90mm trail - not uncommon to hear of high speed instability.

All that being said its just semantics at this point, and we're basically in agreement here. We can bench race all day long. at the end of the day you gotta build it and get out there and test. This is why so many race bikes have adjustable geometry -  rake, offset, rear axle height, swingarm length, and swingarm pivot, etc..

Callum - if you want to go deeper I highly recommend "Motorcycle Handling and Chassis Design - the Art and Science" By Tony Foale. As far as technical books go its a light read and has so much great information including his crazy experiments on zero rake bikes.

 

Yes, all good Doc, sometimes its hard to not sound argumentative when typing rather than talking, On the Sanctuary bikes they recommend reducing rake about 1 degree and reducing offset by about 5mm when changing from 18 inch to 17 inch wheels.  We could have some very interesting conversations about these early bikes and what was done to make them handle. We had some scary fast bikes built here in that time period, And  look What Gardner did on the Moriwaki modified Kawasaki MKII 1000 against guys like Barry Sheene, on their home turf, who was racing Suzuki GP bikes...I love this stuff...



 

wrote:

wrote:

Thanks zed, I'll come back to this when the forks are apart but that means I can go to 20-25mm and have some room to move on the rake. I assume you haven't noticed any extra flex in the forks? I'll probably be putting a brace on so it shouldn't be an issue anyway.

EDIT: Removed brain fart

No extra flex or any other issues.
Have done this to both RWU and USD forks into Z frames.
I have used GSXR, RF600, ZXR ,ZX7R, 9R, ZX10R, FJR1300 forks to name a few with their matching yokes and all handle very well.
Despite the differing offsets etc the main factor is keeping the frame stance and consequently the rake close to stock.
Having the back end jacked too high or the front to short results in shopping trolley handling that will tank slap to death and try to kill you once you are approaching 100mph.


 

Not trying to be argumentative but its a balance, The rakes used in the 70's and early 80's were all over the place, Kawasaki went from relatively normal, with poor offsets on their clamps, early 900's and especially the first Z1R's, to majorly conservative with the early GPZ's, 1000R's, 1000J's  and the like, they were much more stable,  but pigs in stock form to get around corners quickly, that's why most people who raced those bikes altered the frame rake. I helped a racer here recently take 4 degrees out of his GPZ1100B race bike by cutting the frame and repositioning the headstock, this was a common mod. His stock rake was 29 degrees. As long as the bike is set up properly there will be no "shopping trolley" effect, that happens when trail becomes dangerously small, raising the rear, or lowering the front is usually done to get the weight balance right, or more weight on the front wheel, you can reduce rake marginally and improve swingarm angle at the same time, again, you have to know what you are doing and stick between the accepted parameters... .....

Thanks to all of you for all your help. Doc: I'll have a look at that book, thanks for the tip.

I think I've pretty much got everything sorted: Mill out the bottom of the top yoke, extend the forks internally (chuck a brace on for good measure) and lay the shocks down a bit. Now I just need to start getting things together to get the work done.

Has anyone used the sanctuary shock bushes ( www.webike.de/en/products/3461329.html )? They're pretty cheap and will be better quality than I can make myself. I'm just not too sure on the back section - smaller diameter than the section for the shocks up front, but probably still easily strong enough? Would like to know the diameter (looks like about 10mm). 

@Zed1015: I'm trying to find an inline guide and cutters to cut the larger holes in the frame. I read somewhere that you got them on ebay but I can't seem to find the right search term. Could you maybe point me in the right direction?

wrote:

@Zed1015: I'm trying to find an inline guide and cutters to cut the larger holes in the frame. I read somewhere that you got them on ebay but I can't seem to find the right search term. Could you maybe point me in the right direction?

I used a 20mm Rotabroach for the cutter and made up my own jig and drill guides etc.  

Rotabroach is trade name for annular cutter if you want to broaden your search

Thanks guys - "Kernbohrer" in German - that was what I needed I ordered some of the sanctuary mounts, I'm pretty sure those guys know what they're doing and the time it would take me to make them can be better spent elsewhere. I'll get a Ruko cutter and make an adapter for the drill when I know what size I need. Time to start planning that jig...

Slow progress over the last while. Between the Zephyr, finally getting away for a short holiday and getting back to normal working hours (plus a few extra), I haven't had much time at all.
Got the jig for the shock mounts sorted. Waiting for the triples to get back so I can see which of the three holes to use for the shocks - probably be the middle one. Then I just need to spend some time on the lathe to make a drill guide and an adapter for the cutter.

 




I have also been collecting some other parts. Not sure about the clutch cover - may be a touch too "boy racer"...

Nice, I'm about to make a shock mount jig of my own , very similar to yours....

That clutch cover is cool! Where'd you get it?