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Author Topic: Fork Tube Length  (Read 1206 times)

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Offline j8ksdad

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Fork Tube Length
« on: April 21, 2020, 09:57:07 pm »
Hello,  Wondering if anyone knows the length of the stock fork tube? I'd like to order extended tubes but need to know where to start.  Thanks!



Offline strummer

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Re: Fork Tube Length
« Reply #1 on: April 22, 2020, 01:18:23 am »
Hi
None of my business and I might get this wrong due the language barrier but...
Why do you want a longer tube?
Can't you extend the original one?
I guess it is cheaper and even a faster and  less expensive job to do.
There is a guy on ebay who sells a perfect match for our forks, luckily for most of you he is in the USA and he will machine all the length you fancy.
I personally chose 1" so I didn't have to extend the cable/wires and to match the 1" longer Hagon socks at the back.
This is just what I think.
Unfortunately I don't know the length of the stock fork tubes sorry.
Peace & Anarchy

Offline srinath

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Re: Fork Tube Length
« Reply #2 on: April 22, 2020, 11:19:00 am »
I'd fit a fork set off a 80's or 90's dirt bike - complete with triples and all.
Like this. This was off a 89 KX 125 if I recall. I fabbed the triples for a 9 degree rake, but that is not required. This bike - a savage - the forks that aren;t offset like crazy ahead of the steering stem will hit the tank - hard. That's one reason plus I wanted the rake.
The advantage is - you can flip the legs left to right and run the axle behind the fork leg - or in front - however it handles better.
You'd need the entire front end - everything steering stem and forward. I believe the bolt is a 28X28 bearings - Yamaha dirt bikes of the 80's and 90's likely were 28X28 as well for bearings. Either way - match those - and all you need to do is fit the bolt's stem into that lower triple if the lengths don't get close.

Cool.
Srinath.

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Offline Sdaniels

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Re: Fork Tube Length
« Reply #3 on: April 22, 2020, 11:35:44 am »
Why not get fork extenders instead of replacing the entire tube?  Much less work, can be easily reversed.
2015 C-spec

Offline j8ksdad

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Re: Fork Tube Length
« Reply #4 on: April 22, 2020, 11:42:41 am »
I'm going for a '70s Nor Cal chopper look. I want to get 6" over tubes.  I currently have 3" extension and like them but want a bit more.  I'm not comfortable with 3" more extended than I already have.  I'd rather just have the complete fork tube.  Franks Forks can make them but I need to know how long the stock ones are so that I know how long to order the new ones at 6 over that measurement.

Offline srinath

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Re: Fork Tube Length
« Reply #5 on: April 22, 2020, 01:10:11 pm »
I'm going for a '70s Nor Cal chopper look. I want to get 6" over tubes.  I currently have 3" extension and like them but want a bit more.  I'm not comfortable with 3" more extended than I already have.  I'd rather just have the complete fork tube.  Franks Forks can make them but I need to know how long the stock ones are so that I know how long to order the new ones at 6 over that measurement.


Oh yea - You need the right dirt bike forks.
BTW you thinking of raking it extra - That way you can machine the triples and slap longer legs, wheel, brake etc etc etc on it. There bearings don't matter, you cut your triples to match the bolt, fork hole dia and spacing to match the bike you're getting it from and heck if you want to use Harley risers - drill 1/2" holes at 3.5" in the upper triple, and drill the lower triple for the headlight etc etc -
I got some pics of the 2 parts I made to make this work on the savage.

This is it - 1 pair of triples. Raked 9 degrees.
The bolt is very much heavier than the savage. I would absolutely not just put longer legs on it - they flex more as they get longer, the rake/Trail/offset etc etc all go to kablooey.

BTW how did 3" longer change the handling ?
You can even get an upside down fork leg set off a dirt bike - they're stiff as hell. I personally don't like them, but something like a DR-Z or a CR500 etc would do awesome.

Cool.
Srinath.

Offline j8ksdad

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Re: Fork Tube Length
« Reply #6 on: April 22, 2020, 01:24:12 pm »
I'm going for a '70s Nor Cal chopper look. I want to get 6" over tubes.  I currently have 3" extension and like them but want a bit more.  I'm not comfortable with 3" more extended than I already have.  I'd rather just have the complete fork tube.  Franks Forks can make them but I need to know how long the stock ones are so that I know how long to order the new ones at 6 over that measurement.


Oh yea - You need the right dirt bike forks.
BTW you thinking of raking it extra - That way you can machine the triples and slap longer legs, wheel, brake etc etc etc on it. There bearings don't matter, you cut your triples to match the bolt, fork hole dia and spacing to match the bike you're getting it from and heck if you want to use Harley risers - drill 1/2" holes at 3.5" in the upper triple, and drill the lower triple for the headlight etc etc -
I got some pics of the 2 parts I made to make this work on the savage.

This is it - 1 pair of triples. Raked 9 degrees.
The bolt is very much heavier than the savage. I would absolutely not just put longer legs on it - they flex more as they get longer, the rake/Trail/offset etc etc all go to kablooey.

BTW how did 3" longer change the handling ?
You can even get an upside down fork leg set off a dirt bike - they're stiff as hell. I personally don't like them, but something like a DR-Z or a CR500 etc would do awesome.

Cool.
Srinath.

Srinath,

Thanks for your input!  That'a way beyond my abilities.  I'm just a bolt-on kind of guy. I may order some 5 degree raked trees to go with the longer legs.  The 3" extensions took a little getting used to but I love them.  No more scrapping pegs in the corners. I also need a longer kick stand now.

Offline srinath

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Re: Fork Tube Length
« Reply #7 on: April 22, 2020, 01:41:28 pm »

Srinath,

Thanks for your input!  That'a way beyond my abilities.  I'm just a bolt-on kind of guy. I may order some 5 degree raked trees to go with the longer legs.  The 3" extensions took a little getting used to but I love them.  No more scrapping pegs in the corners. I also need a longer kick stand now.


Its beyond mine too - that's why they have machinists.
A few mins of math would tell you the offset you need for the distance between the top and bottom triple.
Its something like top of upper triple to top of lower triple X cos (angle of rake).
Then you center punch those holes.
Then a hollow mill and a milling machine - both available at machinist and the angle finder - I used a couple of blocks of wood, that way you can easily punch through and not hurt anything.
And my kick stand is stock on that bike. Its ride height only went up a tiny tiny bit.
I got a 21" aluminum wheel instead of a 19" steel wheel, and the bike handled a lot better.

The 2 parts I fabbed allowed me to fit everything else around it. I fit the stock headlight by drilling the lower triple to match it, Harley 1" risers and bars by drilling the top triple for HD risers.
I'd rather do that then mess with fork leg innards. Of course that's just me.

If you're really inclined to do just longer legs, get fork legs off a dirt bike that match the diameter of the bolt. Bolt is what 41 or 43 ? Both are common sizes in the 80's and even 90's. That fork I used may have been a direct fit in the bolt's lower - it was a 41 if I recall. Kx250-500 - last version before they went upside down, and even the kdx200 for a few yrs after that - look it up. The trick is finding a good quality one after 30 yrs.

Cool.
Srinath.

Offline srinath

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Re: Fork Tube Length
« Reply #8 on: April 22, 2020, 01:59:03 pm »

Its something like top of upper triple to top of lower triple X cos (angle of rake).



D-uh - That's how dumb I am. Its tan of the angle. Opposite side/Adjacent side.

BTW I used a thicker upper triple - if you do that you need to measure from bottom of the upper triple to top of lower and add the thickness of the plate you're using.

Cool.
Srinath.

Offline srinath

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Re: Fork Tube Length
« Reply #9 on: April 22, 2020, 02:17:12 pm »
There you go.

https://www.kiwavmotors.com/en/faq/about-fitment/motorcycle-fork-tube-size-chart

Looks like those KX's of the 80's and the kdx are 41's.But remember Don't buy one of those that had upside down forks. The upper is much much larger, like 55mm or something on those.

Also look into bikes like those Vulcan's which had axles run in front of the forks. That means the inner goes down a lot deeper into the lower because the damping rod is at the bottom 2-3" below, not 2-3" above the axle like it is on the bolt. You are more likely to get a pristine fork leg if its been sitting in oil protected from the elements.

Cool.
Srinath.

Offline srinath

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Re: Fork Tube Length
« Reply #10 on: April 22, 2020, 03:42:19 pm »
Thinking this through a little bit more -

When your suspension compresses, the fluid gets compressed out of the lower leg through the holes in the damping rod into the tube.
The bolt at the bottom of the damping rod needs to be the same size/thread - else you'd have to mod the hole at the bottom of the fork etc etc. Best to get that sorted out in a parts fiche before hand. Or you'd be machining a little here and there. Not too big a deal.

I'm nearly certain a whole damping rod+outerleg will function in the bolts lower leg in terms of functionality. Matching travel would take you 1 step closer, though with a different rake etc etc you'd likely need more or would the flex cover a bit of that and you can get by with less ? No idea.

Now, damping rods have different lower profiles including cups and sleeves and what not they sit in.
Matching those would lead to a easier swap. You could modify that part also. But try to get one that fits and you may just fly home free.

Then here is one important aspect to remember. If the donor bike had a drain hole at the bottom - don't get that one's forks. Bolt doesn't have drain hole - and while this may seem inconsequential - those drain holes sometimes (like in a katana 600 also 41mm forks) fit inside a notch in the mushroom cup the damping rod sits in. That mushroom has a D shaped hole the damping rod is machined to fit.

If you are taking apart a katana 600 fork, you would be thankful you knew that.
The procedure is to drain it on the bike with the drain bolt. Then reinstall it. Then you can take off the bolt at the bottom holding the damping rod. No need to make those weird long tools to go through from the top to hold the damping rod. But you cant use that drain or the "keep from spinning" anyway, best to avoid machined together parts.

So IMHO - find a fork that matches these with similar travel and 41mm and give it a good measuring all around. Good luck, and post back.

Seriously at this point I want to do this - except I still have 2 savages, one of those I am de-raking and de offsetting. So I am doing it, in reverse.

Cool.
Srinath.

Offline srinath

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Re: Fork Tube Length
« Reply #11 on: April 22, 2020, 05:04:00 pm »
Go to a junkyard with an accurate caliper that can spread the width of the bolt's forks.
Find a 41mm dia with the same spacing and the length you like and the entire thing with the wheel etc etc should be a drop in to the bolts triple trees. Slam dunk. I'd start with an XT600 - looked like it was made form 84-96 and pretty common I think. In fact there is like 20 xt600 forks on ebay - ask those sellers what the spacing between the forks is. If its the same as the bolt - you're pretty much set.

If you don't want to give up the bolt's front wheel for a spoked aluminum one of 21" dia - You need the same axle spread and diameter as the bolt  - all you need to make is spacers and brake adapter. Might be an easier way.

The axle diameter can be worked around by swapping wheel bearings, but your forks should come from a bike with a larger axle - which is rather unlikely IMHO - the bolt's axle is pretty fat. But we do this to SV rear wheels to fit em into a GS. 20mm ID on the SV, we knock those out and fit the GS'es 17mm ID ones.

This has been a fun exercise - but IMHO - the bolt is as heavy and as hard to steer as anything I'd care to ride, I am not raking it or putting longer forks. Now this gives me crazy ideas for the savage. But the offset of the bolt wont let it fit the savage without clobbering the tank.

Cool.
Srinath.

Offline lunkhead

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Re: Fork Tube Length
« Reply #12 on: April 24, 2020, 01:48:39 am »
When I lowered the front end of my Zero FX about 5", rake went from 25.4 degrees to about 23 and trail 4.1" to about 2.5". Adding 6" longer forks to the Bolt will get you about the same amount added to rake in the other direction but trail will increase even more than that. Going to a smaller diameter front wheel would help take away some of that added trail. Going from a 21" to an 18" on the Zero made the rake 22.5 degrees and trail only 2". Handles like a bicycle. ;D
C-SPƎC

Offline srinath

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Re: Fork Tube Length
« Reply #13 on: April 24, 2020, 10:39:04 am »
Trail is just about the most important number for how the bike will feel. I've seen a monster rake chopper with a twisted girder FE, with a massive near foot long - what really looked like a Klingon Batleth the axle went through - and it had some massive 26" spoked wheel if I remember - I swear that thing would have weighed as much as a pickup truck. And that was just the front end - LOL.
The guy was on youtube - not a famous build off type show - he was just describing his bike - and he said trail is near 0 and hence it handles like a feather.
I went like WTFFFFFFF - How you get 0 trail - your trail looks like its measured in feet not even inches.
But on closer examination - a lot of the rake was in the neck. The massive twisted steel rods that are part of the girder FE were just parallel to the neck. In fact they may have deraked it a bit compared to the neck. The massive wheel and the batleth compensated for each other leaving 0 trail.
Look up you tube for 0 trail chopper. It probably is still around.
However you lose control of the trail if you only make 1 part - longer fork legs leave you with the stock neck, and raising it does increase the rake, but you cant take some of it out as the forks go, you cant put wheels and axle arms of your choice.
That's why the dirt bike FE option appeals to me. You atleast get the choice of flipping the legs to run the axle trailing the leg, likely turning a potential negative trail into a positive one.
I know on my savage the difference was night and day. It looked awesome fitted the right way - but handled worthless.

Cool.
Srinath.

Offline lunkhead

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Re: Fork Tube Length
« Reply #14 on: April 24, 2020, 12:52:13 pm »
More trail is good for leisurely touring and making a flexible chassis safer. With a stiff enough frame and a steering damper, a bike can safely have a less trail with the right tires. The FX chassis is super stiff and it can't go over 85 mph so I don't need a damper. Anyone building a chopper can''t worried about handling or going fast.
C-SPƎC