homemade fork extension...hum....

[klingklang]

By this project I wanted to be able to play a bit more with my fork height. My back and front suspension is perfect (for me, since this is suggestive).  Fork springs, rear spring and stock shock rebuilt by stadium suspensions here in Quebec.  The only thing is that even if the sag is perfect I always got the feeling that my nose is a bit down.  Since I want to keep my preload like it is and the fact that at 6'4", my mind refuse to get anything to go down. So I went with homemade fork extensions o try.  I did a 18mm upper section since the top clamp is 36.  Then at worst, half of the cap is lock in the lamp.   Choppers use 2,3 4 inches long extension then it should be strong enough. Unfortunately, can tell the feeling yet since the temperature is not really the favorite one to test up here!!!!  hate that crap.  Can't stand to test that!....kk

[Chris Canning]

Well I'm not sure if that ranks as interesting or totally bizaar,the fact that tigers steer like a super tanker with the forks set fluch with top of the yoke,and every thread I have ever seen talks about how much to drop the forks through,from 5mm and upwards and you have put an extension on :shock:,it'll steer like a chopper.

[klingklang]

yes but it is totally different with new shock and mostly offroad.  tell me, using the gas cap as reference and the bike on a flat an at level, is your gas cap is at level? (or parallel to ground) mine was 3 to 3.5 deg with the front nose down and that strange feeling. anyway, will try.

[Chris Canning]

You've lost me a little I'm afraid,when I ran a 19" front wheel(I now run a 17") I started with the forks flush in the clamps,the steering was that slow,to say I was unhappy would be an understatement,in the end dropping the forks through the yokes 10mm resulted in a reasonable result.

If riding offroad is your priority I'd have thought working how to fit a 21" front wheel would have been the better option.

If you feel that you need to lift the front,I can only assume you have some how put a long shock in.

[cosmo]

There is a wee problem with your 'chopper' logic:

True, there were extensions sold like those to which you referred.

They are not sold now.

This is because they are hugely dangerous, as they broke.

Frank's Forking sells new forks, in any length.

Proceed along the present path without my blessing.

Cosmo

[Bixxer Bob]

I can add to Cosmo's piece.  The internal threads in the fork tubes mean there's slightly less than half the thickness of metal that there is in the main tube  body.  There are then two things acting against you.

First - in the standard forks, the weaker, threaded part, is well supported by the top yoke (triple tree) clamps and the fork cap being screwed in place.  This has the effect of turning any bending force into a shear force. which is much easier to resist.

Second, the modified, extended cap puts the weaker part at the bottom of the yoke turning the shear force into a bending force which tries to 'burst' the tube top, which is much easier to do.

I've produced a couple of rough sketches to illustrate what I mean.  And just to correct some of my text, due to the rake of the forks the suspension forces act in the opposite direction to what I've drawn.  The bigger, more serious forces such as any sort of frontal impact, are as drawn.   Over time, age and stress of suspension movement would result if cracks and eventual breaking on the opposite side of the tube.

[Mustang]

My 2 cents input on this .............
years ago I had a 650 bonneville chopper , it had 10 inch over fork tubes because it was raked to 10 degrees beyond what a stock triumph was

in this the only digital pic of the bike it hasthe honda 750 front end on it that was extended the same way as ops . it had 10inch extensions that screwed into the original fork tubes ........never I repeat never did it have any problems in many thousands of miles .



Would I make something like that fora customer?..............absolutely not

But like I said it performed flawlessly on the chopper for lot's and lot's of miles

I kind of miss that bike it was as comfortable as a Goldwing and had a 11.5:1 compression motor with race cams and big ass Keihin carbs and only weighed in at a little over 300 lbs wet .

It would embarass a lot of Hardly Abelsons

[Bixxer Bob]

I was concerned more about strength in a frontal impact rather than normal wear and tear although I stand by what I said,  if you are going to have any problems it'll be stress cracks where I indicated.  To know whether that would actually happen, you need to know the forces involved ie actual bending force from the suspension, resistance to twisting of both yokes and, assuming for this purpose the frame is rigid, length of the forks.  After that it's just simple fulcrums and levers.  Once you have a number, you check the tables for the material used to see whether it's below the infinite stress figure.  If it is, then in theory it'll never break.  (I have seen this go wrong though - we had a trailer under a transportable radar built in Syracuse NY that was under the theoretical infinite limit.  All the theory in the world isn't gonna help when it collapses in a heap...).

It would be interesting to do the engineering calculations I think, just to see if they are weakened as much as gut feeling tells you they are. To do that we need ID of tube, OD of tube, cap and thread details (all of which I can get from my bike) then the details of the extension (length above threads, material used, whether the threaded portiuon is solid, and how far they protrude above the yoke.  I'd make a couple of assumptions regarding the grade of steel and alloy for the standard parts (unless someone knows what is actually used..)

With that information I could calculate the failure strength as a percentage of the original strength.  That way there's no need to know the actual loads involved.

[Mustang]

Yoshimira likes em ..................

http://www.yoshimura-rd.com/t-kit_fork_ ... p_set.aspx (http://www.yoshimura-rd.com/t-kit_fork_tube_cap_set.aspx)

[Bixxer Bob]

Maybe I'm just an old-fashioned stuck-in-the-mud engineer then.  $265 for 20mins in the machine shop could part a guy from his principles though... :lol:

[Chris Canning]

Regardless of who likes what,those extensions on his tiger!! will make it feel like that old chopper :( ,and they don't steer all that quick in the first place,e'r thats the Tiger not the chopper  

[Mustang]

those extensions on his tiger!! will make it feel like that old chopper :(

well it won't be that bad ( and actually the chop was a very decent handling rig except for parking lots you needed as much room as a bus to turn around )no matter  what works best for you , me, or anyone else though  ,the op might like it that way   ..........just sayin