USD forks, altering front geometry???

[900streetfighter]

Hi all, as mentioned in a prev post Im trying to fit a SSie front end. Im getting bits together and will be trying them in a few months, The legs are 60mm shorter, I can get back 30mm with a modded top yoke, drop them in the yokes a further 5-10mm so the fronts dropped by 25mm max. The USD's have less travel so I shouldnt be hitting the rad with the wheel. Overall this should quicken the steering, no bad thing.
The yokes have a longer offset putting the legs further forward by a few mm, this is handy as the legs and yokes should clear the tank, but what will this do to the handling?? cheers, Keith.

[Tamburinifan]

Have you calculated rake & trail? Would give a hint on rideability.
25mm f sure sounds like a lot...

[900streetfighter]

Ive ridden 907 with 20mm lower forks and it was fine apart from rad/tyre contact under hard braking.

The offset difference is about 5-10mm but I dont think the wheelbase will alter much as the shorter legs will counteract the offset but with steeper steering angle. K.

[907pasonut]

I dont have any engineering qualifications in steering geometry
...but in my opinion the handling would change a little and possibly for the better, and at the same time inherit a more sportier (lower front 25mm) riding position
then there is...the individuality factor
...GO FOR IT KEITH

[paso750]

the lower the front and the quicker the steering, the easier you get front wheel wobble at high speeds and you also put more pressure on the wrists.
Most forks can be lengthened by 10-20mm by inserting a spacer at the bottom or in case of an USD fork on top of the tube. If it´s a nonadjustible you can also have higher endcaps machined. Many full adjustible forks can also be lenghtened I heard, but that is probably a bit more complicated.



G.

[900streetfighter]

Thanks for replys and encouragement, I'm passed the point of no return now having bought a few more parts to go ahead. I'm trying to do this on a budget and use as many excess parts as I have in my garage. The adjustable SSie legs and yokes were in stock and a pair of 65mm calipers were too cheap on ebay to pass on. That leaves a wheel, discs and spindle to find, although I can rob these from my SS for a trial. I'll modify a top yoke to lower the legs over the next month or so and put it all together in spring. Ive got a selection of clip ons that i can try and I intend to mount them above the top yoke.
Ive spent a lot of time thinking about how its going to ride, and have convinced myself it'll be OK. The yoke offset is +4mm, which'll move the front wheel 2-3mm forward. The forks will be 25mm shorter, that'll steepen the steering by maybe a 1/2 degree and bring the F wheel back again by a couple of mm perhaps, giving bigger weight bias to the front. I dont understand much about rake and trail but I think this will give it more front end feel. I dont think anything drastic is going to happen, to me the steering on 907's round town is a bit vague, with luck this will improve. I dont think high speed stability will suffer as they're a long heavy bike.
The new forks are about 10 kg heavier, which is a bummer but the new wheel will be a bit lighter.
I'll learn to post piccys and show you how it turns out. Cheers Keith.

[ducbertus]

Hi Keith,

what about the with of the lower yoke. has it sufficient room to move in the fairing? there is not mucht space between the leg and coolant reservoir or electronix on the left side.

Bertus

[higgy]

http://en.wikipedia.org/wiki/Bicycle_an ... e_geometry

The basics of Fork geometry

[900streetfighter]

[quote="ducbertus"]Hi Keith,

what about the with of the lower yoke. has it sufficient room to move in the fairing? there is not mucht space between the leg and coolant reservoir or electronix on the left side.

Hi Bertus,
I havent looked further below at clearance but the showa yokes back edge is the same as the marzocchi from the stem, so hopefully wont foul anything.

[paso750]

I`m curious to see some pictures or drawings. As the fork is already a bit shorter I don`t understand how it would be possible to mount clip ons above the upper triple tree. To do that you would have to lower the whole front even more.

I dont think high speed stability will suffer as they're a long heavy bike.

That doesn`t matter. An extreme example of a steep head angle combined with lots of rake are shopping cart wheels.

G.

[900streetfighter]

[quote="paso750"]I`m curious to see some pictures or drawings. As the fork is already a bit shorter I don`t understand how it would be possible to mount clip ons above the upper triple tree. To do that you would have to lower the whole front even more.

I have a cunning plan, Im going to have some leg clamps(cut from a st yoke) welded below the top yoke that will hold the fork legs below the yoke. The empty upper clamps can hold a 60mm high piece of 50mm dia tube, to which I fit the bars.

[907pasonut]

The new forks are about 10 kg heavier, which is a bummer but the new wheel will be a bit lighter.
I'll learn to post piccys and show you how it turns out. Cheers Keith.

are you sure... the M1R forks including top and bottom clamps are just under 10 kg,
that makes the USD forks 20kg...thats a lot of metal

[900streetfighter]

The new forks are about 10 kg heavier, which is a bummer but the new wheel will be a bit lighter.
I'll learn to post piccys and show you how it turns out. Cheers Keith.

are you sure... the M1R forks including top and bottom clamps are just under 10 kg,
that makes the USD forks 20kg...thats a lot of metal

My mistake, the M1R leg I weighed had no oil, + my scales must've misread. Not much difference. Happy.
I'll put both complete forks on the scales at some stage for a proper comparison.

Im hoping the Ign/steering lock will fit onto the SSie top yoke fixing and work. That'd be a real bonus.

Thanks Higgy for the link, I need to study the subject a bit but the proof will be in the riding.

[paso750]

I have a cunning plan, Im going to have some leg clamps(cut from a st yoke) welded below the top yoke that will hold the fork legs below the yoke. The empty upper clamps can hold a 60mm high piece of 50mm dia tube, to which I fit the bars.

Honestly I would call that crazy. It would be easier and for sure a lot safer taking a full alloy tube, machine a thread on to it and use that as an extension for the fork.

G.

[907pasonut]

Keith...I know you're the one skinning this cat...and good on you for having a go
G's idea with the tube extension makes a lot of sense, and if you have the joint in the centre of the top triple tee you can gain at least 10mm without loosing metal strengh...then fit the clip ons on top...
PS: make sure the top extension does not spin around when stressed by the clip ons ie R/H side during emergency braking and L/H side by quick accelleration...as this may have dire consequences