Topic: Paging Industrial Cafe: Way overdone front forks... but will they work?

[camelman]

I bought a set of 2005 SV1000N forks the other day (for $100, I couldn't pass), but just realized the offset is only 35mm.  I'm planning for these to go on a CB400F or maybe a 550F.  The stock offset is 60mm.  Does anyone have experience shifting offset this much?  Am I in for a ride that I won't like if I do this?  My other option is to have a set of billet triple trees with 60mm offset made to take these.

Thanks,
Camelman

[domer]

you should ask i.c. what the offset on his gixxer forks is... i know its less than stock since he added bolts to the steering stops. nice find by the way!

[camelman]

Thanks, I was pretty stoked.  It even came with the axle and calipers.

Who is IC?

[scottly]

I put a set of Kawi 550 forks on my Seeley with much less offset than stock, and I had to modify the stops to the point that I have the turning radius of a school-bus. It feels like a high-side just waiting to happen.

[Retro Rocket]

Who is IC?

Forum member "industrial cafe"

Mick

[camelman]

IC,

I couldn't find an e-mail addy for you, but I'm curious to hear your thoughts.  I'm worried about Scottly's comments about instability.  Can you fill me in on how your forks have held up?

Thanks,
Camelman

[scottly]

My issues were due to the reduced turning radius. I had to modify the stops to keep the upper part of the fork tubes from hitting my gas tank (it's a Seeley, and I really, really, don't want to dent the tank). Other than not being able to turn very sharp, the forks worked very well. I either need to make/get new triple clamps, or put on a different tank. There may have been a wee bit more head-shake with the lessened offset, but I can't be sure about that...  

[camelman]

Scottly,

Are you running a steering damper?

[scottly]

No. I think it would help, but I think it would have helped with the 750 forks as well...

[Retro Rocket]

I spend a lot of time setting up my bikes suspension and on all the Honda4's i have owned i have run steering dampers,without them i have had head shakes over 100mph, scary #$%*, with dampers i have never had one. These bikes all had stock forks, i am running GSXR forks on my current build and will use a damper for sure.

Mick

[scottly]

What sort of dampers do you run, Mick?

[Retro Rocket]

The Black ones that were common in the seventies and eighties, i see them sold as Z900 steering dampers from time to time or universal steering damper. I am not sure of the brand but i bet someone on here would know for sure, they were almost exactly like the one in this link.

https://shop.pingelonline.com/iwwidb.pvx?;multi_item_submit?comp=PEI

Mick

[754]

I think Z1 sells them, around 80 bucks

 They were an option on triples and Z1,s.

[camelman]

I picked one up on eBay a few months back.  Nothing terribly high quality, but it definitely dampens.  Just not sure if it is going to survive the San Francisco fog without rusting.  It looks the same as the link posted above, so it should be easy to install.

What will decreasing the fork offset do to handling?  Stock is 60mm, and this unit is 35mm.  It seems to me that it will be less lively, but more stable at higher speeds.

Camelman

[scottly]

What will decreasing the fork offset do to handling?  Stock is 60mm, and this unit is 35mm.  It seems to me that it will be less lively, but more stable at higher speeds.

Camelman

I may be wrong, but I believe it's the other way around....

[Retro Rocket]

I believe you are probably right Scottly but if i remember correctly it ids the same rake trail and offset as the early GSXR's and they were pretty stable, but in saying that, mine came standard with a steering damper...

Mick

[MaaseyRacer]

I think you will be fine, I have seen a couple guys on this forum running R6 forks from around 2004.  This guy is even racing them:
http://forums.sohc4.net/index.php?topic=63938.0

On the third page of that thread if you look at the upper triple tree the off set is not too far off from yours.  However, that being said it appears he is running 17inch sport bike wheels.

[camelman]

Okay, it looks like Maasey and I are going to do some dynamic system modeling of my future 400F speed/handling magi.  Stay tuned for the next exciting episode.

Camelman

[mystic_1]

All other things being equal, reducing offset will increase your trail value bu approximately that same amount, but that is a HUGE oversimplification.  There are a lot of other variables that will affect trail also such as the exact fork length, wheel sizes, rear shock length, etc. all of which will affect your true steering angle.


http://www.rbracing-rsr.com/advchoppercalc.html

You can do a lot of math on paper to figure it out or you can slap it all together and measure.  Really you should do both.  I made accurate scale drawings from the dimension drawings in the book and then made changes digitally.  My figures came out pretty close to reality when I put them to the test.  It was a lot of work though.

Honda seemed to like trail figures right around 4 inches.  The CB750 is 3.75 inches.  My bike currently has stock offset but 8" over forks, giving about 5.3 inches of trail and steering is a bit heavy as you'd expect

mystic_1

[mystic_1]

Ah, I see you're going with the "grueling math" approach   Have fun!

mystic_1

[Retro Rocket]

I think Z1 sells them, around 80 bucks

 They were an option on triples and Z1,s.

Thats the one's...

Mick

[camelman]

Ah, I see you're going with the "grueling math" approach   Have fun!

mystic_1

I misspoke.  I'm going to lay out my front suspension with a static analysis like you showed.  I'll model full compression and full jounce just to make sure it isn't too far off, but I'm planning on modeling at "ride height".  I really need to set up an Excel spreadsheet to model this.  Hmmmm, maybe I can make some money at that, especially if it is dynamic.  ;-)

Camelman

[Industrial Cafe]

sorry to take so long to get here-

the trail on my GSXR front end is 35mm.
   The only thing I've noticed is when I turn around I end up 3 feet in the grass whereas before I was just touching the edge of the road.
I'll probably end up having a new set of trees made with a better offset, but it's really just something I'll have to get used to. Sorta like going from driving an 89 toyota to a full size duallie.  ;not so much "unsafe" just surprising until I know my limits.
   It feels stable all the way up to about 80, but that's all I could get up to in front of my house. (I'm still not done with the brakes or speedo on the swap)  



I would say, if you got the money, take your trees to a machinist and tell him you need some new pieces made out of a good quality alloy that give you a 50mm offset or whatever you think would be best.
   personally I think 50mm would be fine since the fork is like an inch wider it will give you the same effect as the stock fork and 60mm offset.


EDIT: this link might help some-  http://forums.sohc4.net/index.php?topic=63784.0

[camelman]

I've been doing some research, and it looks like the shorter offset will result in more trail.  The greater the trail, the greater the stability of the bike at speed... but the less "flickable" it is.  Next thing to see is how 17" wheels will affect the system.  I think I have some math staring me in the face.

Camelman

[Industrial Cafe]

I think it's the opposite.
I don't know, I haven't finished testing.

[Rick4004]

Go to my website for a fully interactive rake/trail calculator for the CB400F. Just plug in whatever wheels/clamp offset/shock length/ swingarm length you want and it will give you the resulting rake and trail.

http://www.denoonsp.com/rake-trail-2.php

Rick Denoon

[camelman]

Dang, Rick, that is a sweet tool.  Thanks for setting that up for the public.

My trail definitely increases, which is going to slow my handling down a bit, but improve high-speed stability.  The front will comes in closer to the engine though, so it will be loaded more.  I guess I just need to build this sucker to see how it performs.