Topic: Struggling to calculate stock trail from other stock specs!

[Alan F.]

I've tried about a dozen trail calculators on the web, using the stock tire sizes, wheelbase, fork length, fork offset, neck rake, etc. I'm unable to calculate the 95mm trail specified in the SOHC home page's Tech/Bike specific specs page  for my 73 CB750K3.   I'll be performing a fork swap and want to calculate the difference in trail before I get too far down the wrong road.

can anyone verify my stock K3 CB750K uncompressed fork length, I've got 29.75" from caps to axle center. 
100/90-19 has a diameter of 26.086" 
110/90-18 has a diameter of 25.795"
wheelbase is 57.283"
fork offset is 1.805"
rake is 27 degrees

and trail listed in the tech section is 95mm?    why won't any of the calculators on the web give me anything different than 110mm give or take???  that's 4.33"

What am I doing wrong?

[Alan F.]

I've messed around with it a little, and if I've measured the fork offset wrong.... I thought I measured from the centerline of the fork tubes back to the centerline of the steering stem..... 

If the fork offset were 2.59 inches, or 65.786mm the trail calculators on the web yield a trail of 95mm like the manuals and tech specs page say....

Does anyone have any thoughts on fork offset measurements?   

The forks and wheel I was planning to swap on will give me a trail increase of nearly 2.3mm  that should increase high speed stability, right?

[Alan F.]

What, nobody?   Does anyone even know what I'm talking about? 

[Tim2005]

Have you tried measuring the current rake and trail to give you a baseline to work from? I've tried this in the past, not on a 750 albeit, and the measurements came out a bit different to the manufacturer's specs. Various factors influence it- rear tyre size, shock length etc all affect the ride height and may have been changed from when the bike was new.

BTW, fork offset is indeed measured from the centreline of the stem to the centreline of the fork tubes (unless you are using leading axle forks). Reducing the offset increases the trail, up to a certain point near 0 offset.

Some of this stuff may be useful too

http://www.motorcycle.com/how-to/chassis-alignment-basics-3444.html
http://www.tonyfoale.com/    (check the articles tab)

At the end of the day, if you do end up with 2mm extra trail, I doubt you'll even notice that.

Cheers

[754]

If by chance you have a 77/78 fork it will be different offset.

[Alan F.]

Insert Quote
Have you tried measuring the current rake and trail to give you a baseline to work from? I've tried this in the past, not on a 750 albeit, and the measurements came out a bit different to the manufacturer's specs. Various factors influence it- rear tyre size, shock length etc all affect the ride height and may have been changed from when the bike was new.

BTW, fork offset is indeed measured from the centreline of the stem to the centreline of the fork tubes (unless you are using leading axle forks). Reducing the offset increases the trail, up to a certain point near 0 offset.

Some of this stuff may be useful too

http://www.motorcycle.com/how-to/chassis-alignment-basics-3444.html
http://www.tonyfoale.com/    (check the articles tab)

At the end of the day, if you do end up with 2mm extra trail, I doubt you'll even notice that.

Cheers

Thanks Tim!
I may just have to mock up the stock setup again and string some lines up to measure baseline trail before my mods, then mock up my new stuff and measure the same way and document the difference.
I measured fork offset on my K3 trees to be 1.805" which figuring stock tire size and the 27 degrees of neck rake gave me a calculated trail of 110mm.  Which conveniently is what many modern sportbikes are using.  I wonder if the 95mm trail Honda spec'd out in the manuals was bogus after all.

Yesterday a friend loaned me Tony Foale's book, should be enlightening at very least.

Topic Summary
Posted on: Today at 02:27:49 pmPosted by: 754 
Insert Quote
If by chance you have a 77/78 fork it will be different offset.

Thanks 754!
I've actually got a set of 78K trees I've been comparing things to, seems like a great trick upgrade part for the early bikes.  But I'm using 41mm Forks from a 90's Honda...so...

[XN]

If by chance you have a 77/78 fork it will be different offset.

Would K7/8 trees increase or decrease trail?

[Alan F.]

more offset will give you less trail.  I think I may have mis-spoken.