emig offset clamps

28 replies to this topic
  • ncmountainman

Posted August 19, 2005 - 02:13 PM


Moving the forks straight back does not alter the rake angle (the angle ahead of vertical at which the forks are mounted). That remains parallel to the head angle. .6 is 24.25% of 2.5, but that has nothing to do with anything we're talking about.

Because the forks move straight back toward the stem, they do so along a line which is 117.5 degrees from vertical (perpendicular to the steering head), or 27.5 degrees to the ground, which is the line through the axle center I mentioned earlier. Moving the forks back 2.5mm moves the axle down this line by that same distance, and the axle's new location on that line is .6mm closer to the ground than it previously was. If you PM your e-mail address to me, I'll send you a diagram of what I'm talking about. If I can get my web site to talk to my computer again, I'll post it there and post a link.

ya i know it doesn't change the rake i just worded that wrong by using "just" i'll go back and fix it. but if your forks had a 67.5' rake, the amount of movement below horizon would be 87.5% of initial movement ....45' rake is 75% below....22.5' is 62.5% below between 22.5' and back to the opposite 90 (0') its rapidly accellerated. i just got done making the models :D just post it here if you can.

  • grayracer513

Posted August 19, 2005 - 09:15 PM


OK, Bob, here you go. First though, let me admit that I was incorrect about the change being .6mm. It's actually twice that, or 1.2mm, which is still only .047", but it is bigger than I first calculated. I crossed myself up by taking one of my typically convoluted approaches to solving what should have been a simple problem.

Anyway take a look at This Diagram and see if it makes more sense to you.

The axle height dimension for the original center was taken from my bike. It's absolute accuracy isn't really important because what we're looking for is the amount of change involved in the offset of the fork. In the drawing, the new axle center is lower than the original position, but in practice, of course it will stay the same, and the difference will show up at the top of the steering head. What you're really doing in that sense is sliding the steering head 2.5mm closer to the forks along a 27.5 degree slope, so it ends up higher than it was. Note that the 2.5mm separation of the two lines representing the original and offset positions of the fork tubes is greatly exaggerated for clarity, and is larger than "to scale". Even at that, you can see how small the overall effect on height is.

So, yes it does raise the bike, but so little that you could compensate by scooting forward on the seat 2 inches. And yes, it does help the cornering to pull the forks up in the clamps. That's always been true because it makes the head angle steeper.

Your modeling of the 45 degree angle was close. It actually calculates to .707, not .75. :D

  • ncmountainman

Posted August 20, 2005 - 04:55 AM


i knew my figures were not precise; as they were measurements from drawings and then a mock up , but i knew that you were nowhere near it, hell you had me calling my mathmagician buddy who told me that 75% at 45' sounded reasonable to him. it didn't have any ryhme or reason compared to what you suggested (0-25%-50%-75%-100%) so in fact even though my numbers weren't as correct as they could be,i was closer to correct than you were :D :D :eek: :eek: :eek:
thats if your even right this time :D(shouldn't © intersect with (a) changing a= ? nice plug for the 756,but i think a maxxis SI would yeild a higher percentage :D oh,btw i could most definately feel the need to lower the clamps,whether it be for this reason or shorter wheelbase or both. you'll see :D

  • RCannon

Posted August 20, 2005 - 06:25 AM


Guys, I dont know much, but I THINK Grayracer went to class instead of hanging out at the local pool hall.

Damn.....that drawing rules. I saved and printed it.

I am guessing here, but I doubt he is working at wal mart as a door greeter.....

How would these figures change if a person was running one of the new 90/100-21 front tires? This tire does measure 10mm higher than the standard issue tire.

I am using the 712fa Dunlop. Its a great hart terrain tire, not so good in the soft stuff!

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  • ncmountainman

Posted August 20, 2005 - 06:37 AM


the figures wouldn't change it's all relative. as long as you don't change the tire size at the same time.

  • grayracer513

Posted August 20, 2005 - 07:54 AM


the figures wouldn't change it's all relative. as long as you don't change the tire size at the same time.

True. As I said before, what the discussion is concerned with is the amount of change produced by the offset. The only two things that have to be accurate are the head angle and the amount of offset in the clamps.

And yes, a does change; from 355.6 to 354.4, 1.2mm shorter. Drawing a second a side intersecting the new axle center would have made the illustration a little messy in the small space available, but I put the dimension there.

  • mikedabike

Posted August 20, 2005 - 08:03 AM


Alright you two stop it already, my brain is starting to hurt :D

  • ncmountainman

Posted August 20, 2005 - 08:14 AM


OK, Bob, here you go. First though, let me admit that I was incorrect about the change being .6mm. It's actually twice that, or 1.2mm, which is still only .047"

i'm a little confused by this math shouldn't it be .47 of 2.5mm?
mikedabike,if i don't tickle my brain cells every now and then they go bad :D

  • grayracer513

Posted August 20, 2005 - 11:09 AM


The ratio of the relationship of the two short sides will vary with the angle. The first formula shown is the one that expresses the rule that applies here. The sine, a trigonometric function, of angle A is equal to the length of the opposite side, a, divided by the length of the hypotenuse, the long side, c.

Since the information we had available was the 27.5 degrees of A, and the vertical height of a, that's the formula we used. BTW, it only applies to right triangles.

It's Trig, dude! :D

You could also build a little triangle where c was the 2.5mm you moved your forks, and A is 27.5 degrees. Guess what the length of a is.... 1.1544mm.

For fun, try http://aleph0.clarku...oyce/java/trig/,

then go to item 10, http://aleph0.clarku...trig/right.html

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