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A confused guy want to change forks(8 posts)

A confused guy want to change forksDerf
Aug 16, 2003 6:22 PM
I tried this on the cyclocross board because it is a cyclocross bike. But, I thought that I would try it here.

I am looking to swap out my fork with rake of 44MM (Fork A) to one with 41MM (Fork B) of rake. I figured that this would not be a big change. But, then I figured that the fork lengths should be taken into consideration.

So, I tried to calculate Trail and see how big the change would be.

Per my calculations a change from a 400mm (fork A) to a 412mm fork (fork B) should change the head angle by -.01183 degrees using the formula arcsin[(old length - new length)/(wheel base)] with a wheel base of 1014.4

This changed the head angle from 72 (Fork A has a rake of 44mm) to 71.988 (Fork B that a rake of 41mm)

Plugging these into the model at http://www.kreuzotter.de/english/elenk.htm and using a tire diameter of 68.5 cm I came up with a trail of 10.7 for fork A and a trail of 10.7 for fork B.

Per this, my steering should not change. Are my assumptions full of it or do the change in the 2 variables give me a ride that is substantially the same?

Thanks for your help!!
Derf
Where's Alexx?the bull
Aug 16, 2003 7:35 PM
He is our fork engineer on this site! :)
You might want to double check your numbers...Nessism
Aug 16, 2003 9:08 PM
I'm coming up with a 0.68 degree change in head angle using your numbers. This amount of change jives with the following link.

http://www.sheldonbrown.com/rinard/forklengths.htm

Fork A: 44 mm rake, 400 mm length, 72 deg. head angle, Trail of 6.5 cm

Fork B: 41 mm rake, 412 mm length, 71.3 deg head angle, Trail of 7.3 cm

Fairly large difference if you ask me.

Ed
re: confused guyAlexx
Aug 17, 2003 6:52 AM
Something is waaaay off on your figures-any decent road bike from modern times will have a trail reading of about 50-70mm. A trail of 10.7mm will be just about unrideable at any speed over 10 mph.

Your ride is not affected by rake, trail, or geometry, only the handling is. If you wish to change the RIDE of your bike, change the tires, not the fork.

As far as your calculations go-I'm far too busy today to look at them. Maybe tomorrow, if I have time.
re: confused guyDerf
Aug 17, 2003 6:35 PM
I am trying to change the weight on my bike and have a good deal on a fork (it's about 1/2 lbs. lighter).

I think that I calculated my change in head angle in radians and not degrees.

I need to rerun my figures also.

All of your input is appreciated and if you have more please help.

Derf
D E R F - F R E D = 0 nmthe bull
Aug 17, 2003 7:38 PM
Thanks Ed and AlexDerf
Aug 18, 2003 10:40 AM
I reran the numbers and came out to the same ones as you, Ed. The arcsin function in Excel gives you radians as I had thought. I also did not have an appreciation of what the outcome should be, hence being "confused".

So, it looks like the fork change could my it ride a bit squirrelly.

I guess that I will have to seek out another fork to help me shave some weight off of my bike.

Thanks for your input,
Derf
Actually...Nessism
Aug 18, 2003 2:19 PM
the change would make your bike steer slower - anti-squirrelly?

Ed