Bike geometry - am i right?

tom_fun

Hi folks, I have a Cannondale RZ140 with a standard fork fitted using the proper reducer headset. When I got the headset, it came with 2 bottom cups. 1 which fitted in the headset so the fork is flush with the frame, and a bigger cup that adds an extra 10mm between the crown race and the bottom of the head tube. I fitted the bigger cup just to make sure that the crown of the fork didn't catch the frame.

I've been thinking about putting the 'flush with frame' cup on but would like to sense check my understanding of geometry with you folks...

My understandings are that currently with the extra 10mm between crown & head tube I have slackened the HA by about 0.5 degree & slightly raised the BB. If I put on the 'flush' cup I will return the bike to its default HA & BB settings and compared to my current set up, put more weight over the front wheel which will have climbing and stability benefits?

Does that sound about right or am a miles off?

DCR00

i wouldnt have said 10mm will do that much. IIRC 20mm does about half a degree on the HT.

The Rookie

It all depends on the A to C of your fork versus the original, you have only slackened it if the fork is the same length or longer than the original, if it's shorter than the original by more than 10mm it will be steeper anyway.

tom_fun

Not sure what fork came as standard as I got the frame second hand but I run a 140 fork and it was designed for a 140 fork. I think the frame came with either a Lefty or a Fox 140 float depending on model. I'll swap the cups over and see what difference it makes in the real world, if I can even notice any difference! Cheers folks

The Rookie

Why not check the A to C's first, different same travel forks can have A to C's up to 25mm different and you will be going to a degree of hassle for nothing.

tom_fun

Makes sense, do you know if there is a database online with that info? If not i'll get out my tape measure and try to pin point the original fork that came on the frame. (A to C is middle of Hub to crown race isn't it?)

The Rookie

Manufacturers webby, and yes that is A to C.

miceden

The RZ140 I think came with a Lefty Max Carbon PBR 140 fork, the A-C on that fork was 510mm... Fox Floats at 140mm have an A-C of 525mm

Did a lot of research into Fox & Lefty forks a while back so believe my figures are true.

tom_fun

Thanks a bunch miceden! I run a DT Swiss XMM140, I found a site selling them that had a description of 'Height: +/- 3mm = 515'

I'd presume that they are referring to A to C.

So i currently have the A to C of 515 + my 10mm bottom cup gives me a relative A to C of 525 which is bang on what the Fox forks are and if i put the small cup in it will be basically bang on the set up if it had the Lefty fork fitted.

Hmmmmm, at least i now know that if i put the standard cup on it will be no worse off than standard!

Thanks for all your help chaps.

miceden

You're welcome... here is a link to DT's tech specs on their forks http://www.dtswiss.com/Resources/Suppor ... _Forks.pdf - the "height: +/- 3mm = 515" does refer to the A-C measurement

Your XMM's are the twinshots? I have the XMM 120, really nice fork.

tom_fun

Yes, twin shots. Great fork, had a Fox 32 Float prior to those and the XMM's are far better!