The head angle on my bike is 68.5 degrees with a 100mm fork.

I’ve read that by adding 20mm of fork travel the head angle slackens by 1 degree….so for example putting a 120mm fork on the bike would put the head angle at 67.5 degrees, obviously i dont want to do this as the front of the bike will sit higher, the seat tube will be canted backwards even further and the bottom bracket will be higher.

So, if i fit an angle-set headset with an extra 1 degree of slackness but kept the 100mm forks then the front would be lower as the forks would be raked out in front of the bike slightly more….but if i have the 1 degree slacker angle-set combined with 120mm forks then the front of the bike would come back up to its previous height, the seat tube angle would be the same as it is currently and the bottom bracket would be in the same position as it is now?

Or have i got this completely wrong?

I’m very definitely not a ‘frame geometry expert’, but as I understand it and based on my own experience of fitting a 2? slacker head-set to an RC405, you’re roughly right in principle, though I don’t know about the exact figures.

A couple of things to bear in mind. It’s the axle-crown (AC) measurement that matters rather than the travel per se. Not always the same thing, some forks have a relatively low AC for their travel and vice versa.

You may also find that some angle-set type things add to the height of the front end if you go from a zero stack headset lower cup to an external one.

You can also tweak things like cockpit details with different stem, spacer, riser height arrangements – you may have to go with a shorter stem to keep steering sharp if you go slacker too. Plus inline and layback posts also give you some scope to tweak things. I found I had to experiment a bit to get things right, but did go from a standard 130mm fork, via a Pike, to a 150mm fork with no problems. With standard angles, the 150mm was too long for the frame, now it feels ‘right’.

Somewhere there’s neat online calculator that’ll do all the maths for you and give you a reasonable idea of what changing the variables will do with head angle, seat angle, BB height etc. Can’t remember where it is, but I’m sure someone’ll have it. Or I may have bookmarked it… hang on.

This is quite handy…

http://bikegeo.muha.cc/

Or http://www.cotic.co.uk/geek/

No expert here but familiar with the maths and have a calc set up that does it for me now –

The idea’s alright but the angleset adjustment won’t lower the frame as much as the 120mm fork height adds. Slackening the front by 1 deg on the head tube will lengthen your front-centre around 10mm, so will lower the bike maybe only 2-3mm? Once sag’s factored in your BB will only be a few mm higher when riding with a 20mm longer fork.

The 120mm fork only makes the bike slacker for the first 20mm of travel as you’ve not adjusted the frame itself. The angleset does adjust the frame. So what you’ll get is a bike that may feel like it’s about 1.5 degrees slacker to start with, but with the added sag and fork movement it’ll mostly ride like a bike that’s 1 deg slacker.

Wrong. honestly CBA explaining why tho.

your bike’s probably absolutely fine as it is.

“CBA explaining why tho” jeez GW have a word

OP I doubt the effect of the +20mm would be as significant/noticeable as you think.

17mm =~1 degree – after sag.

huh?!!

OP CBA telling us what frame/size it is, on never mind it’s full geometry so why should I BA?

FWIW I CBA giving vague/pointless replies like yours either Al.

You can sort-of compensate with various adjustments (as I have), but it will have an affect, possibly even a positive effect. For example, if you can budge your saddle forward and down a bit or replace a laypack post with a straight one. Run a bit more sag on the fork as well – more sag is good. Move some spacers or flip your stem to keep your hands low. If your bike had a low BB to start with, it’s not going to ruin it IMO.

GW my point is your post smacks of the whole “I am right but my time is too important and valuable to use it explaining why to you, you internet minion”…almost as bad as a certain stw poster’s “Do not reply to my post!”

Anywhy how is the frame size etc material?

Oh and your post was way more vague than mine 😎

Anyway I should have finished my new fat-cross bike next weekend so expect a FB PM re an evening ride.

Anywhy how is the frame size etc material?

________

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Pretend the lines above are the top tube lengths on a small, medium and large frame. Imagine raising the right side of each line by 20mm. Which line will be the steepest? Obviously exaggerated for effect.

Blah.. balh blah.. whatever Al 🙄 but you are right my time was more important than replying in full (then) especially as jameso had pretty much covered it already.
RE: size. When fitting a taller A-C fork or angleset a longer frame’s geometry will be affected slightly differently to a shorter frames geometry. Also, chainstay length tends to stay the same throughout the entire size range on most manufacturers frames so front-centre to rear-centre bias will be affected more slackening out a shorter frame no matter how it’s done.

no idea who the cretain stw poster you refer to is and am not really interested TBH. <EDIT> mistyped “certain” I’ll just leave it tho as it’s prob 50/50 it’ll still be accurate 😉

you’re welcome to come along to any of our nightrides (there was a group of us out all 7 nights last week)
or post up a ride of your own for us to join you if you want.

Jameso explained it for me the best so far….the other replies are getting complicated.
If it helps the material is aluminium, the frame is a 17 inch Kona Caldera with a 600mm top tube….from what i can gather so far it would seem to be ok fitting a 120mm fork as we’re only talking a couple of mm difference in bottom bracket height at most?

you seem to want a 120mm fork so just fit it, you can always lower it if it’s horrible.

Euro and GW…yes the top tube and other dims vary between frame size of course.

But not enough to matter for this discussion. Do the maths yourself if you think I am wrong. Cretainly I might be 😛

Fit the 120mm forks, if it feels wrong, then consider the angleset. Also bear in mind that forks have different axle to crown lengths for a given travel, so nothing is set in stone.