Numpty fork / geometry question

dom stockton

Hi all,
I know that this question may have been asked many times already, but anyway... ;o)
I currently have a 100mm fork up front which I'd like to replace because it's a bit gash. if I was to replace it with a 120mm fork am I going to do anything noticeably bad to the way my bike rides? I used to ride with a 2.35 HighRoller on the front and a 2.0 Conti Vapour on the rear and there was about a 15mm height difference between the wheels - that didn't affect things negatively. I've since moved to having HighRollers front and rear so am "level" again in that respect.
Cheers in advance for your help.

Bike is a 2005 Cindercone with Marzocchi MZ Race fork btw and I'm getting back into biking after a 15 year (!) lay off.

nicklouse

every inch/25mm in axle to crown height changes the head angel by about 1 degree.

thelonegroover

I think the OPer is asking what effect adding 100mm to the folk length will have on the handling.

Anonymous

Thelonegroover wrote:

I think the OPer is asking what effect adding 100mm to the folk length will have on the handling.

No, he's asking what adding 20mm would do.

Not sure what the frame is warranted for, but it shouldn't ruin the handling - it's not an insurmountable change.

Pudseyp

I guess a one degree angle increase to 69 will not make a significant difference. Having a 5 inch headtube should be strong enough...

nicklouse

Thelonegroover wrote:

I think the OPer is asking what effect adding 100mm to the folk length will have on the handling.

no he is asking about changing forks and as I have said it is the axle to crown measurement that needs to be looked at.

for example there are 180mm forks out there that are taller than some 200mm travel forks.

there are some 100mm travel forks that are taller than some 120mm forks.

what he IS asking is what difference a 20mm change of travel might make. the answer is some change or no change or.... like i said for every inch of change in the axle to crown the head angle changes by about one degree.

The Rookie

yeehaamcgee wrote:

Thelonegroover wrote:

I think the OPer is asking what effect adding 100mm to the folk length will have on the handling.

No, he's asking what adding 20mm would do.

Or the 13mm once you dial in 1/3rd sag?

As Nick says its the axle to crown height that counts, a 120mm with the same height as his current 100's set to the same mm sag will make no effect at all.

Simon

supersonic

The flip side is that at full compression the bike will be steeper. Not usually a problem, but worth noting.

nicklouse

and the tyre may also be closer to the downtube....

thelonegroover

OK, so I typed 100 instead of 20, sorry folks, let’s not get too pedantic please.
The question is, how is adding 20mm going to effect the handling. (All other things being equal).

Maybe the PO should clarify.

supersonic

It adds 1 degree to the angles, slightly increases BB height, front centre and wheelbase.

Slacker head angles make the bike more stable, but if is very slack or steep to start with then the difference may not be that noticeable. You will also be at slightly further behind the BB with less weight over the front end.

thelonegroover

So, he might find it easier riding hands free (without holding the handlebars), but might find the front wheel drifting on steep inclines.

supersonic

Quite possibly - subtle, but will be there.

dom stockton

cheers for al the help guys, will be getting the tape measure out, i'd kind of assumed that forks of a given travel distance have standardised overall lengths - i'm glad i know better now.