# Changing forks and geometry

#### Will1985

##### Über Member
This is a long question. Recently I won a TT frame on eBay which has a 700c rear and 650c front.

Firstly, the rear seatstays are so narrow that it can't take a 23mm tyre...no problems there I think because a 20mm will certainly fit and possibly a 22mm.
Secondly I already have Zondas and Carbones, so swapping out the 650c fork for a 700c would save having another wheel clogging up the place. I'm no mathemetician, so I am unable to calculate roughly how much of a seat angle is lost by raising the front up by an inch....does anybody have a rough idea? I drew a diagram to work out the increase in wheelbase, but not sure if that can help with calculating the decrease in seat angle.

I also have a Thomson setback/forward seatpost which could counteract any loss of angle (I'm not bothered about the style critics who say that is wrong).​

#### Keith Oates

##### Janner
If you mean by how much will the nose of the seat rise up, I would say not very much. However this can easily be adjusted down again. Most riders say that the seat should be horizontal to the road so that can be easily noted just by the eye although a spirit level could be used if you were really worried about it. You may however find that you prefer the nose of the seat to be up or down a little so trial an error is the best way to find out what is comfortable for you!!!!!!

OP
OP

#### Will1985

##### Über Member
Hi Keith - thanks for the quick reply.
I was actually thinking more along the lines of the angle of the seatpost in relation to the BB and losing the "steep" angle.....it is 76° untouched, but I'm interested to find out if raising the front of the bike alters the seatpost angle to less than a standard road bike (around 72.5°). It's hard to imagine if 3.5° really makes much of a difference - more comfortable on a 25 perhaps.

Also to clarify the tyre situation I'm wondering - if it fits - whether a 22mm Conti Attack is suitable for the back...

#### mickle

##### innit
Raising the head tube by an inch will change the head angle by about one degree (and the BB height by close to half an inch). Not a particularly big deal, one may adjust the saddle to accomodate the change of seat angle and position relative to the BB and drop the bars. The real issue is what happens at the contact point of the front wheel on the road relative to the center of steering. You'll need to install a fork with more offset (or "rake") to maintain the correct amount of trail. The bike might end up looking like a tourer.