Fork length and frame angles.

[brocklanders023]

Not sure if this is strictly retro chat so mods feel free to move

Anyway.... Does anyone know how much say a 10mm change in fork travel will affect the angles of a frame. My brain is small and can not compute these things.

e.g frame has 74 degree seat angle and 70 degree head angle as stock. What would these figures change to by adding 10mm :?

Thanks.

 

[Dr S]

Very little. Do you have an iPhone?? There are a few free apps that measure angles.

There has been plenty of debate over on my Yo Eddy thread on fitting longer forks- take a look.

If you look at a modern long forked frame you will find the head angles to be between 69 and 67 degrees (67 being the head angle of a Schwinn Excelsior btw, so we are full circle). Fitting a longer fork to a 90s frame with steep racy angles will still result in a head angle of around 69-70deg- hardly slack by today's standards. If the steering does become a little lazy just fit a shorter stem- around 90-100mm instead of the crazy 130-140 stems usually found on our retro stuff.
Another thing to consider is Bottom Bracket height but even with a 20mm longer fork you are only talking an increase of about 10-12 mm- handy on low slung 90s race bikes and hardly noticable when you take sag into consideration.
One final consideration is aesthetics. Some frames can look awkward with a longer fork although 10mm won't look any different. Anyway who gives a toss what it looks like when you are enjoying the ride!

Fitting a 100mm fork on a Yo is the best improvement I have ever made to a bike. It's still sharp in the turns and so much quicker over rougher sections. It's a bike I can (and do) ride all day now.

 

[Anthony]

The nth degree of accuracy depends on the wheelbase and how much sag you run, but for an average-length bike and 25% sag you can reckon that the head and seat angles will diminish by 0.45 deg (sagged) for every (un-sagged) cm that you add to fork length and the bottom bracket height will increase by 3mm.

i.e., if you were talking about rigid forks, it would be 0.6 deg and 4mm respectively.

 

[brocklanders023]

Cheers chaps.

Was thinking both modern and retro. Retro side, sticking 80mm forks on an mid 90's Orange. Modern was for my 5. Listed as 70 degree head angle as stock with 130mm forks on a 2006. Mine came with 2007 140mm forks so I wondered what difference the 10mm would make. The 2010 is slacker so wasn't sure if mine would be closer to the new angles. I guess not really.

Seen your Yo thread Dr and am watching with interest. It's your fault I'm looking at sticking modern forks on a retro! :roll: