How do I calc correct fork length for a frame conundrum?

So this is an age old problem that's had me boggled for ages, is there a way of fairly easily calculating what length fork A-C a given frame is designed for?

Thankfully most of my collection runs in the good old 390 A-C, however I've had situations where I have a frame and cannot work out its age etc and what fork its designed around, the only expensive/time consuming ways of calculating is through trial/error or looking back through the archives to see what fork it came out of the factory on, and then often further research into what A-C given fork runs.

I've got a feeling its going to require measuring lengths and angles but does anyone know a method that for sure works? This has all come about through a recent thread on here about a Torus type frame that a members building up and also my own quest, with a couple of late Klein frames that I've picked up and i'm building up.

Cheers for any help

BUMP... anybody know of a way besides trial/error??

I've just downloaded the Protractor app for my phone, seems pretty useful tool to give out head angles but only once a fork is fitted and you know the head angle...

TOMAS":123w3dhs said:

BUMP... anybody know of a way besides trial/error??

I've just downloaded the Protractor app for my phone, seems pretty useful tool to give out head angles but only once a fork is fitted and you know the head angle...

Click to expand...

I have often wondered if it is possible to accurately measure the Head Tube angle on a bike. Going back to my early days of bike building, I did indeed upset the geometry on a frame that was pre-suspension corrected by fitting some Amp Research F3 forks. I was really looking forward to riding a bike with some front suspension, but couldn't figure out why it was wallowing at slow speed :facepalm: .

The problem as I see it with measuring the actual Head Tube angle, is that it is such a short amount to measure against - i.e. 110 / 130 mm or so. I have wondered if a Smart Phone app (or any other method) would be able to get enough accuracy from such a short "base line".

Frame Geometry by Philip Mock, on Flickr

If you are doing this on your own, I would think that you could lay the frame on its side with just the rear wheel in place. Then line up the base of the rear tyre on a line along the ground, and swivel the frame around the rear axle, until you achieve a desired angle of the Head Tube. I think that you would need to get up on some steps to look down from a decent distance.

The alternative would be to do it against a wall with someone holding the front of the bike up, then raising and lowering the front of the bike until the desired angle was measured at the Head Tube. Remember that you need to measure the Head Tube and not the forks as these will have a rake / offset. This isn't obvious in the picture below, as my camera is lined up with the centre of the bike, rather than over the forks.

Muni Mula Geom by Philip Mock, on Flickr

I will say that even with using the crude method of measuring with a protractor on my monitor, and and taking into account a bit of sag with me sat on the bike, I managed to measure 71-ish degrees for the Head Tube, and 73-ish degrees for the Seat Tube which is the recommend angles for a 1997 Muni Mula. The forks were set to the shorter 63mm travel / 430mm C-A, so about 415mm with sag.

Pip.