Framebuilders - Geometry - confused at the front end...

So, I'm designing my DIY frame jig and I'm drafting it all up. I've worked out how I'm going to locate and adjust everything that I'm going to be playing with.

What is hurting my head is working out where the bottom of the steerer tube should be located.:twitchy:

Obviously the wheel size (700c) is a constant, but the rake of the fork is one variable that is making it hard for me to conceptualize and locate the steerer tube bottom edge.

Is it possible to just say that the bottom of the steerer tube is at Xmm from the centre line of the hubs all the time, simply following the line of the steerer tube? Or does the selection of fork screw with things on a frame by frame or fork by fork basis?

Is there just a single standard fork length (in terms of position of the bottom headset bearing)? Is it really as simple as just measuring a fork or using the wheel sizing info?

It seems to be something that is not discussed in the resources that I'm looking at - I'm getting the idea that it is something that should be known already. Perhaps I'm just looking at it the wrong way, or maybe I'm just thick.

The rear triangle seems easy!

There is no standard for the axle to bottom headset height. There are too many variables- tires, fenders, brakes, standover height, etc.
For the rest, as always, it depends!
Depends on what kind of jig you're building/depends on what kinds of bikes you're wanting to build.
My jig has fixed reference points for the front and rear axles and adjustable head tube/bb heights. This way it's super easy to set up and can build all sorts of frames.
I've used several styles of jigs and I think this rail type is the best!

My jig design will allow a great deal of adjustment - I'm just not sure where I need to adjust it to!

I am also using a rail design (but might modify a couple of parts now that I've seen yours!)

Rail jigs are the best.
You might like to check out this thread too http://www.mtb-news.de/forum/showthread.php?t=387748&page=2
it has some good pics of DeKerf's rail jig in use.

What often happenst while travelling down the path of developing the tooling etc... is that the last thing one wants to spend time and money on is the parts of the bike that come later in the process. If you had a wheel and fork in stock for this design, or enough experience to know all the dimensions, you would have your answer. I have been in the same situation myself and seen it many times. You either just have to estimate on the basis of bikes you have etc... with the possibility of being out a few mm, or you need the parts in hand. If you have the parts then you just do a full size drawing incorporating your rake requirements and you are ready to do. For the number you want, you don't even need to draw the whole frame out. Look forward to seeing the jig.

Thanks Peterpan.

No particular parts in mind. The fork would have been purchased later and would be a "standard" - whatever that is! - road bike fork.

If being out a few mm is no big deal, then I'll just go with some basic numbers from my other bikes or I might go to the LBS and measure a few possible candidate forks.

I'll post a drawing of the frame on the jig when it is done. Don't have a CAD program, so I'm using Adobe Illustrator and drawing at 100% scale.

Most forks will give you a crown height amoung the specs. There can be some variation in the same fork class, obviously a lot with something like a suspension system. But with fixed forks you have the numbers to go by. You also need the stack height of you lower bearing for the headset. On Patenek's site there are plans for a dummy headset you can use to mock up bikes. If you can pick a frk before hand, even if you don't buy it, that would be ideal. It's a pain to be off a little bit when you went to all the trouble to do a custom build.

Why not build your own fork...it's the easiest part of the whole project. Much easier than the rest of the frame.

I'm not sure there is such a thing as a "standard" fork, especially if you want to build custom.

There aren't any standards based on size, but there are things like "I am building a touring bike and I am going to use something like a Surly LHT, or a Tange". Standard as to brand and general features, but still variable in size.

Actually there is probably in some regards too much standardization. If one decides on an odd HT geometry, one could find it pretty difficult to get a stock for that worked...

A lot of builders regard the fork as a particular challenge, because getting it wrong is really bad for your bridgework; it requires some bending that isn't all that difficult, but it's one more thing , and one more jig (I have some backgroud in metal bending and bender fabrication, and managed to embarass myself on my first fork brlade...); Sometimes the brazing is pretty straightforward nice fat socket joints, but the heat control is a different thing from other joints in the bike; Some of the copes can be compound angles; some builders never make their own forks.

Road bike forks fall into a pretty narrow range of variance. Common lengths are 363 - 375 mm, and rake ranges from 38 - 53 mm. Length is measured parallel to the steerer tube. If you design your fixture to accommodate these ranges of sizes I think you will be okay. Of course, if you are talking about MTB the situation is completely different.

Road bike forks fall into a pretty narrow range of variance. Common lengths are 363 - 375 mm, and rake ranges from 38 - 53 mm. Length is measured parallel to the steerer tube. If you design your fixture to accommodate these ranges of sizes I think you will be okay. Of course, if you are talking about MTB the situation is completely different.

Thanks for this range information.

I've got three different road forks here (on bikes) so I'll measure them all, check it to this range, and design for something close to the average.

Quick clarification on the measurement. You say parallel to the steerer tube, so does this mean that I measure the rake 90deg from this imaginary straight 'length' line? This would make the intersection of the 'length' line and the 'rake' line below the level of the hubs. Correct?

Quick clarification on the measurement. You say parallel to the steerer tube, so does this mean that I measure the rake 90deg from this imaginary straight 'length' line? This would make the intersection of the 'length' line and the 'rake' line below the level of the hubs. Correct?

Correct. Check this link for some more info... http://www.sheldonbrown.com/rinard/forklengths.htm

Correct. Check this link for some more info... http://www.sheldonbrown.com/rinard/forklengths.htm

What is it they say about a picture and a thousand words?:thumb:

Thanks. Much clearer now (wish I had found that one earlier).