Matching fork to frame

[winjim]

Say I have a frame, how can I go about working out what would be a suitable fork to go with it? I'm not so much thinking about steerer diameters and such, but more about the geometry, so rake and axle to crown height. Are there any basic principles that can be put into practice? Can I reverse engineer the frameset design, and what are the consequences should I use a fork with different dimensions than those intended by the frame designer?

Let's assume for the sake of discussion, a road bike frame designed for 700c wheels.

 

[Yellow Saddle]

A rule of thumb would be that any fork you want to fit will work so well that you will not be able to say whether it has a little bit more or a little bit less offset or rake than another. As far as offset goes, all forks will allow you to ride no-hands and, easily steer whilst standing and pedaling uphill. Those being the two ends of the spectrum of fork positioning.

As far as crown height goes, it is the same story. No fixed fork will have a crown height so large that it will affect the steering. This doesn't hold true for suspension forks since some forks could alter length by up to 200mm. Keep in mind that brake calipers have a very small adjustment range, even if you spread the range between the shortest on a short-reach set and the longest on a long-reach set.

That's why 99% of the talk on marketing brochures and bike magazines about bike geometry is nonsense.

 

[winjim]

OK cool, that answers the question in a practical sense and for anything I might actually do in the real world. But just for interest...

When you say I wouldn't be able to tell if there was any difference in the offset, is it that I wouldn't be able to tell, being just some bloke on the internet trying to learn a bit about bikes, or is there really no discernable difference at all? If I blinded a professional cyclist and then replaced their usual fork with one with slightly different offset, what would they notice?

And, although it might make no practical difference, is there a way of telling, for a given frame, which fork would theoretically work the best? Or is it all just in the frame designer's head and down to their personal preference?

 

[TheDoctor]

There's so little difference between road bike forks that they'd be interchangeable. I've swapped forks between bikes and still ridden them no-hands.

 

[Ajax Bay]

is there a way of telling, for a given frame, which fork would theoretically work the best?

When I saw the Op and this elaboration, I immediately thought of Tony Oliver's graph in his excellent 'Touring Bikes' book, plotting head tube angle against offset.
I can't find that graph online but this article link discusses the relationship. I'm trying to answer the 'theoretical' aspect of your enquiry: @Yellow Saddle has opined that there is little difference practically. In theory if you matched a tandem fork with a road frame the steering would be sufficiently more sluggish (aka 'stable') to be noticeable, and conversely (and OT) a tandem pair would find a 'normal road' fork twitchy.

 

[Yellow Saddle]

When I saw the Op and this elaboration, I immediately thought of Tony Oliver's graph in his excellent 'Touring Bikes' book, plotting head tube angle against offset.
I can't find that graph online but this article link discusses the relationship. I'm trying to answer the 'theoretical' aspect of your enquiry: @Yellow Saddle has opined that there is little difference practically. In theory if you matched a tandem fork with a road frame the steering would be sufficiently more sluggish (aka 'stable') to be noticeable, and conversely (and OT) a tandem pair would find a 'normal road' fork twitchy.

No, you are confusing rake with offset. Rake is the angle of the fork steerer axis wrt the horizontal. (Think "rakish angled hat"). Offsest is how far ahead (or behind in the case of a supermarket trolley wheel) the wheel axis is from a straight line drawn down the steerer axis.
Many references to rake are just plane wrong, including the one in the link. Rake is not offset. Offset is a distance, rake is an angle.
Edit: In fact, this link has so many mistakes it boggles my mind. It also confuses head tube angle with steering axis. Steering axis is a line. Rake is the angle of that line.
Best to ignore the wisdom in that link.

 

[r04DiE]

This is all rubbish. Me and some blokes I used to work with changed the forks on a Raleigh Shopper and it made a massive difference. Don't listen to any of these people on here

 

[winjim]

No, you are confusing rake with offset. Rake is the angle of the fork steerer axis wrt the horizontal. (Think "rakish angled hat"). Offsest is how far ahead (or behind in the case of a supermarket trolley wheel) the wheel axis is from a straight line drawn down the steerer axis.
Many references to rake are just plane wrong, including the one in the link. Rake is not offset. Offset is a distance, rake is an angle.
Edit: In fact, this link has so many mistakes it boggles my mind. It also confuses head tube angle with steering axis. Steering axis is a line. Rake is the angle of that line.
Best to ignore the wisdom in that link.

Are you measuring offset horizontally or perpendicular to the steering axis? Is the steering axis drawn as a continuation of the headtube, and if so how can that be the rake, if rake is a property of the fork?

You may strictly speaking be correct about the difference between rake and offset, but remember we live in a world in which forks are sold with rake measured in mm.

 

[T4tomo]

This is all rubbish. Me and some blokes I used to work with changed the forks on a Raleigh Shopper and it made a massive difference. Don't listen to any of these people on here

My cousin welded extensions into his chopper forks to similar effect

 

[Ajax Bay]

No, you are confusing rake with offset. Rake is the angle of the fork steerer axis wrt the horizontal. (Think "rakish angled hat"). Offsest is how far ahead (or behind in the case of a supermarket trolley wheel) the wheel axis is from a straight line drawn down the steerer axis.
Many references to rake are just plane wrong, including the one in the link. Rake is not offset. Offset is a distance, rake is an angle.
Best to ignore the wisdom in that link.

"No, you are confusing rake with offset." I do not mention rake and thus am not confusing one with the other. The two relevant independent variables are head tube angle (aka 'rake') and offset.
"Rake is the angle of the fork steerer axis wrt the horizontal." Yes: ie the head tube angle. But it's clear from many articles that many people erroneously use 'rake' when they are referring to 'offset'.
"Offsest (sic) is how far ahead (or behind in the case of a supermarket trolley wheel) the wheel axis is from a straight line drawn down the steerer axis." Yes
"Many references to rake are just plane wrong" Like the homophonic use there!
Agree that link has several errors of definition and nomenclature and that it was a poor choice of reference.
Perhaps, @Yellow Saddle you'd like to address the 'theoretical' aspect that OP has emphasised and suggest a better article?

There is a spectrum between 'twitchy'/fast response/racing and 'slow/stable response' (eg suitable for tandem) and that Tony Oliver graph of (as I said) head tube angle (aka 'rake') against offset is the best representation I have seen. It shows limits of combinations outside which a frame builder/fork choice should only stray with due and deliberate consideration.

Are you measuring offset horizontally or perpendicular to the steering axis? . . . forks are sold with rake measured in mm.

Perpendicularly (see diagram in 'poor' reference).
Many people erroneously use 'rake' when they are referring to 'offset'.

 

[Yellow Saddle]

Are you measuring offset horizontally or perpendicular to the steering axis?

You may strictly speaking be correct about the difference between rake and offset, but remember we live in a world in which forks are sold with rake measured in mm.

Only a fork can have offset.
Only a frame can have rake.

Offset is a distance (in mm if you like) between two parallel lines. The one line goes down the centre of the steerer (called the steering axis) and the other though the centre of the hub axle but parallel to the steering axis. It is easy to visualize offset on a suspension fork because everything is straight. On a suspension fork it will be the distance that the two stanchions are set off ahead of the steerer plus the distance the drop-outs puts the hub axle forward of the stanchions.

A fork cannot be sold with rake because it has no rake. Many, many bicycle companies make this mistake in nomenclature and that makes it difficult to interpret what they say - which is their point, I suspect.

 

[Yellow Saddle]

"No, you are confusing rake with offset." I do not mention rake and thus am not confusing one with the other. The two relevant independent variables are head tube angle (aka 'rake') and offset.
'.

Then I apologise. I assumed you are in agreement with the Calfee link you posted where confusion is rife.
What are your assumptions regarding the (geometric) differences between tandem and road forks.

 

[Ajax Bay]

Say I have a frame, how can I go about working out what would be a suitable fork to go with it? I'm not so much thinking about steerer diameters and such, but more about the geometry, so rake and axle to crown height. Are there any basic principles that can be put into practice? Can I reverse engineer the frameset design, and what are the consequences should I use a fork with different dimensions than those intended by the frame designer?

although it might make no practical difference, is there a way of telling, for a given frame, which fork would theoretically work the best? Or is it all just in the frame designer's head and down to their personal preference?

What are your assumptions regarding the (geometric) differences between tandem and road forks.

Apologies for having taken time to get back on this. A tandem must steer only where the handlebars dictate: you don't want a tandem to 'self-steer' like a road frame. The machine must not alter its line just because the stoker leans or sways when pedaling hard (say). Tony Oliver suggests that steep (73/74o) head angles are required, with longish offsets, typically 60-75mm. A road racing bike with 73o head angle would have a fork with an offset of 45mm to have steering which is neither too quick nor too slow. I hope this reproduction of the graph from Tony Oliver's book 'Touring Bikes' attached comes out.