pstake 

I'm considering replacing the stock carbon fork (alu steerer) on my road bike with a steel touring fork. The geometry of my bike is pretty traditional "road" with short chainstays, and it's compact and stiff, although it's steel. I'm wondering what rake and design differences exist between a bona fide "road" fork and a purpose-built "touring" fork, besides the obvious things like eyelets and rack mounts. I don't need mounts for front racks.

I don't race road bikes very often but I like having a light and fast bike for long distance rides, that's still comfortable.

Cyclist0108

You might also want to take a look at other carbon forks, like the Spork:

https://www.rodeo-labs.com/rodeo-spork-2-0/

I almost bought one, and then decided to buy a steel touring frame and fork (Soma Saga disc), as it was less expensive than just this fork.

A steel fork can be much more flexy, and the trail may be different depending upon the model.


https://www.somafab.com/archives/pro...c-touring-fork


This will be stiffer:


https://www.somafab.com/archives/pro...-for-wolverine

Wilfred Laurier

A touring fork will almost certainly be longer, probably about 30 mm or so, which, according to my calculations, will slacken the head angle of a medium sized road bike (~1000mm wheelbase) about 1.7 degrees. This will make the bike less quick to jump into corners, and will likely add some straight-line stability. This change might be perfectly acceptable and you will get used to it in about two minutes of riding, but there is a small chance it might work well and you might find riding more tiring trying to get the bike to do what you want.

If the new fork has greater rake (distance the axle is offset from the centre line of the steerer tube) than your stock road fork, this will counteract the decrease in head angle by a bit.

Also, there is no way to predict if a replacement part will be more or less stiff solely by changing the material. Steel forks can be made *ahem* rooster stiff, and aluminum forks can be made overcooked spaghetti flexy. The material is less important than how it is used.
Most people use carbon forks in the belief that they will absorb more road vibration than steel or aluminum, but the same thing applies - you could make a carbon fork with the ride quality of an oak log or like a willow branch, but it's the construction that makes this difference, not the material.

duffer1960

What little I know about it suggests to me.. The problem with making an aluminum fork flexy is that aluminum will fatigue if it moves at all. Steel has some spring and can move a certain amount without fatiguing. Are there any aluminum springs out there? All or almost all springs are made of steel. Aluminum forks have to be made stiff else they'll fatigue and fail.

aclinjury

^This.

OP,
touring fork, in order to accommodate bigger tires, will almost certainly have a higher axle-to-crown (AC) height, which has the effect of slackening the head angle, which makes handling slow in corners but a little more stable at higher STRAIGHT LINE speed (is does not more stable at cornering speed)..

If you wish to maintain the same handling feel, you'll also want the new fork to have a greater offset (rake) than the current fork, as greater offset will decrease trail (decrease trail makes bike turns faster into corners and hold lines better thru corners, but a little more twitchy in straight line speed).

If new fork has the same AC, and you wish to maintain same handling character, then new fork should have same offset.

In general,
"racing road bikes" should have shorter trail for quick cornering and stable cornering
"endurance/gravel bikes" should have longer trail for straight line speed stability and more stable going over rocks and potholes.

Wilfred Laurier

This is an oversimplification of 'fatigue failure' and of 'fatigue limits'.

First, the fact that 'steel has a fatigue limit but aluminum does not' is oft repeated to imply that every aluminum part will break and every steel part will last forever. This is not accurate - 'Fatigue limit' refers to the amount of stress the material can theoretically experience an infinite number of times without failure, but because a material has a fatigue limit does not necessarily mean the stresses it experiences are below this limit. The design and construction of the part, as well as the type of use it sees, have to be taken into consideration. A 120lb person on a Surly LHT will almost certainly never experience fatigue failure of the frame, but a 240lb person on a superlight steel frame from the 70s is more likely to experience fatigue failure.

As for aluminum, at low stresses, yes, it will still theoretically be a candidate for fatigue failure, but at low stresses you'll be talking about billions (x10^9) of stress cycles, which in the case of a bicycle under extreme use where one pedal revolution is a stress cycle, for instance, could potentially allow for fatigue failure somewhere in the range of hundreds of thousands of km of such conditions, and bikes I have had died from misadventure long before that point.

Lastly, the idea that noticeable deflection means the part (fork) is seeing stresses at a level that will cause fatigue failure in a meaningful time is not necessarily correct. The design of a part can result in very low stress that, again, brings us to billions of stress cycles before fatigue failure is likely.

I've got about 15000 kms on my aluminum fork and I definitely see deflection over bumps and under braking. I am considering replacing it, but only because I want disc brakes on the front of my touring bike, not because I have any concern that the fork is going to fail.