Carbon fibre disk brake touring fork

[belgiangoth]

So ... Maybe best would be a carbon fibre fork, 45mm rake and provision for v brakes. Ditch the whole disk brakes for the moment. As long a it will take guards and 35mm 622s it should work.

[Freddie]

Look, I don't want to be rude, but you seem to have made your mind up before you asked the question. Brucey and I have given good reasons for pursuing something other than your original idea or a hodge podge of ideas (see Brucey's second paragraph in his last post), but you seem determined.

You are free to do what you want, but why pose the question if you aren't open to palpably better solutions. I don't get it.

[Brucey]

So ... Maybe best would be a carbon fibre fork, 45mm rake and provision for v brakes. Ditch the whole disk brakes for the moment. As long a it will take guards and 35mm 622s it should work.

that could work out OK but it still leaves you with a potential corrosion issue on the CF fork's aluminium parts. I guess the devil is in the detail; a CF fork meant for loaded touring might be just as stiff as the one you have at present.

cheers

[belgiangoth]

Brucey,
Why is alu corrosion an issue with forks but not frames or crank sets (etc)?
Also, to be clear, I want a fork that will take wide tyres and mudguards and v brakes - which is why I called them touring forks. I won't be using them for load carrying (I doubt the combined rider and luggage mass will exceed 100kg at any point).
...
Also, I'm sure you suggested in a previous point that a different steel frame would not be significantly lighter than the one I have now (but then it was a while ago and I haven't slept since so I may be confused ...)

[PH]

My Tange steel touring forks are 790g, if anything I consider them to be over engineered.
What forks have you got that are half as much again?

[Brucey]

Brucey,
Why is alu corrosion an issue with forks but not frames or crank sets (etc)?

that is a good question. CF frames an cranks etc are by no means immune from this either. I think it is worth restating that this is an issue for a commuting/winter bike more than some others because it will likely see lots of road salt, wet weather and perhaps little cleaning. Essentially aluminium parts that are bonded into carbon structures are liable to become anodic with respect to the carbon; they are also rather small parts which means that they will tend to corrode away/disbond from the CF more quickly than they might otherwise. Similar effects can occur if you have a small chunk of aluminium bolted to a larger chunk of stainless steel; thus hub flanges can corrode near stainless steel spokes, and so forth. The possible presence of salty water and crevices makes for a 'perfect storm', corrosion-wise. Part of the trouble is that people don't think that corrosion will afflict a CF part, so they don't look out for it; it isn't as if it goes brown and horrible looking, like steel, and gives you plenty of warning...

Also, to be clear, I want a fork that will take wide tyres and mudguards and v brakes - which is why I called them touring forks. I won't be using them for load carrying (I doubt the combined rider and luggage mass will exceed 100kg at any point).

understood; however getting the features you want without it being built for a heavier load might be tricky.

Also, I'm sure you suggested in a previous point that a different steel frame would not be significantly lighter than the one I have now (but then it was a while ago and I haven't slept since so I may be confused ...)

no sleep... .?... that'll do it. I don't remember the context of this but if you have (say) a pompino frameset the fork is kind of heavy but you would struggle to shave more than ~3/4lb off the frame itself -and still keep it steel- but it is still quite a stiff frame; IIRC the top tube and the down tube are slightly larger diameter than traditional steel frames and this does increase the stiffness a fair bit; my 'back of envelope calculations suggest that in a 'traditional' steel frame about 1/3rd of the flex in the fork/front end may in fact come from flex in the DT and TT; modern frame designs often take that away, at the expense of ride quality.

cheers

[jk49]

Another thing I've learnt from my 'experiment' is that the carbon fork is more flexible than the steel unicrown v brake fork they replaced. I didn't expect that. I know sometimes we start to believe our own propaganda, but I really think this has improved the ride of the bike I use everyday. As I said earlier, I fully accept, its not for everybody. I would never sell this bike, its trigger's broom, I bought it new in 1997, its been stolen once and then returned by the police 6 months later, so I have alot of affection for it! BTW I still have the original fork so I can easily reverse all this.

[Brucey]

Another thing I've learnt from my 'experiment' is that the carbon fork is more flexible than the steel unicrown v brake fork they replaced. I didn't expect that....

I don't disbelieve that; but then your old fork was probably rather stiff.

In an ideal world there would be a database of fork stiffnesses but as it is, it is a bit of a pot luck job.

Weight and dimensions are a fair guide to steel forks but with CF you could be buying 'mostly matrix' or 'mostly fibre' and there is a world of difference between the two.

cheers

[Freddie]

In an ideal world there would be a database of fork stiffnesses but as it is, it is a bit of a pot luck job.

How would stiffness be quantified, other than in subjective terms. I understand what makes a fork stiffer, but how would you measure stiffness (displacement at the dropouts for a given input?).

[belgiangoth]

Stiffness is generally measured in force per distance (how hard you have to pull to stretch it a given distance). This would be a good measurement for forks, as it would take into account the shape of huge forks.
Alternatively you could use the Young's modulus, but that is dependent on the material and independent of the shape.

[belgiangoth]

Brucey, you're suggesting that you would only reduce frame mass by 0.75 pounds, which is just over 300 grams, but that you could cut fork mass by almost twice that?
...
Or did you mean 3-4 pounds? (I.e. Just shy of 2 kg)?
One seems shockingly low and the other extremely high. I don't see how you can reduce fork mass significantly more than frame mass if the frame has triangulation to stiffen it and the forks have a gentle curve to help them fall apart.

[belgiangoth]

My Tange steel touring forks are 790g, if anything I consider them to be over engineered.
What forks have you got that are half as much again?

Sorry, 1025g, not 1250g. My bad.
Still, 790g in steel, you would think you could cut some off that (say down to 700g) if you didn't want it for loaded touring and still not have some sort of death trap.

[PH]

My Tange steel touring forks are 790g, if anything I consider them to be over engineered.
What forks have you got that are half as much again?

Sorry, 1025g, not 1250g. My bad.
Still, 790g in steel, you would think you could cut some off that (say down to 700g) if you didn't want it for loaded touring and still not have some sort of death trap.

Yes, I've never weighed the forks on my Hewitt tourer, but I wouldn't be surprised if they were lighter. I've seen reasonable prices quoted for custom forks, a decent builder should know how light they can go for the intended use and possibly come up with something lighter still.
The Tusk Trekking fork someone suggested earlier comes in at 740g. I was considering it before I bought the Tange, but was told that the crown down to below the brake bosses is carbon wrapped aluminium.
I too sometimes feel the itch to try disk brakes, went as far as buying a disk wheel a few years ago, but then realised I've never felt the brakes i do have were not as capable as needed. But the itch is still there...

[belgiangoth]

Honestly, the disk brake appeal is more about easier wheel swap, not needing to re-align my brakes.

[Brucey]

Brucey, you're suggesting that you would only reduce frame mass by 0.75 pounds, which is just over 300 grams, but that you could cut fork mass by almost twice that?....

I don't see how you can reduce fork mass significantly more than frame mass if the frame has triangulation to stiffen it and the forks have a gentle curve to help them fall apart.

Realistically, going on experience in relation to your service, a steel frame under about 4lbs is going to be difficult to find and potentially vulnerable to knocks because of its thin tube walls etc. ~4.5lbs would be about right, I'd suggest. By contrast a steel fork weighing ~750g (plus a bit for bosses and extra length perhaps) is (or perhaps I should say was... ) a very standard item, well proven for this service and worse.

Honestly, the disk brake appeal is more about easier wheel swap, not needing to re-align my brakes.

if your brakes need realigning every time you refit the same wheel then I'd suggest there is something wrong with your V brakes. FWIW I have more such alignment problems with disc brakes than rim brakes, I suspect because the hub need only move a few thousandths to cause a disc to rub.

BTW a standard fork stiffness test would (say) comprise a loaded cantilever bend wherein the (fore-aft) deflection would be measured in a simple jig that holds the fork roughly horizontal. This up/down (fore-aft) movement would be a useful proxy for 'overall fork flexibility' simply because it can be measured reasonably easily. I'd suggest a ~150mm separation of the two load points on the steerer and the deflection to be measured at the fork tip under a standard load of (say) 75lbs or something like that.

cheers