Cycling UK Forum

Hello

I currently use this Trek FX2 as my regular do everything bike.

https://www.trekbikes.com/gb/en_GB/bike ... 2/p/17455/

I would prefer a carbon fork on it for my planned LeJoG trip next year.

Will this Trek FX3 carbon fork fit ok?
https://www.tritoncycles.co.uk/frames-f ... ork-p20757

I realise the carbon fork is a disc fork and I would buy a TRP Spyre disc brake for the fork.

I just wasn't sure how the 50mm rake on this carbon fork would affect the handling compared to the 45mm alloy fork that the bike has at the minute.

Thanks for any info you can provide.

I found it hard to source post mount Spyres. Many sites said they had them but I was eventually advised they were out of stock when checking out. Some of the eBay offerings are quoting extra long delivery times and postage rates of £50 etc.

newtotouring wrote: ↑21 Aug 2021, 10:08pm Hello

I currently use this Trek FX2 as my regular do everything bike.

https://www.trekbikes.com/gb/en_GB/bike ... 2/p/17455/

I would prefer a carbon fork on it for my planned LeJoG trip next year.

Will this Trek FX3 carbon fork fit ok?
https://www.tritoncycles.co.uk/frames-f ... ork-p20757

I realise the carbon fork is a disc fork and I would buy a TRP Spyre disc brake for the fork.

I just wasn't sure how the 50mm rake on this carbon fork would affect the handling compared to the 45mm alloy fork that the bike has at the minute.

Thanks for any info you can provide.

My god 13 pages of forks! Good find, but the search function is whack

I have been trying to solve this dilemma viewtopic.php?f=1&t=147347

It'll be a bit more twitchy and harder to ride hands-off. On a heavily laden bike, this is a good thing as it reduces lean steer. Go for it.

If you increase fork offset and leave the head angle the same (ie. same fork length) then trail will be reduced.
Short trail is a feature of racing bikes, so that they turn quickly in a peloton. Watch the Tour stage winners slalom the photographers with their arms held aloft in celebration....short trail means that the bike turns quickly in response to leaning.....whether the rider has their hands on or off the bars.
Touring bikes tend to have longer trail than race bikes, so that the steering is not perturbed by random inputs from side-winds, road camber, or bags moving about. A touring bike ridden no hands wants to go "straight on" and requires exaggerated angles of lean to make it deviate from straight ahead.....the faster you go, the more stable (less inclined to turn in response to lean) it becomes.
As far as I know, only Jan Heine claims that low-trail bikes make it easy to flick the bike round a pothole, but at the same time will take you home with no wobbling when you are shattered.....a clear case of having your cake and eating it.
Thats the straightforward version.

To match up forks, there 2 sorts of things you need to match......mechanical stuff, and steering geometry stuff.
The steering geometry stuff I mentioned above;
45mm is by far the commonest offset for carbon forks. You should be able to get a 45mm offset fork that will suit.
Axle to crown race seat (fork length) also needs to match fairly closely. A long fork will slacken the head angle, 20mm extra length gives about 1 degree slacker on a bike of average wheelbase.
Mechanical stuff.
Brake mounting....its easiest to get a fork with the same brake mounting, unless an adapter is available.
Axle type.....I think the fork you linked is thru' axle, whereas your bike (wheel) is a standard Q/R?
Steerer....I think both your bike and the fork you linked have inch and eighth straight (alloy) steerer; carbon steerers are usually tapered, inch and a half to inch and eighth.

I don't think that's quite right. Touring bikes should have less trail to avoid lean steer. They generally have more offset than road racing bikes, but that's because they have slacker head angles and the two cancel out. Road racing bikes are among the least flickable of all, and the easiest to ride hands-off (TT bikes are sometimes even more stable, because the rider has less steering control on aero bars). Road racing bikes tend to have 73/74 deg head angles and 43-45mm fork offset because the important thing is that they are all similar, and the bunch can turn as one.

The terms "quick" and "slow" steering mean opposite things to different people and are best avoided.

Anyway, a bike with supertanker-like handling, where the steering has a strong straight-ahead tendency and naturally tries to carve a wide radius curve (which I would call slow), is improved by fitting a fork with a bit more offset. I have a Holdsworth where the original fork has been brazed badly with the steerer leaning back in the crown, reducing the true offset to about 25mm. It was horrible to ride, having to be manhandled around bends, until I fitted a different fork with about 45mm offset, and now it is absolutely fine.

rogerzilla wrote: ↑22 Aug 2021, 7:17pm I don't think that's quite right. Touring bikes should have less trail to avoid lean steer.

My understanding of lean steering is that it is counteracted by self centering.

As I understand it trail is the lever that acts together with the friction between tyre and road to self centre the steering. Increasing or decreasing the length of the lever increases or decreases this effect. In a similar way anything that increases or decreases the friction between road and tyre will also increase or decrease it.

All bikes have self centering steering. A well designed bike should have sufficient to fit it for purpose and ought not to need improving.

It surprises me how quickly we adapt to different steering geometries. I once had a long chat with a Sustrans Ranger in Scotland who was riding an old steel drop bar bike that had clearly been ridden into a brick wall at some time in its life. When I asked him if it affected the steering he was adamant that it didn't. On another occasion I met someone who was happily riding a bike with the forks fitted back to front.

Imagine a supermarket trolley castor, the trail is easy to see and as the trolley moves forward the wheel spins round about a vertical steering head so the trail is opposite to the motion.
If you then were to tilt the steering head the trail will be placed ahead of the steerer but still behind the turning axis. In fact you can move the axle ahead of the steering axis at the hub & it will still be behind at the point it meets the ground.
Off setting the axle from the turning axis will reduce or increase the amount of trail.

This offset and steering head angle give a cycle the appearance of being opposite way round to a supermarket trolley castor but in fact its not.

I had a trawl of the internet searching "steering trail for different bike types".
A depressing experience; most of the stuff out there is fanciful, incomplete, or just plain wrong.
About the best I found was;

"As a rule, more trail results in more stable steering and a greater ability to hold a line on rougher terrain.
Less trail makes for a bike that steers more sharply, but too little can result in a twitchy feel."

They go on to compare Trek Madone (road bike) with 58mm trail with Trek Boone cross bike with 67mm trail.
....Madone to have agile handling required in road racing, and Boone keeping the steering straighter on the rough.

Thats "Road cc.....Bike geometry 101; Learn why frame angles and trail matter"

I would prefer a carbon fork on it for my planned LeJoG trip next year.

I am assuming that you want to do this because a carbon fork will offer more shock absorption. I commute on a specialized Sirrus with a steel fork and my wife had the model up with a carbon fork. Hers was a slightly smaller frame so not a direct comparison but there was not much difference in feel between the two. A fork designed for a disc brake will also need to be stiffer than one designed for rim brakes so I would expect the difference to be even less in your case.

Of course if you want to swap forks for some other reason then this may not be relevant.

Steve

Very many thanks for all the replies here. I measured the fork length of the alloy fork on my Trek FX and it appears to be 40mm axle to crown.

Am I right in thinking then that this Surly Cross Check fork would be a good replacement?

https://www.bikemonger.co.uk/surly-cros ... gKvlvD_BwE

Thanks again for any replies.

You mean 400mm for the Trek fork?

I came across a useful piece on fork length, etc....https://www.sheldonbrown.com/rinard/forklengths.htm....includes exactly where to measure length.
So, just check...length and offset...similar.
Regular Q/R axle.....inch and eighth straight steerer. The Surly fork supplied as a spare will have the steerer un-cut, so there is the opportunity to leave the steerer an inch longer than the current one....just buy a few spacers.
Cross check set up for cantilever brakes, as I recall?

If you are looking for more comfort, I suspect a bigger tyre at a lower pressure would have more effect than changing the fork.

531colin wrote: ↑25 Aug 2021, 3:38pm You mean 400mm for the Trek fork?

I came across a useful piece on fork length, etc....https://www.sheldonbrown.com/rinard/forklengths.htm....includes exactly where to measure length.
So, just check...length and offset...similar.
Regular Q/R axle.....inch and eighth straight steerer. The Surly fork supplied as a spare will have the steerer un-cut, so there is the opportunity to leave the steerer an inch longer than the current one....just buy a few spacers.
Cross check set up for cantilever brakes, as I recall?

If you are looking for more comfort, I suspect a bigger tyre at a lower pressure would have more effect than changing the fork.

Thanks Colin. I appreciate you taking the time to get back. I agree about the bigger tyre idea. I'd rather have 40 Marathon Supremes on the bike but it will only take a 40mm tyre on the front without a mudguard. I might try to squeeze a set of 38mm Marathon Greenguard or 37mm Continental Tour Ride tyres on to see if that improves thing from the 35mm tyres I'm currently using.

Many thanks for your help everyone

Can you make up a front guard which doesn't actually go through the fork?
ie finish the guard at the back of the fork crown; if you want a separate bit in front it will need its own stays.
You used to be able to buy a sort of gaiter to keep the muck out of the headset on bikes with no guards, but I suspect a bit of old inner tube could be made to work.