Groasters

I am getting a new all carbon fork for my custom Ti rim brake road bike, to gain larger tyre capacity.

Current fork (Easton EC90 SL) has 43mm rake. From memory head angle is 72.5 degrees so it's not naturally a twitchy geometry. It feels responsive and stable. I live in a hilly area so downhill stability is a priority.

New fork will be a Columbus Futura available in 45mm and 50mm rake options.

The natural choice is obviously the 45mm.

I just can't help wondering what would be the effect of getting the 50mm version instead?

Super stable descending? Or just dulled responses and a more boring bike?

I realise this is a first-world dilemma but I'm a bike geek and just curious!

Troul 

In theory, this could be tested out with a change in tire sizes front to rear. IE: smallest acceptable profile series tire in the rear & the largest acceptable profile series tire in the front.
I would expect less steering control with increased speeds, increased harshness, more drag as speed increases & slightly less neck strain if the rake is changed as suggested.

dsaul

With a a 72.5 degree HTA and a 43mm rake with 28mm tires you will have 60mm of mechanical trail and 18mm wheel flop. With at 50mm rake on that same bike, you will have 53mm of mechanical trail and 16mm wheel flop.

People who are unfamiliar with bicycle design often mistakenly believe that an increase in fork rake leads to an increase in trail, while the opposite is actually true. I would expect a slightly more responsive steering and less wheel flop at very low speed. It won't be a night and day difference and you will probably get used to it very quickly.

Groasters

The front tyre is basically max 23c and minimum 23c so this couldn't work sadly.

Also I don't think a difference in ride height front to rear of 2 or 3mm would replicate a rake difference of 5mm.

Thanks for the theory though, I get what your thought process is.

Troul 

assuming you have a 700c setup, you might be able to try tossing on a somewhat fitted 26" rear dia wheel. Extreme change to show the differences. Sadly, until the details are all known, it is impossible to describe the changes that you might be expecting.

unterhausen

What dsaul said is correct, all things being equal. 5mm increase in rake is going to make the steering a little faster and it might take a ride or two to get used to that. Lots of people like it. I think it makes the bike feel more lively.

But the OP didn't tell us what changes in a-c would be between the two forks. Assuming the new fork is longer, then the head tube angle would decrease and so the two forks may feel exactly the same.

Groasters

That last point is interesting. I think the axle to crown mm are the same but I'll check. . . I follow your logic though.

On your first point, would a longer rake not make the steering slower, less twitchy and more stable like in a gravel bike for example with a less steep head angle?

dsaul

The longer rake moves the axle forward and that moves the tire contact point closer to the steering axis, which results in less trail and faster steering response.

A longer rake fork does the opposite of a slacker head tube angle. The slacker head tube angle moves the steering axis forward, increasing the trail(distance between the steering axis and the point where the tire contacts the ground).

You can see a visual representation of this and put in your own numbers here Bicycle Trail Calculator | yojimg.net

pdlamb

Possible typo here? You started with the premise that the tire size would change:

Most of the time when someone changes a fork to change maximum tyre (tire) size, they're looking at something more drastic. I was expecting a change from 23 to 32-35, for instance. I'd have to run the numbers since I can't remember the formula; you might keep the same trail or drastically change the trail with a fork rake change and a large tire size change.

Groasters

Dsaul and pdlamb, and others, I really appreciate your input.

Although in my head the actual effect of larger or smaller trail is counter intuitive, I realise I had it back to front, in my previous understanding.

On the subject of tyre size, my typing wasn't incorrect, as the max tyre size I can fit into the rear triangle is 25c. I currently run 25c rear 23c front.

The 23c front only just fits. A 25c will not.

Crazy as it sounds, on our rough roads, the improvement in comfort (and probably rolling resistance) in going from 23c to 25c is noticeable so I'm almost convincing myself that this 2mm difference is worth blowing £200 on a new fork!

As far as rake goes, if I do get a new fork, I shall stick to roughly the same 43mm rake rather than an increase to 50mm.

The caveat is the point about relative crown to axle differences between the two forks.

Crikey, it's complicated but thanks for tutoring me!

Great thread here too. . .https://www.roadbikereview.com/threa....266944/page-2

Groasters

. . .in hindsight, when I commissioned the frame in 2014 I should have specced tyre capacity. I can get a new fork but I can't raise the rear brake bridge so I'm kinda stuck with 25c max! 😫

63rickert

Slow down. Don’t do anything until you have an accurate measurement of head tube angle. Since it is a custom your builder should have a record.

Your builder could also very likely move the brake bridge. Ask him. Ti is not so simple as steel, it should be possible.

Neutral trail is 57mm. Pretty much all high end race bikes are 57mm or very close to it. Neutral means the bike does nothing until you tell it to and then it responds quickly. Responsive. Racy. Fast.

High trail makes the bike more stable. The bike wants to go straight. Just a couple millimeters is enough to slow down handling a lot. It will take a bit of muscle to make the bike move. More than a couple mm the bike feels sluggish and dead. But there is more. How much do you weigh? Race bikes are designed for whippets. The bigger you are, the more momentum you have, the quicker the bike responds. A 150# rider wants neutral. A 200# rider probably won’t care at all about 3mm of high trail and might prefer it. The big rider may see 60mm as neutral.

Lower trail is harder to describe. Twitchy is one of the least specific adjectives in the English language. Not that many people have ever been on a low trail bike. If you ride production bikes the easiest way to get a low trail bike is to ride antiques. Old is not necessarily low trail, it was however allowed to happen without too much thought or lawyering. Custom builders are reluctant to go low unless the customer knows just a lot about what he wants to ride.

As you move to high trail each millimeter makes a big difference. Go down from neutral and you have to move a lot numerically to make a difference at all. A 60mm high trail bike is very noticeably different than neutral. A 54mm low trail bike most would never know. At 63mm anyone would know that was a slow handling bike. At 51mm most would just be beginning to tell the difference.

The only way to know for sure how you would respond to a few mm difference in rake and trail is to ride lots of bikes lots of miles. While knowing what you are riding. And sometimes replacing forks.

If this were steel forks I would say the longer rake is more comfortable. Gauge, taper, bend shape would all make a difference too. Carbon? Depends on how the maker did the layup. My guess is they default to making them all equally stiff. Easton might have something more to say about that. You could ask them.

Myself I would go for the 50mm. If for whatever reason that is not the right answer for you, don’t worry too much. The bike will not change all that much. It will be a rideable bike.

Trakhak

Before you spend the money on a carbon fork, try using two layers of cork (or equivalently cushioned) tape on your handlebar. The first time I did that, I was amazed at the difference in hand comfort.

Groasters

Cheers guys!

63rickert, interesting stuff! I'm 154 so may notice differences.

The extra bar tape might be a sensible option

The builder is in Russia and designer 400 miles from me, so frame mods are not practical!

TiHabanero

Play with a bicycle trail calculator online. Plugging in various head angles, wheel and tire sizes along with fork rake will show how each element affects trail. Remember longer trail equal stability in a straight line. Enter in geometry from various bike models published on a manufacturer website will be a good quick lesson.