Handling with 40mm vs 45mm rake

[Jimbo]

Hello All:
I recently bought a Merlin Agilis largely because its geometry is spec'd by Tom Kellogg who I respect from my visits to T Town.
That bike is supposed to come with a fork with 40mm rake but mine arrived with a 45mm.
Can any of you bike geometry experts tell me how much differently it will handle with the additional 5mm?
Thanks,
Jim
Here is the geometry chart. Mine is a medium. web.archive.org/web/20010207144252/http://merlinbike.com/html/road/agilis.html

[tsarpepe]

I can tell you about an even smaller difference. The 40mm fork on my IF Club Racer suffered in an accident, and I replaced it with a 43mm one. I sensed the difference immediately after getting on the bike. Much more twitchy handling, the ride felt less stable and predictable. This was 3 years ago, and I've gotten used to it; now it's my new normal. I know that there are folks who say that the difference is barely noticeable,. In my case, the two forks were from different manufacturers, slightly different crown-to-axle distance, etc.; this certainly had an amplifying effect.

[Heisenberg]

It'll reduce trail. Your bike will steer from the handlebars faster than stock, if cockpit/tire setup remains the same.

Longer rake fork=less trail.

The chainstays on that bike are long enough to balance zippier handling nicely. In my mind, that=more fun! Plus, you'll have less toe overlap.

[smead]

I picked up a Salsa La Raza a while back that came with a 40mm fork instead of the 45mm fork that came stock with the frame. I did notice the handling was a bit slower (and on rails at speed) but not significant enough to worry about. I later put a 44mm fork on it and again noticed a difference, and preferred the sharper steering, still plenty stable at speed. Bottom line is that I doubt the difference is enough to really worry about, especially if you prefer quicker steering, which is what you'll get with the 45mm fork on your 40mm designed frame.

You can put in your numbers here

http://yojimg.net/bike/web_tools/trailcalc.php

and see where you land - most road bikes are in the upper 40s to low 60s, if you land near the lower end, you might find it a bit twitchy.

[Mark McM]

A description of how the change in fork offset (and its resultant affect on trail) will depend on what kind of stability and handling you're used to, and what kind you want. Some might say the decrease in trail with this fork would result in the handling being more "twitchy" and less "stable". But I'd refer to it as giving a "lighter feel from the handlebars" and "more responsive".

One of my favorite handling bikes, a 54cm Ritchey Road Logic, has the same head angle as the Agilis (73.5 deg.), and came with the same fork offset (45 mm). The rest of the frame dimensions are also quite similar. I find I like the handling of this bike, because it only requires a light touch on the handlebars for control. I dislike bikes with large trail dimensions, because they require heavier handlebar inputs to change direction, and when very rapid maneuvering is desired, bikes with larger trail feel like they are fighting against your steering inputs.

If it were me, I'd probably be happier with the 45mm fork on this frame.

[weisan]

I like Mark pal's explanation. Incidentally, it described my Merlin Agilis exactly.

[unterhausen]

I built a fork with 55mm rake and the steering felt much lighter than with 45mm. I thought it made the bike feel sprightlier. I'm sure there would be noticeable difference with 5mm, but I'm not sure how much.

Note that getting a shorter A-C on a fork would have a similar effect even with the same rake. This is due to steeper head angle.

[Jimbo]

Thanks everyone!
Using the calculator for a 40mm fork and assuming it was originally intended to use 23mm tires, I get a trail of 58mm.
With the 45mm fork and the 28mm tires I will be riding I get 55mm of trail.
So the original spec would have had it very slightly more stable than the 57mm trail that seems to be considered a sweet spot. With the current set up it will be slightly quicker steering than that.
I'm much calmer now. Was really seeing red when I realized the bike had come with an inappropriate fork. Seller said he never checked and thought it handled just find. Based on these numbers he was probably right.
Cheers,
Jim

[tv_vt]

55mm of trail would be way too twitchy for me. It's something you can customize on your bike if you find it not to your liking. Just swap the fork out for one with a lower rake and that will make it a bit less twitchy, assuming the axle to crown distance is the same.

[charliedid]

Try it, you might like it.

The fork rake is not the bike...

[nicrump]

Med? 73.5 HTA? i'd put a 43 enve 2.0 and enjoy it.

edit, i missed that you had a 45. enjoy it. it'll be fine and not at all extreme.

Hello All:
I recently bought a Merlin Agilis largely because its geometry is spec'd by Tom Kellogg who I respect from my visits to T Town.
That bike is supposed to come with a fork with 40mm rake but mine arrived with a 45mm.
Can any of you bike geometry experts tell me how much differently it will handle with the additional 5mm?
Thanks,
Jim
Here is the geometry chart. Mine is a medium. web.archive.org/web/20010207144252/http://merlinbike.com/html/road/agilis.html

[Bonesbrigade]

On my English I had built last year, I intentially went with less trail - 72.5 hta and 50mm rake. It’s an all-road bike that uses 700x38 or 650x48 tires. The 48mm slicks stick to the ground a bit more, so a lighter feel on the steering really aids the handling. In contrast, low trail and 23mm tires wouldn’t be my favourite!

[Jimbo]

To me, my 2001 Trek 5500 handles perfectly under all conditions. Just found an archived brochure showing that it has a 73.8 head tube angle and a 43mm rake. With 28mm tires the calculator puts the trail at 55 just like the Merlin will be. All good.
Thanks everyone and particular thanks to Smead for the calculator link.
Cheers,
Jim

[OtayBW]

55mm of trail would be way too twitchy for me. It's something you can customize on your bike if you find it not to your liking. Just swap the fork out for one with a lower rake and that will make it a bit less twitchy, assuming the axle to crown distance is the same.

Go slower! :D

[572cv]

About a year and a half before Serotta closed, I picked up a second hand Meivici frame w/o a fork, on the ‘bay. Not thinking much, I came up with an F3 fork, also second hand, as part of the build. I didn’t pay any attention to the rake, except that it seemed like a commonly mentioned one. I rode it for a season, but especially on descents, it was a little too twitchy, a little off. By that time I had figured out that forks have different rakes, and it makes a difference. I went to a Serotta dealer, and they looked up the frame geometry and the fork rake it was designed with. I bought the ‘right’ fork, and it was delivered two weeks before Serotta closed up shop. What a difference in handling that made! Smooth, secure, responsive. I still have the bike. Getting the relationship in geometry to work well has a major effect, and is worth sorting out.



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