Fork trail question

[thwart]

I've got a question for the bike geometry experts on the Forum.

I have the opportunity to buy a 50 mm trail Serotta F1 carbon fork to replace the somewhat generic Profile Design carbon fork that came stock on my Colorado III. I'm not sure about the trail measurement of that fork, but it probably is around 45 mm.

What would this do to the handling of my bike?

The more I read about this topic online, the more confused I am... :confused:

Thanks in advance for any help.

[David Kirk]

Here's a good calculator.

Dave

http://www.kreuzotter.de/english/elenk.htm

[fierte_poser]

I think you are using the term trail when you are really talking about rake. Rake is a measurement you can make on the fork alone. Trail is a measurement derived from the rake, the HTA, and the wheel diameter as is shown in the webpage that David posted.

I'm no expert, but I believe that changing to a fork with a larger rake (ie. 50 vs. 45) will 'quicken' the steering. The bike might become 'twitchy'. Someone correct me if I'm wrong!

Kent

[thwart]

I think you are using the term trail when you are really talking about rake
Thank you. You can see how I am a bit confused about all this :crap:

Intuitively, it would seem that increasing the rake would give you an effectively longer wheelbase, and less twitchy steering...

So much for intuition.

[cadence90]

I've got a question for the bike geometry experts on the Forum.

I have the opportunity to buy a 50 mm trail Serotta F1 carbon fork to replace the somewhat generic Profile Design carbon fork that came stock on my Colorado III. I'm not sure about the trail measurement of that fork, but it probably is around 45 mm.

What would this do to the handling of my bike?

The more I read about this topic online, the more confused I am... :confused:

Thanks in advance for any help.
You are mixing two different terms here.

Rake is the offset of the fork dropout relative to the fork steerer axis and is only a factor of the fork geometry.

Trail is instead the distance between the intersection of the axis of the head tube with the ground and the intersection of a line from the center of the front hub perpendicular to the ground, and is a variable affected by head tube angle, fork rake, and wheel size.

In your case, given the same wheel size and head tube angle, an increase in fork rake from 45mm to 50mm will decrease trail; as Kent points out above, the bicycle may handle quicker or twitchier in this case.

I hope that helps.

[Grant McLean]

trail chart:

g

(sorry, it doesn't go up to 66mm of rake boys...)

[michael white]

an interesting side note is that it seems that dealers often stress toe clip overlap over the notion of a "sweet spot." My Merlin's recommended fork rake was 4.0 (spec'd in the chart by Tom Kellogg); it has a 73.5 ht and so that works out to the 5.9 trail figure in your chart. However, I didn't get a 4.0 fork, for the dealer supplied the more common 4.3 without telling me. I caught this while working on the bike--it surprised me when I saw 4.3 on the steerer, and asked them. They said, oh, people don't like toe clip overlap, so we always supply the 4.3. I decided TK knew more than the dealer (a very good dealer, btw) so I demanded a trade, and got it. The bike does feel pretty sweet to me.

[michael white]

But it's pretty easy to think of famous builders whose geo doesn't match this chart. Pegoretti, for instance; as I recall he uses a 73 ht in my size and a 4.5 fork. . . according to this, that's a bit much rake.

[mosca]

I think LeMond is (or was) putting 44mm forks on all their bikes, regardless of HTA. Most people probably would not notice the difference.

[michael white]

yeah I remember noticing that LeMond likes lots of rake, steep ht and shallower st. . . just his style.

[thwart]

Does anyone by chance know the stock head tube angle for a Colorado III? Didn't get that information when I bought the bike, which was not a custom build.

According to the chart kindly supplied by Grant, for a 72 degree head tube (which is current stock geo. for the Fierte---the Colorado of course is no longer made), that 50 mm fork leads to a rake not too far from the magic 5.9... actually closer than the "stock" fork currently on the bike.

[slowgoing]

I remember someone saying that all F1 forks had a 43 rake. Are you sure the F1 fork you're looking at has a 50 rake?

[dauwhe]

... and my new Tournesol will have 60mm of rake.

...that's all I'm sayin', as discussions of trail can easily get out of hand!...

Dave

[thwart]

"I remember someone saying that all F1 forks had a 43 rake. Are you sure the F1 fork you're looking at has a 50 rake?"

The owner thinks it's a 50... something tells me measurement/verification here might be difficult.

FWIW, currently Serotta offers multiple different fork rakes, even up to 52 mm.

[RPS]

Recently I borrowed a 50 MM Reynolds fork from one bike (73 degree HTA) and installed it on a 74 HTA frame for a few test rides, and it wasn’t as bad as the table above would suggest. It reconfirmed that given a choice I personally prefer too little over too much trail. It’s not for everyone, but works for me.

[Ken Robb]

FWIW, my Profile Design AC-1 has 43mm rake according to the sticker on the steerer. If you drop your fork out you may find the info on yours.

[Grant McLean]

But it's pretty easy to think of famous builders whose geo doesn't match this chart. Pegoretti, for instance; as I recall he uses a 73 ht in my size and a 4.5 fork. . . according to this, that's a bit much rake.

hi michael,

it's just a chart. the 'sweet spot' can be pretty big.
there's more than one way to get there.
In my opinion, trail is relative to wheelbase. the longer
the wheelbase, the less trail you need to feel 'right'.
Stability and manouveribility are both necessary in the
geo package, and there are different combos that work.

In my experience:
short wheelbase+higher trail= good
longer wheelbase+lower trail= also good

YMMV

g

[Serotta_Andrew]

spam spam spam eggs and spam...


don't forget about the span!! the F1 has a 36.5 span. i do not know what the profile is....

you have a frame and 2 forks with the same rake (say 4.3) ... but different spans the trail will be different!!!

[michael white]

[QUOTE
In my experience:
short wheelbase+higher trail= good
longer wheelbase+lower trail= also good

YMMV

g[/QUOTE]

as in, say, Colnago in the first case, Rivendell in the second?

[Grant McLean]

as in, say, Colnago in the first case, Rivendell in the second?


good examples!

My Richard Sachs is another example of low trail-longer wheelbase geo too.
It's long and low, and handles like a dream.

g

[michael white]

My Richard Sachs


shoulda got me one of them when his wait was like a year. . .

[Archibald]

spam spam spam eggs and spam...


don't forget about the span!! the F1 has a 36.5 span. i do not know what the profile is....

you have a frame and 2 forks with the same rake (say 4.3) ... but different spans the trail will be different!!!
Different "spans?"

I'll bite.

*** is a span?

:confused:

[Grant McLean]

My Richard Sachs


shoulda got me one of them when his wait was like a year. . .


mine came in a can of spam...

:)

g

[Grant McLean]

Different "spans?"

I'll bite.

*** is a span?

:confused:

length from crown to peaches?

g

[michael white]

yeah what is that, length crown race to dropout?

[michael white]

my spam is suspension-corrected!

[Keith A]

But it's pretty easy to think of famous builders whose geo doesn't match this chart. Pegoretti, for instance; as I recall he uses a 73 ht in my size and a 4.5 fork. . . according to this, that's a bit much rake.IF is another one that uses a 73º HTA and a 4.5cm fork rake for their stock geometry in medium sized frames.

[tv_vt]

I swapped out a Time Equipe fork with 43mm rake on my Merckx ti with a new Reynolds Ouzo Pro 45mm rake fork. (same axle to crown height) It really helped settle down the low speed twitchiness of the bike (it used to really wander/wobble) and didn't affect the high speed stability. It rides nicer because it really tracks well. I think the hta on the Merckx is something like 73 degrees in my size 60cm. Tom Kellogg also recommended that I consider a 45mm raked fork for my frame.

I think I just like bikes that hold their line at slow speeds. It's nice when climbing steep hills, etc.

In other words, I think the chart is a only a guide. I say ride some bikes and see what you like.