Fork rake-handling question...

[SoCalSteve]

Hi all,

Just curious...My Ottrott has a 74* head tube angle and a 43 rake fork.

What happens if I get a fork with a rake of 47? Does it make the steering faster or slower?

I was always really bad with geometry/math in school. A real simple explanation would be the best for my feeble brain.

As always, thank you all in advance,

Steve

[Smiley]

its getting faster , check your build sheet , I think the head tube angle on the more recent Ottrott's as well all Serotta's is 73 and the earlier ones were 73.5 degree's NOT 74 .

You look at the trail based on the head tube angle and the fork rake . So where was the frames original trail number IF the head angle was 74 and the rake was 43 ?? It would appear to me to be in the 5.5 range .

[e-RICHIE]

Hi all,

Just curious...My Ottrott has a 74* head tube angle and a 43 rake fork.

What happens if I get a fork with a rake of 47? Does it make the steering faster or slower?


Steve

faster

[SoCalSteve]

The bike has a 74* head tube angle (custom-big frame 60 x 62 with a 74* seat tube as well) and a 43 rake F2 fork. So, that would be a 5.5? I havent a clue what 5.5 means. Please explain.

And then...Please explain what a 74* heat tube angle and a 47 rake fork is and what it would do.

Thanks so much for putting up with my feeble mindedness,

Steve

[Dave]

The bike has a 74* head tube angle (custom-big frame 60 x 62 with a 74* seat tube as well) and a 43 rake F2 fork. So, that would be a 5.5? I havent a clue what 5.5 means. Please explain.

And then...Please explain what a 74* heat tube angle and a 47 rake fork is and what it would do.

Thanks so much for putting up with my feeble mindedness,

Steve

Check out the drawing below. The "r" is the fork offset (rake). The other two links have more explanation of rake and trail. At the last website, trail is the N dimension.

http://www.trialtir-usa.com/2006-colnago/colnagoinfo/colsizecharts.htm

http://www.phred.org/~josh/bike/trail.html
http://www.kreuzotter.de/english/elenk.htm

Rake is the perpendicular distance between two parallel lines, one through the center of the hub, and one through the center of the steering tube. Trail is the horizontal distance between the tire contact point and a line through the steering axis. The more trail, the more stable the bike (slower steering). The less trail, the quicker the steering. Both fork rake and head tube angle affect the amount of trail. Steepening the head tube angle or increasing rake will decrease trail, reducing stability and quickening the steering. The formula for trail is as follows, where R is the tire radius, and H is the head tube angle. Trail = (R/ tan H) – (rake/sin H). As an example if R = 33.65cm, H=74, and rake is 4.3cm, trail = 33.65/tan74 – 4.3/sin74. This calculates to 5.2cm.

If you increase the fork offset to 47mm (4.7cm) then the trail will decrease to only 4.75cm. This wouldn't be a good idea, IMO. Even on a big frame, that's not enough trail.

[Smiley]

You need to get a shorter rake fork , I think Reynolds makes a 41 and Serotta has a 41 too , that should increase trail to a managable amount . The previous owner wanted a fast racing front end . All new serotta's are designed around a trail of 5.9 and I can attest that this is just right for road riding . Dave sent you all the geek stuff , suffice to say thats why just buying a used frame is not enough to know ST and TT and STA , there's more to it then that . Smiley

[SoCalSteve]

You lost me at: It can't be too simple!

But thanks to the Good ol' Captain.

Actually, the frame feels just right now. Not too fast for me. Or maybe I am just used to fast steering front ends...

When I saw the F3 with the uncut steerer tube and 8.5, I got to thinking about upgrading to an F3. Thats why I asked the questions.

Thank you all for answering my question. If an F3 comes up that is a 8.5 and a 43 rake or less with an uncut steerer, I will know to jump all over it.

Steve

[Smiley]

SoCal , what's the trail on all your bikes if I may ask. If you get a chance go ride a serotta with a 5.9 trail that fits you and then tell me what you think :)

[SoCalSteve]

It feels a bit sluggish to me in comparison with my custom Ottrott. Very stable though. I like them both!

Steve

[cpg]

With low trail numbers (30-45mm) stability is increased at low speeds. Virtually no wheel flop. If you study bike history lots of bikes were built around those numbers. With these low trail numbers as speed increases stability decreases. The reverse is true of high trail numbers (65+mm). At low speeds high trail creates a great deal of wheel flop and subsequently reduces low speed stability. As speed increases stability increases with high trail numbers. That sounds good but with high trail numbers stability can increase to the point of a lose of manuverability. The point is there's a range of trail numbers that impart a good balance of slow & fast speed stability and manuverability. It's not a dire as Dave makes it sound. Plenty of bikes are ridden with those trail numbers. But with racing bikes trail numbers within the range of 50-62mm seem to work best for most riders. Many framebuilders (myself included) have strong opions about where within that range they design their bikes to be. You're welcome to experiment for yourself about what you like. Personally if I was designing a bike with a 74 degree head tube I'd build the fork with 40mm of offset. But that's just me.

Curt

[SoCalSteve]

If I changed the fork to a 4.0 offset, what will I notice differently as opposed to the 4.3 I have now?

Thanks,

Steve

[Smiley]

SoCal , your problem will be to find an acceptable carbone fork with a 4.0 rake , the closest thing will be Ouzo Pro at 41 or the new F3 at 41 , try that one , I think like Curt , you'll be happier .

[Needs Help]

(1) Trail : At speed, if the trail gets smaller, the handling gets quicker. If the trail gets bigger, the handling gets slower.

(2) Rake: If the rake gets bigger, the trail gets smaller. If the rake gets smaller, the trail gets bigger.

If I changed the fork to a 4.0 offset, what will I notice differently as opposed to the 4.3 I have now?
Now, you should be able to figure it out for yourself:

Look at (2), and determine what is happening to the trail.

Look at (1) and determine what is happening to the steering.

[sg8357]

[snip] Many framebuilders (myself included) have strong opions about where within that range they design their bikes to be. You're welcome to experiment for yourself about what you like. Personally if I was designing a bike with a 74 degree head tube I'd build the fork with 40mm of offset. But that's just me.

Curt

Hi, Curt.

What do think about pneumatic trail ?
If a customer promises to always fit fat tires, do you reduce the
the geometric trail ?

At The Cirque Jan H. was sprinting no hands on a Herse Camping, loaded
bar bag and all, neat demo of a low geometric trail bike.
Bike had what looked like 38mm tires, Jan had no trouble keeping up with
the racing bikes, maybe it was the huge fenders ? ;-)

Scott G.
Chris Kvale 2005

[cpg]

I think Jan would keep up even if he was riding a Huffy and I'm not talking about a Serotta rebadged as a Huffy. I don't want to cross the line in talking about my own business but to suffice tire width is part of the decision process.

Curt

[davep]

With low trail numbers (30-45mm) stability is increased at low speeds. Virtually no wheel flop. If you study bike history lots of bikes were built around those numbers. With these low trail numbers as speed increases stability decreases.Curt

Curt,

What do you mean by "low speed"? At what speed does the stability decrease mean a hard to steer or even dangerous handling bike? Or is that the wrong way to think of it?

[cpg]

Low speed would be sub 15mph. There is no magic number. Also one man's ceiling is another's floor. So at one point one rider will find the handling scary. Another will find it responsive. In general terms, descending at 40+mph with a trail numbers in the 30-40mm range with skinny tires (20-25mm) most riders would find the front end handling to be too responsive in other words scary. In more general terms descending at the same speed with the same tires but with trail numbers in the 65mm+ range most riders would find the front end handling lacking responsiveness to the point of scariness. Especially the farther the trail numbers are from 65mm. Again these aren't numbers set in stone but in general terms this is true. Also this isn't new news. Framebuilders today don't have to sort this stuff out. Most of it was pretty well fleshed out at least 50 years ago.

Curt