Fork question

[PoppaWheelie]

Generally speaking, how much difference would a change of 1 or 2mm in my fork rake make...? I have a slightly tricky fork replacement...and can either have one made (better option) or go with a Ouzo Pro that can be had for a song (cheap option). I don't want to find out the hard way that every mm matters when dealing with rake/trail...

Thanks -

[Serotta PETE]

What frame are you putting it on and what came off the bike. I am sure one of our frame builder friends can offer some "food for thought".

It does not have to be a SEROTTA........

[Steve Hampsten]

Generally speaking, how much difference would a change of 1 or 2mm in my fork rake make...? I have a slightly tricky fork replacement...and can either have one made (better option) or go with a Ouzo Pro that can be had for a song (cheap option). I don't want to find out the hard way that every mm matters when dealing with rake/trail...

Thanks -

Assuming that the crown race height is the same, 1-2mm in rake will be barely noticable and will be quickly adpted to.

[J.Greene]

I'm guessing about the same difference between what a 22 and 25c tire would make. But that is just a guess.

JG

Generally speaking, how much difference would a change of 1 or 2mm in my fork rake make...? I have a slightly tricky fork replacement...and can either have one made (better option) or go with a Ouzo Pro that can be had for a song (cheap option). I don't want to find out the hard way that every mm matters when dealing with rake/trail...

Thanks -

[PoppaWheelie]

I've always thought it would be great to have access to a number of otherwise identical bikes that had slight geometry tweaks in order to get a good sense of how the changes really impacted the different ride qualities. Maybe my wife will let me do this after the second or third time we win the lottery...

[MadRocketSci]

Assuming that the crown race height is the same, 1-2mm in rake will be barely noticable and will be quickly adpted to.

just curious then if there's another reason why a fork manufacturer like reynolds makes 43mm and 45mm rake ouzo pro's, and why some builders like ~56mm trail (73 HTA / 45 mm rake) and others like ~59 mm (73 HTA / 43 mm rake)?

[cpg]

Different strokes for different folks. Really! Both combinations work. It's just some like one, some like the other and some can't tell the difference. There's no wrong answer.

Curt

[Dave]

Generally speaking, how much difference would a change of 1 or 2mm in my fork rake make...? I have a slightly tricky fork replacement...and can either have one made (better option) or go with a Ouzo Pro that can be had for a song (cheap option). I don't want to find out the hard way that every mm matters when dealing with rake/trail...

Thanks -

The fromula for trail is simple: R/tanH - (offset/sinH). A 2mm change will make about a 2mm change in the trail since 1/sin73 = 1.045.

Just as important is the axle to crown distance, that's often overlooked. Stock forks range from 365 to about 374mm. That range can change the head tube angle by about .5 degree and trail by a little more than 3mm.

It's possible to double or cancel out an expected change in trail with a change in the fork length.

[thwart]

The fromula for trail is simple: R/tanH - (offset/sinH)
I knew I shouldn't have slept through that geometry class :crap:

Thanks for making things a little easier for us laypeople to understand.

I have a 50 mm rake F2 fork I'm thinking of using but have heard it may make the handling a little twitchy (compared to a stock 45 fork). Does that sound accurate?

[Dave]

I knew I shouldn't have slept through that geometry class :crap:

Thanks for making things a little easier for us laypeople to understand.

I have a 50 mm rake F2 fork I'm thinking of using but have heard it may make the handling a little twitchy (compared to a stock 45 fork). Does that sound accurate?

More rake will reduce the trail and make the bike steer quicker.

[superunleaded]

More rake will reduce the trail and make the bike steer quicker.

Dave, wouldn't this be the other way around? Less rake = steer quicker and twitchy?
Does more rake means you have to make wider turns? I noticed before that the more rake i have on a bike, the wider and more lean I have to do on a turn. Does this sound right?

I know I should not have skipped those geo classes too like thwart.

PS
I am about to put a 45 Ouzo Pro on my Spectrum which was equipped with a 43 Kestrel fork.

[Dave]

Dave, wouldn't this be the other way around? Less rake = steer quicker and twitchy?
Does more rake means you have to make wider turns? I noticed before that the more rake i have on a bike, the wider and more lean I have to do on a turn. Does this sound right?

I know I should not have skipped those geo classes too like thwart.

PS
I am about to put a 45 Ouzo Pro on my Spectrum which was equipped with a 43 Kestrel fork.

To repeat what was already posted:

The formula for trail is: (R/tanH) - (offset/sinH). A 2mm change will make about a 2mm change in the trail since 1/sin73 = 1.045.

Notice that the first half of the equation is the trail without the effect of fork offset (rake). The second half is the reduction in trail from fork offset. Smaller amounts of trail make for quicker steering response. Of course there is a small increase in wheelbase with more offset, but it's a very small percentage, compared to the change in trail. The bike will steer a bit quicker with more fork rake.

This all assumes that the fork length is the same and not changing the head tube angle (H).

[Grant McLean]

wouldn't this be the other way around? Less rake = steer quicker and twitchy?
.

no, less rake is more stable, and more rake = less trail.

Check out the drawing. More rake moves the red line closer to the green line
(less trail)

Remember what "trail" is...
"trail" is really just the size of the gap between the steering axis and the contact
patch of the tire at the ground. The steering input acts on the tire patch
in order for you to turn. If you think about it that way, it's easier to get.

The further away the steering axis (green line) is from the tire patch, (red) the slower the steering,
and the closer the steering axis is, the quicker the steering.

Because the two lines cross over... they're reversed, that why it's confusing.
(if you're not more confused now...)
-g

[superunleaded]

more confused now
-g

ahhhhh my head hurts :)
maybe i should take this headphone off my ear with AC/DC screaming "Highway to H....

Thanks for the pic BTW.

anyways, if let say the pic shows a 43 and I pop in my 45, would the axle move forward away from the green line?

[Grant McLean]

anyways, if let say the pic shows a 43 and I pop in my 45, would the axle move forward away from the green line?

nope. you're over thinking it.


the blue line (45mm fork) is just 2mm ahead of the red line (43mm fork)
so the trail is just 2mm less.

The average Trail on bikes ranges from about 5.6 to 6.3 cm's (more than 2 inches)
so a few mm's change in rake isn't going to close the gap entirely.

-g

[superunleaded]

nope. you're over thinking it.


the blue line (45mm fork) is just 2mm ahead of the red line (43mm fork)
so the trail is just 2mm less.

The average Trail on bikes ranges from about 5.6 to 6.3 cm's (more than 2 inches)
so a few mm's change in rake isn't going to close the gap entirely.

-g

Thanks G,
now I get it with the 2nd pic. more rake = less trail.

thanks again,
-gas

[Louis]

more rake = less trail.

Within the limits of most forks.

A very large amount of rake will result eventually in an increasing distance between the location of the steering axis and the tire contact area.

[Tom Matchak]

more rake = less trail.

Within the limits of most forks.

A very large amount of rake will result eventually in an increasing distance between the location of the steering axis and the tire contact area.

Yup, beware of any fork with more than 100mm of rake.

The "eventually" scenario you describe is possible in a mathematical sense. But a steering geometry with the tire contact point located _ahead_ of the steering axis intersect would, on a single track vehicle, be an unridable design.

Not really a concern, eh?

[Tobias]

Within the limits of most forks.

A very large amount of rake will result eventually in an increasing distance between the location of the steering axis and the tire contact area.
Louis, by definition wouldn’t you consider that extreme case to become "negative" trail?
As such it would still be less, right?