Trading 43 rake for a 50 rake fork?

[NYCfixie]

Hi,

This may not make sense but please answer if you can....I would really appreciate the help.

Assume I have a 53cm top tube road frame with a 71.5 head tube angle and the company who produced the frame just spec'ed a 43mm rake fork across all sizes of the frame (i.e. 53, 54, 56, 58) which does not seem to make sense because not all frame sizes have the same HT angle. The 43 rake fork only seems to make sense on the 56 and 58 because they have 72.5 HT angles which results in 61mm of trail which is what they are going for.

The 53cm with a 71.5 head tube angle with a 43 rake fork and 23mm tires results in 68mm of trail....sort of slow/sluggish.

If everything else stays the same, but I install a fork with 50 rake (let's say it currently has an Enve 43 and will have an Enve 50 so the a to c measurement stays the same) this should result in a more desirable 60mm of trail.

1) Is this new trail number (60mm) correct and will it be a good for a very stable and neutral handling bike...and maybe even a bit quicker steering?
2) Because the a to c measurement is the same on both forks, wouldn't the front of the bike be lower with the 50 rake fork because theoretically it pushed the front wheel farther out?


Thanks in advance!

[GrantM]

Because the a to c measurement is the same on both forks, wouldn't the front of the bike be lower with the 50 rake fork because theoretically it pushed the front wheel farther out?

As you discovered, if forks were actually measured as you suggest, each rake would
result in a change to the head angle of the frame, but i don't believe that's how the
forks are really measured.

It might help to visualize the fork not as a curve, but and "L" with two dimensions,
vertical and horizontal offsets. Length being the vertical, rake being the horizontal.

The axle to crown measurement given by the fork makers is actually
measured parallel with the steerer tube, so it's a constant dimension that
does not change with the variation for different fork rakes, and all the various
rake options in a given model have the same distance from ground to headset.
Only thing that changes is the horizontal offset.



-g

[rabo]

If I add the numbers I come to 61 mm. Don't ask me about the subtleties in handling by arriving at a trail through changing the rake that dramatically. It will feel faster for sure.

[Jonathan]

Grant is right, as usual.

[NYCfixie]

As you discovered, if forks were actually measured as you suggest, each rake would
result in a change to the head angle of the frame, but i don't believe that's how the
forks are really measured.

It might help to visualize the fork not as a curve, but and "L" with two dimensions,
vertical and horizontal offsets. Length being the vertical, rake being the horizontal.

The axle to crown measurement given by the fork makers is actually
measured parallel with the steerer tube, so it's a constant dimension that
does not change with the variation for different fork rakes, and all the various
rake options in a given model have the same distance from ground to headset.
Only thing that changes is the horizontal offset.



-g

So then I am going with the thinking the this change would make sense:
- swap the 43 rake fork with a 50 rake because the head tube will stay in the same place
- The trail will decrease and steering will become quicker than it is currently
- This is probably what was originally designed into the frame had the manufacturer offered fork rakes that matched the head tube angles for each size in their range.

Thanks!

[thollandpe]

Can you try this out first? And maybe with a 47 as well?

[GrantM]

So then I am going with the thinking the this change would make sense:
- swap the 43 rake fork with a 50 rake because the head tube will stay in the same place
- The trail will decrease and steering will become quicker than it is currently


Thanks!

Yeah, a 50mm rake fork makes sense. I'm small too, so i've experienced exactly what you are describing,
my Colnago C40 in a 52cm has very high trail. Changing from 43 to 50 will add 7mm of wheelbase to the
front centre, and that will change the feel a bit, as will the reduced trail. If the seat angle is 74 degrees or
less, then for sure the geometry will benefit from some more front center. Personally, i really dislike very
short front center bikes, and the trend of all sizes of production bikes getting 43mm forks is just plain lazy.
If you think 50 might be too much, there will still be a noticeable improvement going from 43 to 47 too.


-g

[Mark Kelly]

Because the a to c measurement is the same on both forks, wouldn't the front of the bike be lower with the 50 rake fork because theoretically it pushed the front wheel farther out?

The axle to crown measurement given by the fork makers is actually
measured parallel with the steerer tube, so it's a constant dimension that
does not change with the variation for different fork rakes, and all the various
rake options in a given model have the same distance from ground to headset.

Actually the effective headset height does change, by about 2 mm.
The actual amount is cos(HTA)*(Rake 1 - Rake 2), in the scenario given this is cos(71.5) * (50 - 43) = 2.22mm.

This is slightly less than the amount that it would change under the OP's scenario where the height change is

sin (HTA) * (SQRT(AC^2 - rake2^2)- cos(HTA) * rake2) - (sin (HTA) * SQRT(AC^2 - rake1^2) - cos(HTA) * rake1)

which is sin (HTA) * ((SQRT(AC^2 - rake2^2))-(SQRT(AC^2 - rake1^2))) + cos(HTA) * (rake1- rake2)

which would be about 3mm for the scenario given if AC is around 370mm. (calculation not shown due to assumed lack of interest)

[Michael_S]

I think a 50 mm rake fork would be even better than a 47 if it's available. The resulting trail would be 60 mm assuming you are running 23 mm tires.
It would be an easy choice for me.

[GrantM]

Actually the effective headset height does change, by about 2 mm.

No, you're making incorrect assumptions about how axle to crown is measured.

The length of axle to crown isn't the distance from the crown to the hub.
It's a straight line to the plane of the hub, parallel to the head tube.

-g

[Mark Kelly]

No, you're making incorrect assumptions about how axle to crown is measured.

-g

I used your stated method of axle to crown measurement, along a line parallel to the steerer tube. The rest is simple geometry.

[GrantM]

I used your stated method of axle to crown measurement, along a line parallel to the steerer tube. The rest is simple geometry.

Huh? How can you measure the same distance twice and get different numbers?
Fork rake is totally independent variable to the length.

-g

[The Bridge Guy]

Take a fork, or even an envelope, anything that has a right angle, tilt it back to emulate the head tube angle, note that as you change the length of the short side (rake), the long side gets lower in relation to the axle which is the fixed point, thus the front end gets lower by a marginal amount. In effect, the fork is sliding backwards and down in relation to the axle.

Something to think about: in a recent article in the new British magazine Cyclist, Mike Burrows said he considers trail to be a minor contributor to maneuverability relative to wheelbase. Thus it is possible that increasing the wheelbase via an increase in rake will more than cancel out the change in trail. It would be interesting to hear more on this from our framebuilder/designers.

[Mark Kelly]

Huh? How can you measure the same distance twice and get different numbers?
Fork rake is totally independent variable to the length.

-g

I didn't. The two forks have different rakes, so they have different effective heights.

Draw it out, you'll see what I mean.

[GrantM]

Take a fork, or even an envelope, anything that has a right angle, tilt it back to emulate the head tube angle, note that as you change the length of the short side (rake), the long side gets lower in relation to the axle which is the fixed point, thus the front end gets lower by a marginal amount. In effect, the fork is sliding backwards and down in relation to the axle.

Your logic is faulty, the hub is not a fixed point, it moves horizontally as rake changes.

Take a hockey stick and trim some length off the tip of the blade, does the handle get shorter? No it does not.
The hockey stick handle length is the fork length, independent of the length of the blade.


-g

[e-RICHIE]

Fork rake is totally independent variable to the length.

-g

This.

When I make two forks with two different rakes for the same bicycle, I make the span (crown race to axle) the same so that the top tube isn't affected.

[Mark Kelly]

Then your two forks would have different axle to crown measurements as defined by Grant.

[GrantM]

The two forks have different rakes, so they have different effective heights.

Draw it out, you'll see what I mean.

What is "effective heights"? We're taking about this:

http://www.velocipedesalon.com/forum...tml#post521166

[Jonathan]

I was certain the lower XY headtube axis should remain constant. To prove it I plugged it into bike cad. Oops. I'm surprised. Fork Length below remains constant, rake changes as does axle to crown but the xy point of the bottom of the headtube moves. hmmmm

[e-RICHIE]

All I can say is that I am so glad I am in the fourth quarter, or near to it atmo. I suspect others are glad too.