# Bicycle Mechanics - Explain fork rake to me.

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View Full Version : Explain fork rake to me.

evilcryalotmore
12-27-10, 07:46 PM
So, im looking for a fork that will shorten the distance from my front wheel to my rear wheel, trail distance?

My stock offset is 28mm, If i get a higher rake offset does that mean closer to the frame? or farther from the frame?

Can i use a 1inch threadless fork on a 1/18 steering tube?

Shims and other accessories?

Shimagnolo
12-27-10, 07:58 PM
http://en.wikipedia.org/wiki/Bicycle_and_motorcycle_geometry

evilcryalotmore
12-27-10, 08:01 PM
So if i try out a 55mm offset fork at the shop it should reduce the trail correct?

fietsbob
12-27-10, 08:04 PM
Rake/ offset is the difference between the centerline of the steering axis, and the center of the hub axle.

BUT that is only PART of the steering Geometry.
with the steering axis being at an angle,
the vertical line thru the axle crosses
the angled line of the steering above the ground-plane
so on the ground the tire touches the ground behind the steering axis line..
that is TRAIL.

trail is reduced as the rake is increased, reducing rake, increases trail

because the 2 lines cross

someone elsewhere on this site has graphics showing this

a+b=c but only if you don't change a, or b to another value..

fietsbob
12-27-10, 08:08 PM
So if i try out a 55mm offset fork at the shopTry it out? you have to make the commitment and buy it [\$\$\$]
before you can install it ..

Shimagnolo
12-27-10, 08:11 PM
What problem are you trying to solve?
It sounds like the goal is a shorter wheelbase, which is the wrong reason for going to a fork with a different rake.
More rake -> less trail -> less stability / quicker turning.
Less rake -> more trail -> more stability / slower turning.

evilcryalotmore
12-27-10, 08:19 PM

Im trying to reduce the wheelbase distance, What would i need to change? All i know off my oem fork is that the rake is 27-28mm

Im going to try a 55mm rake offset because at the shop someone owns one, Im not going to purchase it at the shop.

zzyzx_xyzzy
12-27-10, 08:41 PM
Rake goes forward. a 55mm rake will increase your wheelbase.

Trail goes backward, it's measured back from the imaginary intersection of the steer tube with the ground. More rake = less trail.

Why are you trying to reduce the wheelbase? If you're trying to change the handling, a 1% change in wheelbase is dominated by other factors: head angle, trail, weight distribution.

fietsbob
12-27-10, 09:01 PM
OP just needs to buy another bike frame and fork to suit the changes they now desire.
Shorter wheel base typically comes off the back of the bike ..
an S bend seat tube is one way to shorten up the chainstays to shorten the wheel base..
Builders have also pressed a dent in the seat tube to make tire clearance.

The Italian RiGi company split the seat tube below the seat post
to 2 side by side tubes to pass the wheel between the 2.

front of the bike is about rider fit,
a steeper head tube can be part of a twitchy steering bike.
Criterium racing and some Track events may favor that .

Al1943
12-27-10, 09:07 PM
Changing rake offset will make very very little difference in handling. As said above, increasing the rake offset will make steering quicker and wheel base longer.

DannoXYZ
12-27-10, 11:09 PM
Im trying to reduce the wheelbase distance, Why? Are you after a handling change? What change?

evilcryalotmore
12-28-10, 12:38 AM
Im Trying to close shorten the wheelbase for a customer, just wonder what i would need to change to accomplish this,

Oem is 27-28mm

The one he wants is 55mm what will this do, Will it make the length from the rear wheel to the front wheel longer or shorter.

fietsbob
12-28-10, 12:55 AM
Given:Rake bends forward Adds 27mm, a bit more than an inch, 55-28=27

You could just run the existing fork backwards ..

Ingleside
12-28-10, 01:13 AM
Im Trying to close shorten the wheelbase for a customer, just wonder what i would need to change to accomplish this,

Oem is 27-28mm

The one he wants is 55mm what will this do, Will it make the length from the rear wheel to the front wheel longer or shorter.

You don't solve a problem by possibly creating a new one. What do they think a shorter wheel base is going to do for them? Are they the type of rider that would even notice a difference? Did they just read Road Bike Action or Mountain Bike Action and decide that they needed a shorter wheelbase because a tester said it was better? You need to sit down with this customer and explain to him what is going to happen if they try to reduce wheel base by changing their steering geometry. A few mm in wheel base at the expense of a proper steering geometry is going to completely upset the ride the bike intended to have. If then understand how their bike will ride after the "upgarade" then sure, sell them what they want.
In the 13 years I've worked behind the counter I can attest to the fact that people typically don't what the hell they are talking about when it comes to bikes. The ones that do typically will do it themselves.

Kimmo
12-28-10, 01:58 AM
Yeah, this is total WTF stuff.

Leave the fork that came with the frame on there; it was specced for more important reasons than wheelbase.

Changing the fork would be a dumb way of achieving a shorter wheelbase. You do that with a frame.

evilcryalotmore
12-28-10, 02:00 AM
Well, he insist that he needs a shorter wheelbase, How would i achieve that for him?

I told him such risks. etc.

Kimmo
12-28-10, 02:40 AM
New frameset.

End of story.

Maybe you should ask him what he's hoping this shorter wheelbase will miraculously do for him.

evilcryalotmore
12-28-10, 02:43 AM
Yeah, i'll suggest he get a new frame set. He is trying to shorten his wheelbase so his bike "wont look so funky"

^^ idk...

So, For further reference, If he gets him self a lower rake fork and wants me to install it, it'll put his wheel closer to the rear wheel correct?

Kimmo
12-28-10, 02:52 AM
Yeah, but it'll have more trail and handle slower. Depending on the clearance of the old fork, it might even make the head angle slacker, causing even more trail.

I'd tell him the gap between his back tyre and his seat tube looks 'funky'. And if he's just concerned about looks, I'm guessing the old forks are sharply curved at the very bottom in the old style, unlike the yellow bike in your sig. Maybe just a set of modern-looking forks with the same rake will make him happy.

DaveSSS
12-28-10, 09:44 AM
A 28mm offset is highly unusual for a road bike. There is no way to change the wheelbase of a bike significantly, without fouling up the steeering. Changes to fork offset (rake) are usually small, to fine-tune the bike's steering. Changing from 28mm to 55mm would rarely make any sense. The trail would be reduced greatly and the steering would become much quicker. Totally bad idea.

Shimagnolo
12-28-10, 10:36 AM
As an example of how sensitive handling is to fork rake, I once bought a used tall frame which was afflicted by the lousy geometry which too many mfgrs put into their large frames.
It would go into uncontrollable high-speed shimmy over 30mph.

After consulting Dave Moulton's articles about geometry, I made a bunch of measurements and ordered a custom-made fork.
The change from a fork with 45mm rake to one with 38mm rake cured the high-speed shimmy.

Al1943
12-28-10, 10:46 AM
What kind of bike is it and how do you know the rake offset is 28 mm? Seems highly unusual. The road bike fork rake offsets I've seen have been in the 38 to 50 mm range.

My experience has been that a change offset 3 mm will make a significant difference.

If the owner is concerned about appearance perhaps straighter fork legs is what he really wants. Fork rake offset is not dependent on curvature.

evilcryalotmore
12-28-10, 10:58 AM
He is going to get a new fork in the same demonsions.. He is looking for a straight fork, no curves...

Well i know that the for is a 28 because the demsions are on the fork.

fietsbob
12-28-10, 10:58 AM
They want a shorter wheel base sell them a frameset with a shorter wheel base,
the whole thing.
steep seat and head tube short chainstays the whole ball of wax .. a track frame
rear facing dropouts are needed , verticals would work, but i suspect this is a Fixie gear type guy..

rawly old
12-28-10, 03:25 PM
Looks to me that bike already has too little rake for a road bike unless he's only doing urban riding.
Maybe you should shop around for a more suitable frame, and get some sizing help from a LBS.

ironwood
12-28-10, 04:19 PM
There might be some one on the Framebuilder's forum who could explain front end geometry. Perhaps you and the customer should study up on the subject before investing any money on a new fork.

Al1943
12-28-10, 06:37 PM
He is going to get a new fork in the same demonsions.. He is looking for a straight fork, no curves...

Well i know that the for is a 28 because the demsions are on the fork.

A straight leg fork can have the same rake offset as a curved fork due to the angle at the crown. If you are saying that the 28 mm rake offset is one of the dimensions and that the new straight leg fork will have the same dimensions then there should be no problem. And I've learned something new, since I've never heard of a 28 mm offset.
If he wants a shorter wheelbase he'll need a new frame, and probably a fork with a different rake offset.

dddd
12-28-10, 07:36 PM
I'm guessing that the 28 printed on the fork steerer is perhaps the steerer diameter?

I've never heard of a 28mm rake figure, but if it's a track frame with a ~77-degree head angle that might well be the right figure.

I've got a 1952 E. Christophe road bike, all original, and the head angle is 77.5 degrees. The fork looks fairly normal, but the rake figue must be significantly lower than 38mm in order to balance out the trail figure with the steep head angle.

Reducing the fork's rake will put the rider's toe in the path of the front tire when the bars are turned during slow cornering, possibly toppling the rider.