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Fork Rake Effect on Stability and Turning Response(21 posts)

Fork Rake Effect on Stability and Turning Responsecogmaster
May 31, 2003 4:19 PM
I am sure this subject has been beaten around before, but which has the effect of making a bike more stable:
1- Going to a fork with slightly less rake(say 4.3cm to 4.0cm)
or
2- Going to a fork with slightly more rake(say 4.3cm to 4.4cm or 4.5cm)

I went to a bike with a 1cm longer top tube and a 73.5 degree vs 74.0 degree head tube and am using the same 4.3cm rake fork. The bike for some reason seems slightly less stable that previous one given same make and exact same set-up. Any thoughts?

Thanks..
You probally wont notice the differance(dont waste your money).the bull
May 31, 2003 4:29 PM
The fork with more rake will be more stable.
The fork with less rake will be more responsive.
There is nothing wrong or uncommon with your set up.
I have a Litespeed with a steeper headtube(73)and am running a 43mm rake fork.It handles fine.
Hope this helps.
Am I high, or...feathers mcgraw
May 31, 2003 4:38 PM
...is everything stated above backwards? More rake=less trail, quicker steering. Less rake=more trail, more stable. 73 degrees is slacker than 73.5 and 74.

I had a 73.5 HTA bike with a 45mm rake fork and it was hard to ride no handed. Switched to a 40mm and it was more stable. Most of the time it didn't feel that different, though, and I never thought the 40mm fork wasn't responsive enough.

If I'm wrong, flame away.
Were both high!the bull
May 31, 2003 5:05 PM
My Litespeed is slacker at 73.

Trail is the distance the contact patch of the front tire lags behind the point where the steering axis intercepts the ground.

Consider the casters on a shopping cart: the wheels are offset behind the vertical steering axis, and the force of the ground on the wheel tries to center the wheel on the direction of travel.

The greater the distance between the steering axis intercept and the contact patch, the stronger the centering force becomes.

For a given steering angle, offsetting the hub forward reduces trail, while offestting the hub backward increases trail. This may seem counterintuitive, since very stable cruiser bikes usually have more fork rake than twitchy track bikes. But the other factor at work is the angle of the steerer -- cruiser bikes have very slack head tubes, so they have more trail despite their fork rake, not because of it.
You're not high.jtolleson
May 31, 2003 5:08 PM
Bull's description was backwards.
Bull's statements are backwardsYoGeorge
May 31, 2003 5:07 PM
The amount of trail, or shopping cart effect, is greater when a fork has LESS rake. The way to measure trail is to put a straightedge in line with the head tube to the ground, and drop a straight edge (plumb line) down from the hub of the wheel. If you increase fork rake with a given head tube angle, it will decrease trail, since the hub centerline will move forward toward the point where the straightedge along the head tube hits the ground.

As you go to a slacker head angle, you can use more fork rake, keep the trail the same, and give a smoother ride. So the first poster's observations seems to be backwards from what I'd expect. But there are other variables, including tires and tire pressure, fork length, etc. that affect stability.

George
Made a mistake at first. nmthe bull
May 31, 2003 5:11 PM
here's the math...C-40
Jun 1, 2003 4:03 AM
The formula for trail is (R/tanH) - (rake/sinH). R is the tire radius and H is the head tube angle. The first half of the equation is the amount of trail without the effect of fork rake. The second half is the effect of fork rake. All fork rake reduces trail. More rake, less trail.

A bike with more trail will be more stable at normal riding speeds, but less stable if most of your riding is around cones in a parking lot at less than 5mph.
Incorrect.Alexx
Jun 1, 2003 8:14 AM
Please, leave the questions regarding vehicle dynamics to those who actually have degrees in the subject. Your response shows that you are profoundly ignorant on the subject.
Read my second post you fu(king jacka$$ piece of sh!t!the bull
Jun 1, 2003 8:27 AM
What's your background Alexx?bugleboy
Jun 1, 2003 9:27 AM
Your pretty smug for not qualifying yourself. Are you the real deal, or just full of it? Inquiring minds want to know.
What's your background Alexx?Alexx
Jun 1, 2003 4:54 PM
I have a degree in Mechanical Engineering. (Bull obviously has one in English Composition.)
And, btw, bull, sorry, but you already stuck your foot firmly in your mouth with the first post, so there was no need to call me by your family title. Most people probably only read the first answer to any post,so, considering that it was SOOO completely wrong, it was my professional duty to make sure that nobody went out and spent beacoup bucks on the wrong fork because of your bad advice.
Hey alexx-this is what you need to do to yourself!the bull
Jun 1, 2003 5:36 PM
http://fotm.rotten.com/fotm/limberoni.html
Thank you for that, BullAlexx
Jun 2, 2003 3:37 AM
I'm sure you must've gone through all your favorites for that one. Still, I'm surprised that you are this limber. Although, since you are able to put your foot in your mouth so easily, I'm hardly surprised of you being able to do likewise with other appendages.
Very proud of ya!the bull
Jun 2, 2003 8:29 AM
Three snappy comebacks in one post!Good ones too! This must be a record!
OK, forget the profundity!Alexx
Jun 2, 2003 9:31 AM
and just go for a ride!

You're allright in my book!the bull
Jun 2, 2003 1:51 PM
Just pissed that you would stick that firecracker in my ass while I still have a foot in my mouth!

Hope to pick on you soon in the future!
are you sure?feathers mcgraw
May 31, 2003 4:48 PM
Typically bikes with longer top tubes have steeper HTA's, to keep the wheelbase in check. Especially if your two frames are of the same make and model. If you're actually going from a 73.5 to a 74 (instead of the other way around), that would account for the lower stability. A 4cm fork would help with stability, as well as shorten the wheelbase, which would make the bike a little more agile as well.
Handlebar width makes a huge differenceSprint-Nick
Jun 1, 2003 4:10 AM
Feeling 44 cm handlebars felt like mountain bike bars after my time on 38's while on the track I put 38's on my road bike. It feels way more responsive... its a super cool feeling.

Nick
Well, Nick,Alexx
Jun 1, 2003 8:20 AM
You seem to be missing the point of the question. What the original post was discussing was due mostly to road input and damping, whereas your observation on bar width concerns rider input. Anybody who has actually taken a beginner course in physics will understand that having a shorter lever arm (in this case, a handlebar) will result in greater angular motion, therefore, quicker turning. OTOH, when you do get vibration feedback through the bars, I'll bet you will wish you still had those wide bars on it!
Both, actuallyAlexx
Jun 1, 2003 8:11 AM
But, as I've told another poster in the last weekk, the fork with LESS rake will be more stable at HIGH speeds, whereas the fork with MORE rake will be more stable al LOW speeds.
From what you've described on your bike setup, the decreased headtube angle, all things being the same, has given you a lower (numerical) amount of TRAIL, which is the primary reason why you notice less stability at higher speeds. You might be advised to get a fork with less offset (a.k.a. 'rake'), but check to be sure you won't have too much toe overlap first.