I had it wrong...
 

This might not make sense to some folks, but I feel the need to acknowledge that someone else knows more about frameset geometry than I do. :)

Some time back I enthused about the way my Bianchi Pista handles and how the head tube angle and the short rake of the forks work to make that happen.

I said with all confidence that the shorter the rake the quicker the handling.

WRONG!

The shorter the rake, the more stable the bike.

The Bianchi has a steep head tube for quick steering and a short rake to increase trail and thus stability.

Or something like that.

I might still have it wrong.

Anyway, someone posted to the effect that shortening rake increased trail and stability, and I dismissed that person as not knowing as much about it as I thought I did.

That person had it right and I had it wrong.

There.

I feel better.

Everybody can go about their business now, unless they want to talk about head tube angles, rake and trail, and this time I'll pay attention and try not to act like such a know-it-all. :)

Everyting I know about steering geometry I learned from Josh Putnam. http://www.phred.org/~josh/bike/trail.html

From the Josh Putnam site posted by mander:

"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."

The above at least explains how I got off in the wrong direction.

The shorter the rake or the "fork curve" the quicker it gets to make a turn (simple geometry)... the heatube angle have an effect in this also. Stuff like for example how soft the ride will get... and the handleling of the bike... Usually bikes whith tight geometries turn quick but at the some time sometimes are hard to handle... For track bike there is an average of 3cms of rake and a head angle of 75 degrees go over that u get a bike hard to handle in the upper banks if u need a to manouver fast.

Well im talking too much...

cya :)

It sounds like you had it right and now have it backwards.. your post is hard to decipher though.

No, you and ultraman have it backwards. Read the page I linked to.

Yeah, that article (and another one from Sheldon Brown) are the two I read when the thread in question was live. It's contrary to common sense and I was so confused at first.

ok, lock this thread... the internet is NOT for admitting that you are wrong

josh putnam? isn't he the dude from a**l c**t?

forget to switch screen names again?

Seth Putnam can't ride a bike, he's a cripple.

I definitely don't understand the whole rake/trail thing entirely, but my experience on my O degree raked track bike with a 74.5 degree head angle is that of an unsafe, twitchy ride. With approximately 20 mm in tire profile, this results in 91.8 mm of trail which according to most would be a very slow handling bike. At speed, however, the bike threatens to snap the bars to the side if I make a quick steering adjustment. This has made me afraid to get out of the saddle to sprint and order a replacement fork with 33 mm rake as the 0 degree fork was a mistake to begin with.

Not necessarily. Just think of a shopping cart wheel. It has negative rake and is very stable as a result. A shopping cart wheel that is turned around to effectively have "positive rake" will be very unstable in that state, i.e. it will tend to want to stop rolling straight and swing back around. Now consider a bike with a 90 degree head tube. What will be more stable, a negative rake or positive raked fork?

More positive rake=less trail=less stability/ more twitch.

There could be something I'm missing here and I will be grateful if someone points it out.

my mistake. i read this thread too early in the morning... gotta have caffein first.

ken pls just stop postig until you have it right ok
?

Thanks Ken, this actually cleared up a lot of mystery with me and trail vs. rake. Now I understand, and understanding is knowledge and everyone knows that:

Knowledge is half the battle, the other half is guns and lasers.

This is a very good thread. I've often wondered about trail and simply hadn't had the time, or the memory when I had time, to look it up.

Thanks Ken and mander.

...and he's gay.

But if you think of it in terms of automotive front end alignment i'ts quite different. Positive caster angle IE: tilted forward at the bottom (like a fork that has more rake) makes for a car that is very stable and tends to roll in a straight line while negative caster (opposite) makes for a very twitchy and evil handling car that wanders all over the road.
The shopping cart wheel when turned forward has nothing to keep it in that position so will obviously turn right around but it is twitchy (notice how the wheels are always flitting about when you run with shoping cart?)

re: "twitchiness"
this is a word that's use all the time to describe handling, and it feels seriously devoid of meaning. it doesn't help me understand other people's experiences with bike handling, so if people would be willing to use some other words to describe what's going on with handling, that would be great.

here's my experience. my pogliaghi has a tight, steep front end - pretty traditional track geometry (but with longer than usual chainstays). i expected it to handle "twitchy" since that's what everybody says, but i found it to be smooth and responsive instead. twitchy seems to be quick and unpredictable, but even with my pog's very quick handling, it's decisive, not about to respond unpredictably.

i'm not sure exactly what to attribute that to, except perhaps that there's the right balance between rake, trail, and angles - perhaps it's a little bit more stable considering the slightly longer chainstays, while being quick as anything with the tight front.

anyhoo, yes, rake and trail are interesting. i'd be very interested in hearing framebuilders chime in with some of the things they consider when they think about handling, bike design, and the way all of these factors interact.

This totally explains why my Panasonic handles so much twitchier than my Nishiki, despite having a longer wheelbase. People told me the Panasonic road bikes were no good for riding in the city because of the long wheelbase but that's not at all what I experienced. I need to calculate the geometry sometime...

Fork offset/rake, in conjunction with head tube angle, determines trail. Given a set fork offset, the steeper your head tube angle, the more "responsive" the steering will feel - ie, it will be easier to turn the front wheel. Given equal trail, you should be able to move from a bike with a really steep HT angle to a bike with a relatively shallow HT angle without much trouble. They won't feel exactly the same, but it won't be too big a deal - the trail is the largest determining factor.

A bike with more trail will be more stable at higher speeds, but feel sluggish at lower speeds - this is why those stayer bikes have "backwards" forks. Coversely, a bike with too little trail will be stable at slow speeds and handle like poo while going fast (again, compare a beach cruiser to a stayer bike).

The chainstays determine weight distribution, which can affect handling. Short chainstays can mean trouble keeping the front wheel down during seated climbs, while relatively long chanstays can make the bike less agile. Touring bikes tend to have longer chainstays to help with loaded weight distribution and stability, especially during descents, etc.

Trail affects the bike's ability to follow lean and to self correct. Track bikes are designed the way they are so that they will go in a straight line, depsite you throwing it from side to side underneath you in a sprint when you are out of the saddle hammering down - think about how easy it is to ride a track bike with no hands. Take something like a Kona Jake the Snake by comparison, and it's really hard to keep that thing going in a straight line if your hands are off the bars. The geometry of CX and MTBs means that they tend to follow the rider's lean more and also tend to self correct more readily - this is important on non-smooth terrain, where you don't want to hit a root and have your wheel stick at 90-degrees. You want to hit bumps and have the fork come back into line. This isn't going to happen on a track bike.

So that's why our bikes need to get up to speed for them to balance right? I've fallen over before going under 5mph trying to get my foot in the toeclip...

Does stability mean the opposite of agility in this context? Or is that rake vs. trail?

Here's a good article on it

http://www.velonews.com/tech/report/...es/7322.0.html