somebodies

What exactly are you kids talking about? "those forks look a bit slack" etc.

Smorgasbord

Typically slack refers to headtube angle (and possibly seatube angle). The less slack/steeper headtube angle means twitcher/more responsive steering. Slack angles mean more stable/less responsive steering.

I would never call a fork itself "slack." Slack typically implies a fair amount of trail - distance between the steering axis intersects with the ground, and the contact patch made by the wheel (directly under the axle) - which is the more stable/less responsive steering situation.

There is tons on trail and bicycle handling, but if my explaination confused you, check out Josh Putnam's explaination.

jimv

Like smorgasbord stated, slack usually refers to seat/headtube angles. While steering assumptions are often made based on the steepness or 'slackness' of the headtube angle, it is not a given. It would be easy to create a bike with a slack 72 degree headtube with far less stable steering than a bike with a 75 degree headtube simply by throwing enough rake into the fork to create negative trail and you have a bike that is unstable at any speed.

Obviously frame builders know this and are designing frames within acceptable and practical limits. It's still risky to put too much stock in a single steering geometry spec, be it headtube angle, fork rake or trail as it is the combination of these 3 (oh, and wheel base I suppose) that determin the steering properties of a given frame.

Track bikes are generally thought of as twitchy/fast/responsive in their steering and while they almost always have steeper headtube angles than their road counterparts, they also have more trail and less rake and it's this combination that really defines the steering properties. Interestingly, the fast/responsive steering that this geometry is associated with really only occurs at low speeds. The higher the speed the more stable (less twitchy) a track bike is. The opposite is true for a long, slack touring frame.

I for one would really like to 'hear' the opinions of some of our resident frame builders on this topic.

Take care...

Jim

Otony

angle of the dangle

Ken Cox

Jimv wrote:

"Track bikes are generally thought of as twitchy/fast/responsive in their steering and while they almost always have steeper headtube angles than their road counterparts, they also have more trail and less rake and it's this combination that really defines the steering properties. Interestingly, the fast/responsive steering that this geometry is associated with really only occurs at low speeds. The higher the speed the more stable (less twitchy) a track bike is. The opposite is true for a long, slack touring frame."

I didn't know that, but it kinda makes sense.
I love the way my Pista handles at slow speeds, and it surprises me that it seems to get more stable with speed.
I'd like to hear that confirmed and explained a little more.

jimv

For another perspective. Marc Muller of Waterford has some interesting ideas regarding frontend geometry. One, which he terms 'steering angle' reflects the the combined effect of rake, trail, headtube, and wheel diameter. There's a bit more here if you're interested.

Jim

cog_nate 

Perhaps this?

Smorgasbord

Are you sure track bikes have both a steeper headtube angle AND more trail? A steeper headtube = less trail. Despite less rake, track bikes typically have less trail than road bikes - due to their steeper headtube angles.

Run some geometry numbers, I think you'll find that track bikes have less trail.

jimv

No need to check the numbers, YOU are CORRECT, I am WRONG. Track forks have less rake and trail so there is less steering correction (moment?) than with touring bikes. My understanding is that trail acts as a lever that applies a correcting force when the bike is leaned into a turn and I believe this is why a track bike feels more stable in an all-out high-speed sprint as the wheel does not try to rotate into the turn as the bike is rocked side to side. Thanks for the heads up. I've read alot of material on bicycle steering and there's alot of contradictory information. Now I've added my share ;-) I would still like to read what some of our frame designing friends have to say on the subject. If I'm way off base on this stuff, I'd like to know.

Thanks and take care...

Jim

cicadashell

i think any bicycle would become more stable at higher speed because of the gyroscopic effect of the rotating wheels. for a track bike with relatively little caster (sorry for the engineering term there) i can understand how the gyroscopic effect would begin to dominate at lower speeds, and you would therefore notice it sooner. but i can't imagine any sort of physical reason why, if you had two bicycles with the same rotational inertia in their wheels, and one (the "long, slack touring frame") had more caster than the other, that the one with more caster would be less stable. unless wheelbase plays a role, that is.

Smorgasbord

It's more that the caster makes turning at high speeds more difficult, requiring more muscle, less finesse, and utimately destabilizing the system.