Why do longer fork bikes have slacker angles ??

the phantom pieman

I can understand from motorcycling that it give stability - but obviously slower steering.
Presumably, it also increases the wheelbase. So why are they slacker 69 or 68 rather than 70/71?

Is it because the real angle tightens up as it goes through all it's travel?

Mancunianfightingcat

The slack angles are measured when the forks are fully extended, when the forks compress, the angles tighten up. The longer the travel, the more the angles will change.
If the angles were already tight, they'd be even tighter when the forks compressed.

.......Well it makes sense in my head.......

the phantom pieman

Certainly that makes sense ....
I guess I wasn't thinking about it being measured from the extended length but at "sag" length

Radioactiveman

Surely as the fork has a linear path the angle does not change? I though head angle was the relationship between headtube and horizontal. By putting a longer fork on you don't change this angle I though the only way to change this is by headset inserts.

Putting a longer fork make the front end higher which as you said makes it handle less well.

Am I confused,wrong or stupid? PS you can answer all 3

dav1

same reason. Slacker head angle give more stability at the cost of steering speed.

With downhill you don't want a twitchey front end so the sacrifice is worthwhile seeing as a faster steering response couldnt be used effectivly due to instability

XC the faster steering is great for the flat and climbs where a s;ack head angle would become a burden.

@ radioactiveman

It does steepen. Think of a triangle of front hub to crown to BB. If you shorten the front hub to crown distance (as the fork compresses) the angle of the line from hub to crown must steepen as the other distances remain constant.

Try drawing it, or (if you wanted) grab a calculator and do some trig.

Radioactiveman

Dav1 wrote:

@ radioactiveman

It does steepen. Think of a triangle of front hub to crown to BB. If you shorten the front hub to crown distance (as the fork compresses) the angle of the line from hub to crown must steepen as the other distances remain constant.

Try drawing it, or (if you wanted) grab a calculator and do some trig.

Cheers Mate

Just got up so not firing on all cylinders.... my excuse anywhere damn wikipedia skewed my thinking http://en.wikipedia.org/wiki/Bicycle_an ... e_geometry

ilovedirt

A lot of DHers like a slack head angle, as Dav1 said, when they're bombing down the side of a hill, they don't want a twitchy front end. When i ride, if i'm on a steep climb, i'll wind my forks down a bit so it's not so much of a slog but wind them back out at the top so that the ride is more stable on the way back down.

paulrockliffe

The way I've always seen it is that having longer forks effectively attempts to keep the bike closer to horizontal on a Downhill course. Makes it a little easier to keep your weight in the right place. Think about your bike on a slope, then put a longer fork on and it's like the slope isn't as steep.

The head angle will slacken compared with a shorter fork and this helps because the important angle is the angle to vertical.

When considering the head angle you want it's important to consider both the dynamic head angle, with your weight on the bike, but also the angle that the bike will be ridden at, because anything other than horizontal ground will give an effect from gravity on the steering of the bike.

Northwind

It's more about purpose. If you were to make a really hard-hitting 100mm trail-bike you'd make it as slack as you would a 140mm equivalent. But as it is, most short travel bikes are more XC-oriented.