Mountain Biking - Please explain the importance of the terms such as stiff..

Hi guys,

One term that I come up across often in mountain bike part discussion is "stiff", and "flex". What exactly does this mean though?

How can a fork, or even a frame be 'flexy', or not stiff. I'm not exactly sure why it's considered to be a good quality.

Can someone please elaborate on these terms? Thanks!

IMO, these are rather general qualitative terms to describe the feel of the frame or fork.

In frames, it generally denotes how malleable the frame is under load and stress. This could be either by design, or materials used in construction. Stiff and flex can also be a postive or negative connotation depending on its context.

For example, a stiff frame can be good in that it results in better tracking in corners, and does not waste your energy input (i.e. it's efficient) which is important for competitive racing. Think of performance race cars. They have stiff suspension which helps in cornering.

However, an overly stiff frame can be very uncomfortable, and jarring. Imagine feeling every bump on the road magnified through the frame into your hands.

Steel like cromoly, and special alloys like titanium is said to have some flex. That is they have an inherent springiness that gives it a natural shock absorbing quality.

A springy fork, or a stiff fork is neither good or bad. Marzocchi is often described as springy, whereas Manitou is described as stiff; just one example. But a flexy fork is definitely bad!

Stiff = rigid... it's not going to move when you're out of the saddle in hard sprints and it transfers every jolt on the trail to your taint. It's good for efficiency, but uncomfortable to ride. I have an extremely stiff aluminum frame. I wish it had a little more flex to absort shock. I usually just run lower tire pressure instead.

Flex is refers to a more forgiving frame or fork. I think a flexy fork is a bad thing. The fork will flex under hard braking and other scenarios.. just not a comfortable feeling.

In regards to forks you have torsional and vertical stiffness. The between the knee wheel twist is supposedly a measure of torsional strength. How far can you twist your wheel. Is is stiff or flexy. QR20 makes this stiffer. I don't think this is a valid way of measuring. Personally if you wanna see how 'stiff' your fork is go through a rock garden (a big one) at mach 10 and watch your fork bend side to side until you are clear.

You also have vertical stiffness (don't know the real term). This is essentially a measurement of the spring (like bentrim said). Marzocchis are plush at the top of the travel and become very stiff throughout the entire travel. Manitous are (or were...the tpv+ I heard is better) stiff from top to bottom.

cheers