Mountain Biking - Why use a 4 bar rear suspension link?

Hi

Why are 4 bar rear suspension used over a more simple design, ie the shock mounted directly from the frame to rear set?

Many thanks

well, i'm not an expert on this, but as far as i understand the mechanics and from test-riding the difference is that most of the 'simple' designs do give you the positives of suspension for bumps and stuff, but also have a lot of negatives like moving when you pedal and other stuff --- i.e. the suspension takes power away from you as you compress and uncompress the suspension resulting in a 'bob' which is annoying as well as inefficient.

the 4-bar minimizes this to a great extent -- if you have a good circular stroke there is almost no movement in the suspension on a flat section (my Specialized FSR XC for example)

all of the single bar-suspension bikes i've seen bounce and bob like mad which really sucks when climbing or on a flat.

if all you do is downhill or free-ride it doesn't make much difference, but for xc or climbing or all-round, i would definitely NOT buy anything but a 4-bar or hard-tail.

i just finished 4 days of mountain trail riding in Italy (over 25,000 vertical feet - awesome!) with my new Specialized and in our group we had 3 4-bars, 2 hardtails and 1 single-bar and you could see the bob of like 3" up and down of the single-bar on the road or a flat section while very little from the 4-bars -- i'd buy a hard tail way before that b/c the benefit is less than the penalty.

dont post a question in more than one forum....thanks. http://www.cs.chalmers.se/~olahe/Bike/Rear/fourbar.html

Thanks, nathank, that's really good information to know. I'm just getting into cycling and have a LOT to learn. I'm doing some lurking on the forums, trying to soak up as much info as possible. :)

Shawn

The four-bar linkage also eliminates "chain-stretch" by moving in an equi-distant arc around the bottom bracket. The rear wheel travels up and BACK, versus up and forward! Many single-pivot bikes cause the effective distance of the rear hub relative to the bb to increase as the wheel travels through it's movement. The chain length single-pivot bikes can be difficult to properly establish because of this.

BTW, pedal induced bobbing is nearly eliminated in four-bar linkages only where the rear pivot is in front of the dropout, along the chain-stay. In otherwords, a true Horst-Link. This is the design patented by Specialized and licensed to many other companies including Intense, Giant, Kona, Rocky Mountain...etc. Some generic frames will locate this pivot along the seat stay and thus bypass the patent, but this design creates problems with the suspension "jacking up" under hard braking!

The only drawback to a four-bar linkage design is in the amount of lateral play that can develope if quality bearings are not utilized.

Hope this helps!