From the time fork suspension was introduced into mountain bike designs, manufacturers have scrambled to outdo the competition. At one point, designers considered using a swing-arm rear suspension that was proving effective on dirt bikes. They then developed a full-suspension pivot frame. In spite of the advantages of pivot-frame suspension on mountain bikes, disadvantages exist with certain designs.
Pivot Frames
Not all pivot frames are alike in terms of frame configuration. This might be because manufacturers attempt to attract consumers with trick designs. Looking at the frames on full-suspension mountain bikes, you will notice that all have the distinctive coil-over mono-shock below the rear of the seat. You'll also notice the rigid frame arms at the rear of the bike have been replaced by swing arms that connect to a pivot point, or housing, that's near the bottom bracket. The mono-shock and swing arms comprise the rear suspension on pivot-frame mountain bikes.
Swing-Arms
The length of the swing arm is important when you select a full-suspension mountain bike. Imagine the pivot point on the frame as one of the points of a triangle, and the outer ends of the swing arms traveling up and down between the outer points of the triangle. If you wanted a stiffer ride, you would select a bike with shorter swing arms that don't have a lot of travel. If you wanted a softer ride, you would opt for longer swing arms that have a greater amount of travel. But the location of the pivot point that houses the swing-arm mechanism might be more important than the particular length of the arms.
Pivot Point
A low pivot point has the housing for the swing-arm mechanism positioned at the same height or below the bottom bracket. A high pivot point is above the height of the bottom bracket. A low pivot point has certain disadvantages in wet, sandy or excessively bumpy terrain. In wet conditions, the housing can take on water or mud that's thrown from the rear tire, causing the swing-arm mechanism to lose lubrication or bog down. In this event, dry-out and lubrication, or cleaning the mechanism, is necessary. In sandy conditions or in loose soil, the housing is vulnerable to invasion that can damage the swing-arm mechanism beyond repair. If you ride in excessively bumpy conditions over long periods of time, a low pivot point causes you to work harder at cushioning impact because the design does not afford maximum suspension travel.
Observations
Full-suspension mountain bikes and pivot frames have all but replaced other designs that once occupied showrooms and mountain bike trails. Most likely, you won't see a lot of bikes that have a low pivot point because of the disadvantages. Riding a bike that has a low pivot point and one with a higher pivot point is the only way to determine which is best for you. However, you cannot make an accurate comparison in the parking lot of a bike shop. Unless you have an opportunity to test both types on mountain conditions, you might be better off with a bike that has a higher pivot point, and setting up the suspension to suit your purposes.
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