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2000, John S. Allen

No mechanical failure of a bicycle is more hazardous than a front fork failure.

I have been retained in a lawsuit over failure of a "Positrack 7000" suspension front fork, original equipment on a Pacific Nomad all-terrain bicycle. In connection with the case, I have also briefly examined forks labeled "Ground Assault," used on 26" wheel ATBs and 20" wheel children's bicycles. From what I can tell without disassembling them, the "Ground Assault" forks appear to be of similar construction, though I understand that not all forks bearing any of these labels may be of the same design as the failed one.

Pacific off-road bicycles are sold in Toys R Us stores and are very common.. They have features typical of more expensive bicycles sold in full-service bicycle shops --  lightweight wheels with aluminum rims, front suspension and sometimes rear suspension.

Below: bicycle lacking the slider and front wheel
assembly which came off.

wholbike.jpg (25846 bytes)

Fork crown breakage has already led to recalls of several models of forks including Pacific forks labeled "Positrack 7000" (see description on U.S. Consumer Products Safety Commission Web Site). But the failure I observed is different.

As with many suspension front forks, each fork blade of the Positrack fork is a telescoping assembly. Posts called "stanchions" attached to the fork crown slide up and down inside tubular "sliders" attached to the front hub.

The lower end of each stanchion is closed except for a small central hole, and presses down on a stack of elastomer "donuts" inside the slider. (Elastomer: a rubberlike substance).

An elastomer suspension like this inherently has a "bottom-out" characteristic that becomes progressively stiffer as heavier loads increasingly compress the donuts.

There must also be a "top-out" assembly to prevent the fork from overextending when not under load. In the Positrack fork, a central bolt extends up through the stack of donuts, through the small central hole in the closed lower end of each stanchion, then through a another elastomer donut and a washer. When the fork is fully extended, the washer compresses the donut against the end of the stanchion. If the top-out assembly fails, the sliders can come completely off the stanchions.

Two publicized recalls of other models of suspension forks relate to central bolts' not being sufficiently tightened or secured: see CPSC, Rand/Ross Bicycle Co. Announce Recall to Repair Mountain Bikes (1998) and CPSC and Giant Bicycle Announce Suspension Fork Recall (1997).

The problem with the Positrack fork is different yet: the top-out washers are made of a pliable (waxy-feeling, possibly ABS) plastic material that is not able to withstand repeated top-out impacts by the small-diameter bolt head. The photos below and on the next page show how the washers failed.

View of broken top-out limit stop washer as found inside right stanchion
after removing end plug. The washer wore, then sheared and split
radially as the bolt head passed through it.

inside.jpg (13974 bytes)

Top of slider showing bent rim (at lower edge)
due to jackknifing of fork as it came apart.
Small-diameter bolt head is visible inside slider.

sleeve.jpg (21427 bytes)

Additional photos


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Contents 2000 John S. Allen

Last revised 28 October 2001