Top Document: Rec.Bicycles Frequently Asked Questions Posting Part 5/5 Previous Document: 10 Off-Road Next Document: 10.2 MTB FAQ available See reader questions & answers on this topic! - Help others by sharing your knowledge by Brian Lee & Rick Brusuelas, 1994 ABSTRACT: Discussion of the differences between suspension stems and suspension forks, and a listing of the pros & cons of suspension stems. DESCRIPTION: The suspension stem discussed here is the Allsop-type, which employs a linkage parallelogram and a spring mechanism to effect shock-absorption. Two models on the market using this mechanism are the Allsop Softride, and a version produced by J.P. Morgen, a machinist based in San Francisco. There is also a version put out by J.D Components of Taiwan (advertised in Mountain Bike Action), however judging from illustrations, this unit does not employ the parallelogram design shared by Allsop & Morgen. Other Taiwanese models may also exist. The Girvin-type stem, which uses a simpler hinge and bumper, will not be directly addressed here, although some of the comments may also apply. The Allsop-type suspension stem (suspension stem) works on a different principal than a telescopic shock fork. Instead of only the front wheel moving to absorb shock, a stem allows the entire front end of the bike to move with obstacles while the rider's position does not change. All suspension requires some form of "inertial backstop" to operate. A theoretical suspension (stem or fork) loaded with zero mass will not function regardless of the size of obstacle encountered. This is because there is nothing to force the compression of the spring mechanism. It is essentially locked out. In a fork system, the weight of the bike & rider both provide the inertial backstop. In a stem system, the rider's weight on the handlebars provides the backstop. Because of this, the two systems "ride" differently. Since most of the weight comes from the pressure of the rider's hands, the stem system encourages a more weight-forward style of riding. Or perhaps placing the stem on a frame with a shorter top tube so the rider's weight is distributed more on the front end. (Shortening the front end has also been applied by frame builders on frames intended for use with suspension forks. Ex: Bontrager.) What does this mean to you and me? It means the suspension stem requires a certain amount of the rider's weight to be on it at all times in order to remain completely active. For the majority of riding, it's just fine. The only difference is in extremely steep descents, where you are forced to keep the weight back in order to keep from going over the bars. In this situation, much less weight is on the bars to activate the stem. Further, if one were to encounter a largish rock on such a descent, what does one do? The instinctive thing is to pull back a bit to unweight the front and help the front wheel over. This removes all the weight from the stem area, and you are now riding a rigid bike again. A fork system is also affected by weight shifts, but not quite to the extent that a stem is affected, because of the weight of bike & rider coming through the head tube to be distributed into the fork. Even if you were to remove your hands from the bars on a gnarly descent and hang with butt brushing the rear wheel, you are still applying weight to the bike through the pedals. All this, of course, is theoretical and YMMV. I, for one, am not always able to react to obstacles coming at me and leave the front end weighted. When that happens, I'm very glad I have suspension. Now enough theory stuff, here's a summary of the advantages & disadvantages of suspension stems: PROs 1) Lighter than a suspension fork. This depends on the existing stem/fork combination. If the current stem and rigid fork are heavy, then a suspension fork may be a better choice. For example, I've chosen the following items for comparison, as they represent the lightest and heaviest of commercially available stems & forks (weights for all stems are for conventional types - non-Aheadset): Litespeed Titanium 211g Ritchey Force Directional 375g Allsop Stem 625g Fat Chance Big One Inch 680g Tange Big Fork 1176g Manitou 3 1360g Lawwill Leader 1588g So say you have a Litespeed stem and a Fat Chance fork. The combined weight would be 891 g. Switching to an Allsop would change the combined weight to 1305 g, while a Manitou 3 would bring it to 1571 g. The Allsop has a weight advantage of 266 g (9.4 oz). OTOH, if you have a Ritchey stem & Tange Big Fork, the original weight would be 1551 g. Allsop stem => 1801 g. Manitou 3 => 1735 g. In this case, keeping the boat anchor of a fork and switching to the Allsop would be a weight penalty of 66 g (2.3 oz.). Of course, YMMV depending on your original equipment. 2) Does not affect frame geometry. A suspension fork retrofitted to a frame, *not* designed for suspension, raises the front end - sometimes as much as 1". This reduces the effective head angle and slackens the steering, slowing it down. This is especially true for smaller sized frames which, with their shorter wheel base, are affected to a greater degree by the raising of the head tube. A suspension stem provides suspension while preserving the handling of the bike. 3) Torsionally rigid fork. Telescopic forks all have a certain amount of flex to them, and the sliders are able to move up & down independently. This aspect of front suspension forks has spawned a new line of suspension enhancing products: stiffer fork braces, and bigger, heavier suspension hubs. All to stiffen up the fork. This is one reason suspension stems are favored by some riders who ride lots of tight, twisting single track. 4) No stiction. Stiction, or static friction, is friction that exists as the fork sliders rub against the stanchion tubes. This friction is an extra force that must be overcome for a fork system to activate. Not a problem on large hits. But more of a problem on small- and medium-size impacts. Because the stem has none, the stem responds better to small, high-frequency bumps (washboard) than many air-oil forks. 5) More boing for the buck. The Allsop stem provides up to 3" of stiction-free travel, at a cost of about $250, depending where you go. The majority of forks in this price range only offer 1" - 2" of travel, and are often heavy, flexy, and fraught with stiction. The fork could be stiffened, but at the additional cost of a stiffer fork brace or perhaps a suspension hub and a rebuilt wheel (e.g. fork brace - $90; hub - $80; rebuild - $100. Plus the original $350 for the fork. YMMV). 6) Better "feel". The stem allows you to have a rigid fork, which transmits more "information" back to the rider. This is a benefit when riding through creeks where you cannot see where your wheel is. 7) Less exposed to the environment. The stem is higher, more out of the way than suspension forks. Thus you can ride through creeks and mud without having to worry about your fork seals, or about contaminating the innards of the fork. Even if mud splashes on a suspension stem, the pivots are less sensitive to grit than sliders and stanchion tubes. 8) Ease of maintenance. There are no seals to replace or service, no oil to replace, no air pressure to adjust, and no bumpers to wear out. An occasional lube of the pivots is all that is needed. An extension of this is the ease of initial set up. For best results, you have to set suspension (fork or stem) to react according to your weight and riding style. With air/oil forks you may have to change oil, adjust pressure or change damping settings (if the fork has them). With bumper forks you may need to swap out bumper stacks and mix-n-match bumpers until you get what works for you. With the stem, the only adjustment is to increase or decrease the spring tension with an allen wrench. CONs 1) No damping. This is one of the main complaints from proponents of suspension forks. The suspension stem will give way to absorb shock, but the return is not controlled and cannot be adjusted. JP Morgen currently makes a suspension stem which employs oil-damping, but Allsop does not. 2) Requires adjustment to riding style. As mentioned above, the stem requires weight to be applied to it to function. This is also one of the complaints applied to the Softride rear suspension beam. The flip-side to this, according to riders of the Beam, is once the adjustment is made to "plant your butt on the saddle" the ride is extremely comfortable and affords excellent control by sticking the rear tire to the ground. 3) Stem "clunks" on rebound. The feeling is about the same as suspension bottoming out, except it happens on the rebound. This is not as much a problem on the Allsop as on the Morgen stem, which uses a hard plastic top-out bumper. This is a subjective complaint, as some riders claim not to notice it. 4) Stem not torsionally rigid. Another trade off. The stem is not proof to twisting forces and may be noticeable in hard, out-of-the-saddle efforts. Allsop has redesigned the top beam of their aluminum stem for 1994 to address this problem. Instead of the aluminum "dog bone" structure for the top linkage member, they've substituted a machined aluminum beam, reminiscent of a cantilever bridge. SUMMARY: In my opinion, a suspension stem is an excellent choice if one is retrofitting an existing bike, which has not been designed around a suspension fork. A suspension stem is also a very good choice if one's primary riding is twisty singletrack, where you need the sharp, precise steering of a rigid fork. There are undoubtedly situations for which a stem may not be ideal, but stems should not be dismissed as a viable form of suspension. The best thing to do is to try both types of suspension if you can, and see what you like better. User Contributions:Comment about this article, ask questions, or add new information about this topic:Top Document: Rec.Bicycles Frequently Asked Questions Posting Part 5/5 Previous Document: 10 Off-Road Next Document: 10.2 MTB FAQ available Part1 - Part2 - Part3 - Part4 - Part5 - Single Page [ Usenet FAQs | Web FAQs | Documents | RFC Index ] Send corrections/additions to the FAQ Maintainer: iglesias@draco.acs.uci.edu (Mike Iglesias)
Last Update March 27 2014 @ 02:11 PM
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