Words: Taj Mihelich
"PING!!!" I fire out of the 90-degree catch berm that I had just slammed into. That sharp metallic noise that is echoing through the woods came from my fork. I can feel that it is suddenly sagging low, and before even hitting the brakes, I bounce the much-too-soft front end to confirm that yet another fork has given up on me (this time, it ends up being a sheared compression damper).
This kind of thing happens way too much, and in a flood of frustration, I curse mountain biking and all its stupid, overly complicated parts. "Why can't they make parts that last!" My short riding seasons in the Upper Peninsula of Michigan are perennially interrupted by failed bike parts. I've never had a fork or dropper survive a summer. I measure the lifespan of my shocks in weeks, not months. Cassettes wilt underneath me.
Maybe I'm doing something wrong? My pro BMX career indeed developed a riding style that my bikes would call "unsympathetic." When things were on the line, my mindset was, "my bike will
bend to my will (or simply bend)."Still, I pushed my BMX bikes hard most every day and they never seemed to break (even though the whole bike cost less than my now limp mountain bike fork). It wasn't until my pro retirement that I got on a mountain bike and all the bike failures began. And that doesn't seem right to me. I'm a 48-year old man with a fused lower back who sits at a desk most of the time– I don't think that I'm riding that hard!
A couple of summers ago, I rode a shuttle up at Copper Harbor with two other big men who I'd guessed were in the same 250lb (113kg) club as me. I jokingly said, "I bet you boys have broken some bike parts." An avalanche of stories with failed suspension and crumpled parts entertained me to the top of the mountain. They were having the same experience as me. It seemed I wasn't alone. It made me wonder if mountain bike designers worried so much about product weight and performance gains that they had neglected long-term durability for the Clydesdale class?
Frustration drained out of me as I cooled down on the slow, mushy-fork ride home. With a clearer head, I admitted it isn't fair to compare my BMX experiences to MTB. For one, thanks to good sponsors, BMX parts were free. To be safe I regularly changed out "wear parts" like handlebars, forks, cranks, and frames before they showed any signs of fatigue. And, perhaps most critically, I had to acknowledge the painful truth that I wasn't this heavy when I rode bikes for a living. I don't know when it happened, but the shift in my life from riding all day to sitting at a desk all day added some pounds. I wondered, "do bike parts have a weight limit?" My Zwift trainer and my pull-up bar both have a 250lb user limit (which means I'm over the limit if I wear clothes while working out).
Contemplating the design challenges faced by mountain bike engineers made me realize they deserve some sympathy. Imagine trying to design a product that needs to work for a rider who weighs 90lbs (41kgs) or one as heavy as me? And what about riders heavier than me? Their task seems impossible—design a product their pro team wants to use, and every other kind of rider too, oh, and don't forget Taj, the overweight, washed-up BMXer who wants his bike set up unreasonably stiff. I decided to reach out to some designers/ engineers to learn more about the seemingly insurmountable challenges they face.
I hit up my friend George French first. He's one of the engineers who helped transform the dangerously weak BMX bikes of the early 90's into the bikes of nowadays that you can throw off a building. I wondered if he could explain why my BMX held up better than my MTB costing ten times as much.
Why are BMX parts so strong, yet so inexpensive (compared to MTB)?Well, I guess we've had longer to evolve is part of it, and strength has always been a big driver. "Back-in-the-day" everything was shockingly weak; as an aspiring designer I had personal experience of multiple examples of possible failure modes so I knew intimately the things that needed to be stronger, I also knew what was insufficient.
Since then the basic "mechanism" of a BMX hasn't really changed so that data is still relevant.
We've learnt that the standardized testing was almost worthless, passing the standard tests was a formality and not a guarantee that things were strong enough. Ultimately we decided as a company to develop our own more rigorous tests, often breaking the test machine in the process, and designing jigs to magnify the load that we could put on a component so it would be a worthwhile test.
MTB design is younger and has changed (and continues to change) a lot. Just to take one obvious example, head angles have slackened out by multiple degrees and suspension travel has crept up, that causes huge changes in the loads we exert of the front end of the bike. But changes in suspension layout are also very significant in how and where loads are applied. There are also variables associated with how the rider sets up their bike and where in the travel the loads are applied.
Another advantage we have with BMX is how compact the bike is and how short any cantilevered structures are. A BMX fork sticks out a lot less than an MTB fork and the bigger wheel means that the contact point is about twice as far away from the head tube as on a BMX, so has twice the bending moment.
Another factor is probably weight, BMX's these days are really very light, and we do chase the weight, but we stick with an amazing material called steel that will take a huge amount of abuse and can handle a lot of energy (we make springs out of it after all). Steel is relatively heavy, but we don't need a lot of it, we don't have lots of extra pivot points and shock mounts that need to take fasteners and would put the weight up a lot. Steel is also relatively inexpensive and because everything is small we have less of it and less welds, all of which helps keep costs down.
MTB designers always have to have weight in the front of their minds, and those fancy machined pivots and shock mounts contribute a lot of weight even in aluminium or carbon because they just have to be quite chunky to accept fasteners etc. This puts more pressure in other areas to save weight.
BMX also doesn't have all the competing sizes and standards to deal with. We do two fork rakes and that is it. RockShox do something like a hundred thousand different possible fork combos, for multiple headtubes, wheel sizes, tyre widths, rakes, axle sizes, hub widths, brake mounts and travels etc. All this choice adds complexity and expense at manufacture, in distribution and for the shops at the end of the line. BMX has (mostly) ONE BB standard, ONE headset fitting, ONE wheel-size, etc etc.
-George French, Mechanical Engineer @ Odyssey and G-Sport
It wasn't that long ago that I thought a seat post that sprang erect at the push of a button seemed hilariously unnecessary. Now having a dropper seems absolutely essential (but still makes me giggle). Droppers live a hard life, though, enduring a rider's weight bouncing around on rough climbs. I shudder to think of the torture my posts have suffered sandwiched between my bulk and a hardtail. I asked Quinn at OneUp Components for his thoughts.
What are some of the challenges of making parts (droppers) that need to work for a huge range of rider weights? The challenge is to make a reliable and affordable post with the shortest possible stack, longest drop, lightest and smoothest action and no play. When it comes to accommodating a wide range of rider weights, there are a couple of specific things we account for. For heavier riders who put more sideload on the post, you want to maximize bushing overlap and get the tightest tolerances possible. For lighter riders, you want to minimize friction and reduce the minimum force required to drop the post. Striking that balance in the confines of a really short stack makes it even harder. On top of that, for every post length and diameter, there are different frames, clamps and ride heights we have to design for. We also offer an oversize pin kit that lets riders “tune” the feel of their dropper and reduce any play that develops over time.
Is there a weight limit on your products? No weight limit.
I noticed this blurb in the instructions for adjusting post travel, "Lowering the post travel also increases bushing overlap and alignment pin length, which is great for heavier riders." Should riders over a certain weight choose less dropper travel to prolong the post's life?This is a tough question because it depends on more than just rider weight. Riding style and bike design are also factors. We all have buddies who just break stuff (some who seemingly take pride in it). And there are still plenty of frames out there with very slack seat tube angles, which increase sideload and the possibility of binding, especially for heavier riders.
What that means on the trail is that an aggressive 170lb rider on a bike with a slack seat tube angle could cause more wear and tear than a smooth riding 260lb rider on a steep seat tube. Of course, an aggressive 260lb rider on a slack seat tube angle who frequently smashes up rough climbs with their post at full extension will accelerate wear and tear further. Our design allows you to increase bushing overlap to suit your needs and riding style.
Quinn Lanzon, Marketing Director @ OneUp Components
Jon Staples, Owner, Engineer, and Designer of OneUp's dropper
I am woefully unqualified to offer product advice of any kind, but I recently got a Manitou Mezzer Pro on the front of my bike and like it a lot. Cranking the pressure in the upper air chamber seems to work well at keeping the fork supportive under me but doesn't feel harsh as it ramps up. I hit up Phil from Manitou to try and learn about how his crew tackles the design challenges of varied rider weight.
Do your products have weight limits?Mountain bike weight limits are typically prescribed by the bike manufacturer. There are several governing bodies that issue regulatory requirements for bicycle safety. Depending on the rider weight and/or duty load of the intended use of the fork, Manitou tests to elevated requirements to ensure safety and best performance of our forks. Rider weight or gross vehicle weight (GVW) is only one aspect of duty load of a bike. A heavier rider on an XC bike may have a lower duty load than a lighter rider on a dirt-jump fork. This is taken into consideration during the design process and backed up by our data acquisition systems that uses accelerometers and strain gauges to correlate g-forces and material stress for various GVW and intended use of the bike.
What are some of the challenges of making a fork for a huge weight range? There are a few main challenges with designing a fork for a wide rider weight range. The first is chassis size and stiffness; in general, the larger the stanchion size the longer travel and more aggressive riding the fork is designed for. Manitou has designed some overlap into the travel offerings of the forks so that the rider can choose a chassis size, stiffness, and weight that meets their specific needs.
After a chassis is chosen, the next attribute we have spent a lot of time designing to be very adaptable is the air spring system. The Manitou Dorado Air system is unique as there is no transfer groove to charge the negative chamber, the positive and negative chambers are equalized during the air fill process. This gives two riders with very different weights the same initial rate and fork feel, this is not the case with other designs. The next level of this tuning is the IRT (Infinite Rate Tune) air spring, which is essentially a secondary positive air chamber that controls the mid to end stroke of the spring curve. The IRT allows a wide range of rider weights and riding styles to have their preferred sag while still using full travel.
Another component that is adaptable for both rider size and riding style are the rebound and compression dampers. First, the dampers are designed and tuned with a specific use in mind, ie an XC fork will have a lockout where a DH fork will not. The rebound dampers are designed to accommodate a large range of riders, on the Mezzer fork the 9 clicks of rebound damping is sufficient for a 100lb rider through a 220+ lbs rider to find a suitable setup. For outliers, pro riders, and enthusiasts with specific needs the rebound dampers are able to be custom tuned to suit their needs.
Let's say a rider is 250, 300, or 400 lbs. How can they expect their fork to perform?As a rider's weight increases over the intended weight and application range a few things may happen. One is the air spring may not be able to fully satisfy their needs, a heavier rider may not be able to achieve their desired sag for the allowable air pressure. With either the IVA or IRT system will help the overall travel use, but the initial stroke feel may be compromised. As air pressure increases rebound damping force must increase to counteract the high spring rate. A heavier rider may not be satisfied with the rebound control as it may be too fast. This would require a firmer shim stack tune in the rebound circuit to shift the rebound range slower. If the heavier rider with a custom rebound lent their bike to their much lighter friend, the rebound would likely have shifted too slow and even an open rebound setting may not satisfy the new riders needs.
To summarize, certain aspects to the forks design like Max air pressure and ISO tested GVW weight rating are non negotiables. The rider would want to find a chassis that is tested to their weight range and needs, but the damper tunes are adjustable by a qualified Manitou service center or enthusiast end consumer to meet the riders needs.
Phil Ott, Product Manager @ Manitou
It was a eureka moment that made me fall in love with mountain biking when I figured out how much suspension could increase grip. Suddenly a dusty and bumpy berm could feel like a smooth, Skatelite layered, bowl corner on a ramp. Diving in and forcing my tires to grip is one of the best feelings I can think of on a bike. Dave and Bronson from Rock Shox were kind enough to answer some questions about how they make that magic happen.
What are some of the challenges you face trying to create a product that a wide range of rider weights can use? For rear shocks, it's tough to make a product that a light rider can set up with 30% sag and still use full travel regularly, and have the same product for a 250-lb rider without bottoming out often. Volume tokens and different air can options are a big help and we spend a lot of time analyzing the data to best accommodate a wide range.
Most forks and rear shocks tie the design of the air spring closely to the design of the chassis. This means that the positive volume (main acting spring), negative volume (between the piston and seal head), and casting volume (what is left over in a fork between the lower leg and the upper tube) need to all work together; an increase in one often leads to a decrease of another. We need to leave enough room in the chassis design to allow for appropriate air volumes, but also ensure we're making a product that is the correct weight and stiffness. For a lighter rider, casting ramp can add too much ramp to the spring and make it difficult to run a comfortable sag point and use the spring effectively. On the flip side, a heavy rider is going to bottom out too easily unless they add more air pressure, which also impacts their comfortable sag point. Tokens are a straight forward solution for this situation, as well as making adjustments to rebound settings - finding the right rebound settings for air pressure and ride style can help make the suspension feel more balanced.
Is there a weight limit on your products? A frame's leverage rate will have a huge effect on rider weight limits and their appropriate setup at max air pressure. We err on the side of caution based on the weight limitations given to us by the OE manufacturers; therefore, there is no weight limit per se on RockShox forks or rear shocks - only an air pressure limit and token count.
How does suspension perform differently for really lightweight riders vs ideal weight (if there is such a thing) vs heavy riders?For lightweight riders, friction plays more of a role in the overall ride feel, and as a result, they are generally stuck with an overdamped setup that has a hard time using full travel. Heavier riders overcome friction more easily but are more likely to bottom out, ride deeper in the travel, and generally experience an underdamped setup. Riders in the middle of the bell curve of bodyweight have suspension designed for them.
Setting up my bike to factory specs at my weight puts me pretty close to max pressure. Coming from the fully rigid world of BMX makes me want things even stiffer and so I'm often right near the limit. I imagine that is why I have so many suspension failures?Running suspension at its maximum pressure in conjunction with a heavy/aggressive rider performing rad BMX moves is hard on product. But in reality, catastrophic failures are very rare and it's usually the result of a crash, casing a jump, or running into something.
Dave Camp, Senior Design Engineer @ Rock Shox Rear Shock
Bronson Stagner, Design Engineer @ Rock Shox XC Front Fork
I've spent my life on bikes, and so it is strange for me to discover that I'm on the edge of who mountain bikes are designed for. There were some hints in the manufacturer's responses about steps to make my equipment hold up better. For part 2 of this story, I'll dig deeper, focusing on practical advice (from the experts) for setup and durability.