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Technical FAQ: Designing frames and components for big riders

In Technical FAQ, Lennard Zinn details the design changes necessary to increase safety and performance for larger riders

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This week, Technical FAQ will focus on a single reader letter touching on a topic important to me: bicycle and component design for large riders.

Dear Lennard,
Biking blogs often contain debates and discussions about road bikes (and bike parts) for “tall” riders, those 6-foot-4 to 6-foot-9, with weights of 220-290 pounds. Given your own experiences and work (including the great design of the KHS Flite 747), I was hoping you could write about the best equipment and technology for tall guys. Apart from custom builds, what frame (steel, carbon) and components (e.g., cranks, bars, brakes, wheels, tire size) do you recommend for, um, real men?

Tell us what you think, even if it’s about a subset of the issues.

Dear Mike,
All right, you asked for it, and I don’t know how to answer this in a couple hundred words.

You’re the only one that would know this, Mike, but I’ve had this question from you a very long time. I’ve pondered many times how to answer it, and my dilemma has not been for lack of information; rather, it’s been because I have so much experience in this area, since I am dealing daily with riders of this size. The way I see it, there is a fun part to this question and a serious part, and it is the latter where I’ve had difficulty finding my voice. I suppose that’s for fear of being a downer when cycling is supposed to be fun. It’s fun talking about performance, and it’s serious talking about durability and safety, and riders of the sizes you are talking about test the limits of both.

Fit: Poor bike fit is the first obstacle big riders encounter when seeking a decent ride. I believe that more than just the frame needs to scale up for the bigger rider. I think that the cranks, handlebar, stem, seatpost, fork steering tube, tires (and, if possible, wheels) should also scale up in size for the big rider. Little bikes for little kids have tiny wheels and very short cranks, as well as tiny frames and handlebars, and as bike size increases, so do the wheel diameter and crank length.

To me, it only makes sense that this would also be the case in adulthood as well. On mountain bikes, good wheels and tires are available from 20-inch all of the way to 29-inch, and, arguably, also in 36-inch. However, the crank length is often the same on all bike sizes, and on the road, the crank-length variation generally available is tiny relative to the leg length range of people. And, while smaller riders have decent choices in 24-inch and 26-inch, a 700C road wheel is the size for everybody else.

Handlebars for mountain bikes come in any length you could want, but for road bikes, other than a tiny smattering of 46cm choices, drop bars stop at 44cm center-to-center width. For this reason, Zinn Cycles sources bars in Taiwan in 47cm and 48cm widths.

Cranks over 180mm length are not easy to find, and even 177.5mm and 180mm are only available in a model or two from the big manufacturers. There are some other options out there at 185mm, like T.A., and Zinn makes integrated-spindle cranks up to 215mm and offers square taper cranks in virtually any length.

Frames obviously need to go up in length and height for big and tall riders. However, simply scaling them up does not make for a bicycle a big and tall rider can feel safe and confident on, and it won’t also offer high performance in terms of translating pedaling inputs into forward motion efficiently. See the sections on stiffness and high-speed shimmy below for more on this.

Forks obviously also need to be stiffer and have longer steering tubes for big riders. First of all, most road forks on the market have a maximum steering-tube length of 300mm, which is insufficient for riders much over 6-foot-5 or so, especially as they age or are overweight and require a correspondingly higher handlebar relative to their saddle. Secondly, one thing that contributes to the flex of a tall bike, and consequently to shimmy issues as well, is flex in the steering tube of the fork, inside of the head tube, since the tube is so long and only constrained at either end by the headset bearings. This is why Zinn Cycles has for decades had carbon forks made for us with 450mm steering tubes that are twice as thick as those of most carbon forks, first by Alpha Q and then by Serotta. Now that Serotta is gone, I’m not at all sure what I’ll do when we run out of our stock. Enve offers forks with 350mm steering tubes, both tapered and straight, and WoundUp offers forks with custom-length steering tubes. On mountain bike forks, 10-3/4-inch (273mm) steering tubes are standard, but White Brothers offers fork steerers in custom lengths, both tapered and straight.

Wheels need to be very stiff and strong for big riders. This means high spoke counts, stiff rims, and steel spokes. Obviously, tires should get bigger for heavier riders.

Seatposts need to be longer for taller riders and ideally should be larger in diameter and stiffer and stronger. Stems generally need to be longer as well and should be stiffened up due to rider weight and strength, as well as the fact that greater length means greater flex.

Flex (i.e., deflection of a part) generally varies with the cube of length. Try it yourself. Clamp a yardstick to a table and put a weight on it a foot out from the table and measure the deflection. Now put the same weight two feet out from the table; the deflection will not double; rather it will be eight times as great. So, a stem of the same construction that is twice as long will have eight times the flex of the shorter stem.

Stiffness: Heavy riders flex weight-bearing members of bicycles more, be it wheels, frames, forks, handlebars, stems, seatposts, or cranks. And for tall riders, those parts need to also be longer in order to fit them properly. However, for the same reasons as mentioned above, namely that flex is related to the cube of the length, this extra length combined with the big loads they will bear really requires a big increase in diameter of those members. Regarding frames, see more on this under the section on shimmy below.

High-Speed Shimmy: Tall riders generally are plagued by front-end shimmy of their bikes when riding with no hands or even with the hands on the bars at elevated speeds. This is due to long, flexy frame tubes unable to withstand the twisting forces generated when moving at high speed or when not constrained by the rider’s hands on the bars. The heavier the rider, the worse the problem, because it’s related to the resonant frequency of the bike and rider system, which is reduced the heavier the rider is. Simply making a frame taller and longer does not cut it.

I have discussed the subject of front-end shimmy a lot in this column going back well over 10 years. Here’s a bit more comprehensive discussion of it from one of my newsletters.

The way I deal with both shimmy and stiffness for pedaling performance on frames for tall and heavy riders is by increasing tube diameters, relaxing the head angle (so the fork transfers less shock into the frame), and shortening the tubes while maintaining the proper relative relationship of the hands, feet, and rear end. The latter I accomplish by shortening the seat tube from the bottom by raising the bottom bracket for longer cranks and by shortening the seat tube from the top by sloping the top tube, and meeting it lower on the head tube. These modifications also shorten the top tube, seatstays, and down tube without affecting the frame’s effective size. Finally, the wheels need to be stiff and true, and the fork steerer needs to be stiff.

Durability and Safety:
This is the part where the discussion can get heavy (pun intended). Big riders can be at great risk when riding on equipment made for much smaller people. They often put much more stress on parts than they were designed for, and they sit high and have a long way to fall if something breaks.

I focus my frame- and crank-making business primarily on tall (and often quite heavy) riders, and some of our big customers break a lot of bike parts. That’s what inspired me to post this explanation of metal fatigue failure on our website with the recommendation for riders weighing over 250 pounds to replace weight-bearing components at least every three years.

A big customer of ours, Ian, who rides a lot and travels all over the world to ride his bike said it well in a letter to me after coming across this gallery I produced on a carbon handlebar I broke last year; here’s the follow-up on that gallery.

Ian is very big, at 6-foot-7 and 330 pounds. He is extremely strong, and, despite a large amount of riding, his weight remains high. However, his power and speed and ability to climb big mountain passes in the Alps and Pyrénées have increased substantially. Since we built him his first titanium bike (a coupled one for traveling), this is the list of parts he has broken, in his words:

Current list of components broken: bottom bracket, crank, chainrings, front derailleur, stem, bar, chain, rear cassette, seatpost, seatpost clamp, hubs, rims, spokes — so [I’m] keen to optimize for strength and reliability. On the positive side, my drink bottle holders are holding up fine.

While I can beef up his frames appropriately, I’m limited in the components I can put on his bikes to what’s commercially available. As you’re probably aware, getting heavier components often does not equate to greater strength and longevity, either. In the bike business, especially on the road, heavier often equates to less expensive parts made with cheaper materials and processes; the longevity of them may not be any higher.

In a letter, Ian described to me multiple broken frames from a major manufacturer and called for an increase in safety standards for frames and components. After switching to titanium, he has begun stripping his bikes every “12 months, throw away all the wear components and replace with new. So, new bottom bracket, stem, bars, seatpost, seatpost clamp, saddle, cranks, chainrings, chain, rear cassette, rims, hubs and spokes. Also to use Ti frames only — hence my order for three bikes from you.”

Many bicycle components undergo an EU-specified fatigue test by the CEN that they must pass in order to be sold through commercial channels in the EU (which thus protects American consumers as well). In my opinion, these tests generally do provide a satisfactory margin of safety for most bike riders — certainly for those under my weight (170 pounds), as I ride hard and generally don’t break parts.

However, Ian is close to double my weight, and, for instance, one EU frame fatigue test requires the frame to survive a load of 1,100 Newtons (N) repeatedly applied for 100,000 cycles to the pedal. Divide that force by the acceleration of gravity, and you find that 1,100N corresponds to a 112kg mass weighting the crank by gravity alone. A rider like Ian, at 150kg, obviously doesn’t need to pedal very hard to produce that 1,100 Newtons of force down on the pedal, and he applies a 33-percent higher load of 150kg X 9.8m/s2 = 1,470 N simply by standing on the pedal without even pulling on the bar! That load will certainly accelerate the rate of fatigue. And, if he’s applying the full EU-specified load on each pedal stroke at 80 RPM, 100,000 pedal strokes is only 1,250 minutes of riding, or 21 hours; that’s not much riding. For me, that’s less than two weeks, even at this time of year.

When it comes to big riders, I have similar concerns about the static strength and security tests specified by the CEN for bicycles.

Also, keep in mind that these laboratory fatigue tests are placed on clean, new, non-corroded parts. But Ian lives near the sea, and all of the steel and aluminum parts on his bike tend to corrode very quickly as well. While one can’t say how much that also shortens their lifespan, one can at least say that it certainly doesn’t add to their longevity!

So, Mike, there’s a very long answer to your question. As you can see, I think that certain design elements are important to produce performance for big and heavy riders comparable to what smaller riders can expect, and I think frequent replacement of a big rider’s bike equipment is a must to ensure safety or at least to spare them the hassle of dealing with broken stuff while out riding.