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Technical FAQ with Lennard Zinn: Why tall bikes often handle poorly

  • By Lennard Zinn
  • Published Jun. 15, 2010

Dear Lennard,

I am 6’6” tall, and I should probably concentrate on basketball, but for better or worse I’m hooked on cycling. I’m also an engineer, so I like to understand how things work.

It seems to me that just by looking at medium-sized frames and extra large frames, the proportions are not the same.

Really big frames seem tall, but proportionally short front-to-back.

I also found by crunching numbers that larger frames (with higher saddle positions) have less offset from the rear wheel (axle) to the saddle (at centerline of seat tube) than smaller frames. 
Medium-sized riders seem to be positioned in a relatively neutral point between the wheels, where tall riders seem to be quite high above the cockpit and skewed towards the back wheel.

I personally noticed a big improvement in feel when I switched from a standard mountain bike to a 29er, where now I’m more ‘in’ the bike and not so much ‘on top of’ the bike.

I am interested in upgrading to a moderately-priced stock carbon frame, hoping for a lively feel that is quick but not unstable, and not a sluggish cruiser.

So I am scanning the geometry charts of the manufacturers, looking for the best fit.

So my question is: are there any generic rules about front-center, wheelbase and saddle position relative to the rear wheel (in proportion to frame size or rider height)? 
How does the position of the saddle relative to the rear wheel affect bike handling?

My current bike (Giant TCR 58.5) gives me only 137mm of offset from saddle to rear wheel due to short 405mm chain stays and slack 72-degree seat angle. Wheelbase is 1021mm.

Most of the bikes I see have the saddle more in front of the rear wheel due to longer stays and steeper seat tube angle. (149mm to 159mm). None of them have the rear wheel tucked under the rider as much as mine does. Does this make much difference in handling?

Is a longer wheelbase, longer front-center, and more centered saddle position important for handling for a tall rider?

I test rode a 58 Specialized Tarmac (no 61’s were available to test) that felt quick, but stable. It has steeper angles and a shorter wheelbase than my current ride …

-Joe

Dear Joe,

You’re living in my world. I also fell into cycling due to a profound disinterest in ball sports and was also 6’6” until I lost a couple of discs in my back. However, most people don’t notice the idiosyncrasies of stock big bikes that make them so much less than ideal relative to what people 6-to-12 inches shorter experience.

After a stint on the national team when I was subject to the whims of tall frames provided by sponsors, I couldn’t stand it anymore and started building my own frames. My tolerance for high-speed shimmy as well as for the saddle cantilevered way back over the rear wheel, as you point out, eventually came to an end.

There is not a good answer to your question about generic rules about front center, wheelbase, and saddle position relative to the rear axle for tall riders on stock bikes, because all of the options you are likely to find in a frame that best fits you make for a poorly handling machine.

The fact is, most stock tall frames have a ridiculously slack seat angle so that the knee can get over the pedal spindle. But that’s because the cranks are not proportional to the rider’s leg length; 175mm cranks are very short relative to somebody with an inseam of around 1000mm, as I imagine yours is.

And most manufacturers slap a really steep head angle on the frame (with the same fork rake as all of the other sizes so they don’t have to make another fork mold). That way, they can reel in the wheelbase and front center (bottom bracket to front hub) dimension, both of which are specs that consumers look at when making their buying decisions (seeking smaller numbers, of course).

All of those things together add up to terrible handling and encourage front-end shimmy at high speeds (or at lower speeds with the hands off).

The 58cm Tarmac felt good to you because it IS good. But you didn’t get fitted to that bike, and any fitter would slam your saddle way back in order to get your knee over the pedal, and the nice performance you enjoyed would go straight down the toilet. Bikes with the saddle way back over the rear wheel are very light in the front end; ride a steep climb seated on one and you’ll be pulling wheelies the whole way up. And in a criterium, with so much of your weight over the rear wheel, combined with a tall stock bike’s steep head angle and too much fork rake for that angle, the bike will oversteer while at the same time washing out the front wheel due to lack of weight on it.

Building my own frames, I was able to eliminate the shimmy and improve the steering and stability with good frame design, but after decades of messing around with alternatives, I realized that the key to the decent weight distribution on the bike was the crank length (and the wheel size, but you can’t fix that on a road bike). With a crank length in a similar proportion to your leg length as top pros ride, you no longer need a super-shallow seat angle and/or a seatpost with massive setback, because with a, say, 215mm crank (if your inseam is 100cm), your knee will come out over the pedal spindle with a seat angle in the standard 73-degree region.

Happily, I also found that if I have a crank in a similar proportion to my leg length as Lance Armstrong or Paolo Bettini ride, I also go uphill faster (while not pulling wheelies on the steep pitches) and feel more comfortable, as my muscles and joints are going through a similarly efficient range of motion to theirs.

Why is it that it makes sense to us that little kids’ bikes get progressively shorter cranks and smaller wheels, but when it comes to “adult” bikes, we’re all stuck with the same wheel size and virtually the same crank length? We can all be thankful for 29-inch (and 650B, 26-inch, and 24-inch) wheels for mountain bikes; they allow any size rider to have a proportional wheel size with a correspondingly rational frame design scaled up or down, allowing, as you say, for the rider to fit “more ‘in’ the bike and not so much ‘on top of’ the bike.”

Unfortunately, on road bikes, we don’t get a bigger wheel-size option, and, to add insult to injury, we even have rocket scientists at the UCI telling us that the maximum front center dimension we can race is 65cm. Tell that to 6’9” guy who needs a 69cm top tube. How are you going to get to 65cm from the bottom bracket to the front hub if you have parallel seat- and head-tube angles (which so many 5’9” riders find to be quite stable and nice handling) when you also have to add on the fork rake? Any small or medium-sized bike has a front center dimension longer than the top tube, not shorter! The only way you’re going to get to UCI legality if you’re 6’6” or more is to make the head angle way steeper than the seat angle. And you can promptly kiss your good handling goodbye; hello oversteering!

Some good news for a rider as tall as you is that if you use a proportional crank length (say, 21.5 percent of your inseam length — inseam length is usually a couple inches longer than your pants length, BTW), you can use a smaller frame size.

With more of your pedal-to-spindle length taken up in the crank, you can sit on a smaller bike, which will have the nicer seat angle you enjoyed on the 58cm bike you rode. Unfortunately, there’s no way you can ride a criterium with a long crank on a stock bike. For that, you need a custom frame with a higher bottom bracket.

And while I seriously doubt that a 6’6” guy can be properly fit with a 60-61cm top tube, which is about as much as you’ll find on any stock carbon frame, I’m also aware that people your height have adapted their entire lives to things that are too small – beds, cars, door frames, etc. and don’t actually realize how scrunched up they are until one day they get on a frame with a 65cm or so top tube and a 46cm-wide handlebar and realize how it feels to breathe and stretch out on a bike.

Very few tall riders notice these things enough to say as you have, “that just by looking at medium-sized frames and extra large frames, the proportions are not the same.” Rather, tall riders very often wonder why they can’t handle their bikes like smaller riders, coming to the conclusion that with their height and high center of gravity, they are doomed to being poor bike handlers.

But I can tell you that the high center of gravity is not a detriment. In fact, it is very rare for me to find anyone who descends a winding mountain road as fast as I do. With the proper bike, a big rider can take advantage of some of the benefits that large size brings, just as top skiing downhillers do (try to find a top downhiller on the skiing World Cup who is not at least 6’2”; the higher center of gravity is not a detriment to their ability to hold a sharp turn at speed).

-Lennard

Readers can send brief technical questions directly to Zinn.

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Lennard Zinn is a frame builder (www.zinncycles.com), a former U.S. national team rider and author of numerous books on bikes and bike maintenance including the pair of successful maintenance guides “Zinn and the Art of Mountain Bike Maintenance” – now available also on DVD, and “Zinn and the Art of Road Bike Maintenance,” as well as “Zinn and the Art of Triathlon Bikes” and “Zinn’s Cycling Primer: Maintenance Tips and Skill Building for Cyclists.”Zinn’s regular column is devoted to addressing readers’ technical questions about bikes, their care and feeding and how we as riders can use them as comfortably and efficiently as possible.

FILED UNDER: Bikes and Tech / No Spoil / Technical FAQ TAGS: / /

Lennard Zinn

Lennard Zinn

Our longtime technical writer joined VeloNews in 1987. He is also a framebuilder, a former U.S. National Team rider, and author of many bicycle books, including Zinn and the Art of Mountain Bike Maintenance and Zinn and the Art of Road Bike Maintenance, as well as Zinn and the Art of Triathlon Bikes and Zinn's Cycling Primer: Maintenance Tips and Skill Building for Cyclists. He holds a Bachelor’s degree in physics from Colorado College. Readers can send brief technical questions to Ask LZ.

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