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Technical FAQ: Bike positioning and more on shimmy

  • By Lennard Zinn
  • Published Jul. 8, 2014
Couplers on travel bikes may actually stiffen the frame and reduce shimmy.

Bike fit and saddle length

Dear Lennard,
I have a quick question about bike fitting. I just recently changed my road bike saddle from a WTB Rocket V (265mm in length) to a Bontrager Paradigm RL (270mm in length.) What should I do in terms of saddle fore/aft and seat height? I was told in the past to move my saddle forward if the new saddle is longer. So I did move my Paradigm saddle forward by 5 mm, and that shortened my reach from saddle tip to center of bar from 50.5 cm to 50 cm. But I did not raise my seatpost, and I started to feel pain in my patellar region 10 miles into the ride. What should I do? I like the Paradigm saddle because of its more minimal padding.
— Joe

Dear Joe,
You would be making a mistake to base the saddle position on the relative lengths of the two saddles. You must do it based on where you sit on the saddle, and that is going to vary according to saddle shape. For instance, even though this saddle is shorter, you may very well sit further back on it, and you would thus want to be moving it back, rather than forward. Or vice versa. And I’m willing to bet that it’s less tall from rail to top than the old saddle, and that’s why your knee(s) hurt; you’ve effectively lowered your saddle.

Before you remove an old saddle, mark where your sit bones contact it. You can do this yourself, and an assistant can make it easier. I use a silver paint pen, sit on the bike in the riding position, reach under my butt and locate my sitbone, and mark that spot with the paint pen. Then I get off of the bike and continue that mark straight across the top of the saddle (i.e., perpendicular to the length of the saddle).

Take all measurements to this line, before you remove the old saddle. Measure from this line to the center of the handlebar, and measure from the center of bottom bracket to the top of the saddle at this line. Ideally, you also find the stack and reach dimensions from the bottom bracket to the top of the saddle at this line, since these will allow you to most precisely replicate your position. I do this using the method I describe in my answer to Justin in an earlier column.

Now put the new saddle on the bike, approximating the fore-aft position and tilt of the old saddle. Sit on the bike, find where your sitbones hit it, and, as you did on the old saddle, mark a line across the saddle at the sitbone contact.

Replicate all of the measurements from the old saddle, using the sitbone line on the new saddle as a reference. At a minimum, replicate the measurements from that line to the bar center and to the center of the bottom bracket. Better yet, replicate the stack and reach dimensions to that line on this new saddle.
― Lennard

Weight shift versus aerodynamic drag

Dear Lennard,
I just read this statement in VeloNews and it made me scratch my head a bit.

“Sagan moved to the front soon after and began flying down the road. He was sitting on his top tube and his hands were in the drops, his weight shifted to the front of the bike to increase his speed — which reached 75kph at one point.”

How does shifting one’s weight forward increase speed? I can understand that he might have made himself more compact and thereby reduced his frontal area or that the weight shift may have possibly stabilized the bike at high speed (?), but faster? That doesn’t immediately make sense, does it?
― Al

Dear Al,
The weight shift did not make him faster. What did make him faster was reducing his aerodynamic drag by getting lower on the bike.
― Lennard

Front wheel stability

Dear Lennard,
I am currently riding Hed Jet 6 wheels. I’m noticing that once over 40 mph, the front wheel seems to be very “light” feeling. In other words, the wheel will be susceptible to any wind or breeze. It’s not stable and “planted” on the road as a Mavic Kysrium would be, for example. It’s not shimmy, but virtually equivalent in terms of scare factor. Any thoughts? I can tell you that there’s been nothing done to the bike that may have adjusted position, weight distribution, or the like. I’ve spoken to Hed and they had no guidance or comments. I’ll be riding the Courage Classic in July, and at this point I will switch the front Jet 6 to an Ardennes to at least reduce the aero effect on the front wheel.
― Dan

From Hed:
It sounds like Dan is feeling differential air pressure from front to back pushing the wheel around a bit. It would be more noticeable at higher speed (at least it is for me) because the faster you are going, the further you deviate from your line before correcting at high speed. I have experienced this over the years on all aero wheels, and we (and everyone else) have decreased this tendency with the latest generation of blunt nosed, rounded side aero wheels.

For me it is most noticeable on a quick-steering race bike. I get this sensation most on my Giant TCR, and barely at all on my heavier steel Hed Triple Crown gravel bike (it is steel and not a featherweight).

To make sure that it is not the wheels, I would ask Dan to check the hub to make sure there is no play from a loose axle or even worn bearings. A quick check of spoke tension would not hurt either. With no tire mounted, or with the bead pushed off into the tire well, tension on a Jet front should be 110kgf. If Dan has an Ardennes SL or FR he could compare spoke tension on the two wheels with tires mounted and inflated. They should be the same.
— Andy Tetmeyer
Hed Cycling Products

Regarding shimmy

Dear Lennard,
I’ve been fascinated with the never-ending discussion about shimmy, so I have to relate my experience. I’m 6-foot-6 and run 107cm from pedal to seat top, so I need a big frame. I have a 68cm Rivendell Rambouillet that, if I took my hands off the bars while riding at 10 mph, would very quickly start to wobble. I had a couple of experiences with high speed shimmy. This, I was told, was just to be expected with such a huge frame. Then I had Bill Davidson add couplers so I could use it as a travel bike. After the couplers were installed, I was surprised to find that I could ride no-hands indefinitely and it was totally steady – no more wobble or shimmy since! Clearly, the couplers changed the harmonics of the frame so it is now a better bike than before. This is of course a steel bike. For comparison I also have a carbon 64cm Trek Madone 5.5, and although only 17 pounds, it rides absolutely solid and is much stiffer than the steel frame. So those fat carbon tubes maybe another good solution for us big guys.
― Jim

Dear Jim,
That’s cool! I’ve always told people when we install couplers on their bikes that a tube is stiffer with couplers in it. This proves it.
― Lennard

Dear Lennard,
Just had to respond to your statement that “no one has tried to speed up” to show that shimmy is resonance.

The frame in question, all steel from 1998, was driven into the garage wall atop a car many years back. But … My Mostowy, custom built using Columbus tubing including a steel fork, had a shimmy in the range of 30-33 mph. It may not be as serious as the shimmy commonly cited in your questions. I could easily sit up at 40 mph, but as I slowed, I had to put a leg on the top tube or place my hands back on the bars to control it through 30 mph. Down below 25 or 27 mph, it was stable again.
― Charlie

Dear Charlie,
Well, that is very interesting and certainly does make the case for a resonant frequency of your bike.
― Lennard

FILED UNDER: Bikes and Tech / 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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