Gear

Technical FAQ: Seat clamp headaches, eliminating speed wobbles

Lennard addresses those pesky seat clamps, and helps one reader fix an annoying and dangerous descending shimmy.

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Dear Lennard,
On my morning ride, I started asking myself why it seems that ultra-modern road bikes have Medieval seat clamp systems. If you move your seat forward or back and then want to level it, I’d love to see you get one truly level using a 4-ft level; it looks great, and then you tighten it down, and it’s out. And don’t even get me started about replacing a seat where you have a half circle with ridges, one or two nuts, and two pseudo clam shells!

Anyways, what is it about the bike clamp mechanism that has not led to a simple system with calibrated level and/or angling?
— Jeff

Dear Jeff,
Well, there used to be a Salsa seatpost that did what you want. The adjustment for saddle tilt was separate from the fore-aft adjustment/tight clamping of the rails. We used to spec that seatpost exclusively on our bikes because of that. I guess we must have been in the minority of shops ordering seatposts from Quality Bicycle Products, who owns the Salsa brand; otherwise, I have no explanation for why QBP discontinued it.

Anyway, that was a long time ago, and, recently, a customer of ours traded in his old bike toward a new one, and the old bike had that seatpost on it. I showed it to everyone who was around, demonstrating the separated clamping and angle-adjustment systems. Such a shame it is no longer available.

And why do seatpost manufacturers seem oblivious to the trials and tribulations of those of us struggling to actually install saddles onto their posts without completely removing the bolts, and then having a bunch of separate loose parts to juggle under the saddle? And why, once you get the parts together, is it highly unlikely that you’ll tighten it down and hit the saddle tilt you are aiming for on the first try? Well, I am guessing it has mostly to do with weight as well as the cost of production.

The clamp on the above-mentioned Salsa seatpost was certainly not as light as the little clamshell clamps with tiny bolts. And, I suppose, the consumer often doesn’t personally encounter all of these adjustment and installation hassles and is motivated primarily by weight and price. So, lightweight posts with “Medieval” clamps could be what ends up selling, despite the protestations of mechanics installing saddles. If the consumer were to vote with their pocketbooks (or their credit cards) for seatposts with saddle clamps that allowed easy installation and adjustment, I think that’s what we would see in the market. Apparently, they do not.
― Lennard

Dear Lennard,
I am at my wits end trying to solve my problem with “death wobbles.” I first experienced this on a downhill section in Hawaii about eight years ago on my S-Works Roubaix (58cm frame size); it turned out the chain stays had been damaged when the TSA didn’t fasten the bike back into its case properly, so I thought that had to be the cause and immediately replaced the frame with a Cervelo RS (also 58cm).

I didn’t encounter the problem again for about a year until descending down a long, straight hill at 40mph in a group ride when it happened, almost with disastrous results. It then begin to happen more frequently. The last time was the most distressing because I was riding in the city and slowing down for a stop light when a bus passed me. It sent the bike into such wobbles I was sure I was going down and right under the bus. Prior to this incident, it had happened on two descents during a Century ride, and that was the last time I rode that frame.

After this instance, I tried three different wheels sets, alloy and carbon (I’ve balance all three so that they spin perfectly smooth), I had the bike thoroughly checked for loose headset bearings and anything else they could think of. I tried keeping my knees on the top tube, which sometimes seems to help. I’ve tried weight on the saddle, weight off the saddle, and I always spin the cranks if not grabbing the top tube, which seems to help the most until my compact crank starts to spin out around 33-34 mph.

The more it happened, the more tense I became, and so I now have to concentrate on trying to stay relaxed, and I’ve gotten to where I dread even small descents.

A couple of years ago, I bought a No22 Great Divide (58cm) and thought I had found my dream bike, super comfortable (I can’t image ever going back to carbon) and easy to travel with inside a PAKGO X case.

I’m almost 63, and a degenerative disk disease made it necessary for me to adopt a more upright riding style (begrudgingly), and I think this has made it worse. I also notice I’m susceptible to having a gust of side wind set it in motion. I’m only 5’11” but my long inseam makes the drop between my saddle and handlebar too great on all the 56cm frames I’ve tried without having to use a huge number of spacers (I already use 35mm with an upturned 100mm stem on a tall headtube, and I still a still have a couple of inches of drop).

I like to ride hard and fast with younger friends, but my inability to descend is getting me uninvited, and the thought of a future limited only to Zwift is depressing.

Is there anything you can suggest for me to do?
— John

Dear John,
The taller you are (the higher your saddle), the heavier you are, and the higher your handlebars, the greater the tendency for speed wobble. The higher the saddle and bar are, the greater must be the torsional rigidity of the frame to avoid dropping into a resonant frequency that can be set up by wind and by road surfaces, once a certain speed has been surpassed. The wheels also need to be stiff and true, as do the fork legs and fork steerer.

The way I preclude shimmy when building a frame 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 top by sloping the top tube up and, when appropriate, by shortening the seat tube from the bottom by raising the bottom bracket for longer cranks. These modifications also shorten the top tube, seatstays, and down tube without affecting the frame’s effective size.

You are not super tall, and you didn’t mention your weight; I assume you are pretty light, since it sounds like you are able to keep up with your younger friends on the climbs and only get dropped on the descents. You are of similar height and weight to many riders who do not have shimmy problems, so it’s possible that some of the issue lies with the death grip you have on your handlebar, as you mentioned.

A few years ago, I built a bike for rider who has “essential tremors” and consequently had constant issues with speed wobble. He is of similar height to you — 6’0” and 176 pounds — and had a number of carbon bikes, all of which shimmied due to his essential tremors. We built him a rock-solid, super-stiff steel frame with oversized tubing, a super-stiff fork, and, perhaps most importantly, a steering damper, which I describe in detail here. 

You probably can’t put that particular steering damper on your bike, because it requires a special fork with a hole in the bottom of the crown big enough for a 12mm socket to pass through. However, something that you could try on your existing bike that will set you back less than $100 is a Cane Creek ViscoSet damping headset. Given that you seem to have addressed your wheels and frame with an eye to eliminating shimmy, adding this damping headset just might be enough to reduce or eliminate your shimmy problem.
― Lennard


Lennard Zinn (https://www.velonews.com/byline/lennard-zinn) , our longtime technical writer, joined VeloNews in 1987. He is also a custom frame builder (www.zinncycles.com) and purveyor of non-custom huge bikes (bikeclydesdale.com), a former U.S. national team rider, co-author of “The Haywire Heart,” and author of many bicycle books including “Zinn and the Art of Road Bike Maintenance,” “DVD, 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 in physics from Colorado College.

Follow @lennardzinn