Technical FAQ: Bike geometry, more about high-speed shimmy, and differing experiences with tire plugs

Have a question for Lennard? Please email him at veloqna@comcast.net to be included in Technical FAQ.

Dear Lennard,
Regarding high-speed shimmy, I am a 71-year-old experienced bike rider who used to barrel down long steep descents at 50mph. For some reason, when riding the same bikes, I can’t do it anymore without the possibility of front-wheel shimmy. I think it’s mostly mental. It depends on the steepness of downhill, crosswind, and roughness of the road surface. I have overcome the fear of shimmy by shifting more weight to the front of bike, and after I run out of my topmost gear 50-11 or 53-13 by putting one knee against the top tube. My 1970s steel bikes are less prone to shimmy than the 2011 carbon fiber bike. Speed doesn’t seem to me to be the cause as it has happened to me at less than 20mph. The bike geometry of the 2011 bike is just one degree steeper but is 9 pounds lighter.

If I do repeats (become familiar with) the downhill, shimmy disappears.
— Philip

Dear Philip,
Sounds like a tremor from you may be passing into the bike. The same thing happens to me when I’m descending in the cold. Riding down Mt. Evans in hailstorms and snowstorms comes to mind. The same descent as I do with no shimmy, but I’m shaking, and so is the bike. Or, if I have a death grip on the bars due to crosswinds and being passed by cars interrupting the side force from the wind, my bike will shake on the same descent on which it normally doesn’t shake.
― Lennard

Dear Lennard,
I am considering buying a new gravel bike, and the one I am looking at has the geometry below. I would like to use an Enve Adventure fork for a few reasons. This fork allows one to change between two rake options, 49mm and 55.5mm. The factory fork has 50mm of rake. With the Enve fork, the fork/crown height changes from 398 to 406mm as it goes from 49-55.5mm of rake. I find the handling aspect of frame design to be confusing.

Would you mind sharing your views on how the Enve fork may feel different from the factory fork at each of the two rake choices it offers (in simple terms, please)?

Head tube angle: 71.0
Seat tube angle  74.5
Chainstays:      43.0
BB drop:          7.1
Wheelbase:      102.6
Front/Center:    60.4

— Scott

Dear Scott,
Well, in lay terms, increasing the fork rake with the same head angle will increase the tendency to oversteer, and vice versa with reducing the fork rake. In other words, same head angle and tire diameter with more rake will make the bike quicker handling and less stable at speed. This can be explained by the fact that fork trail will decrease with increased fork rake. Fork trail can be thought of as the lever that rights the bike—more trail results in more self-righting capability of the bike.

There is of course some added complexity with changing fork length. You don’t say how long the stock fork is, so I can’t comment on whether the head angle will change with either configuration of the Enve fork. If the height of the bottom of the head tube goes up due to a longer fork, the head angle and seat angle decrease (thus increasing fork trail and stability), and if the height of the bottom of the head tube goes down due to a shorter fork, the head angle and seat angle increase (thus decreasing fork trail and stability).

I have an entire chapter on this subject in Zinn’s Cycling Primer.

If the head angle stayed at 71 degrees with all three fork configurations, the fork trail, with a 700 X 38C tire, would be 67mm with the 50mm-rake stock fork, 68mm with the Enve in the 49mm-rake configuration, and 61mm with the Enve in the 55.5mm-rake configuration. Unless there is a significant difference in fork length (more than 10mm—more on this below) between these two options, I don’t think you’d notice the difference in handling between the stock fork and the Enve fork in the 49mm-rake configuration.

Fork axle-to-crown length and rake are linked. For instance, the height of the bottom of the head tube changes by less than 8mm when switching between the two Enve forks. Even though the difference in axle-to-crown length is 11mm, due to the change in fork rake, it changes the height of the head tube considerably less than that.

You likely won’t care about this, but there is another consideration for slow riding: it becomes easier to keep the bike from weaving back and forth at slow speeds the less trail it has. This may be counterintuitive, but the bike that is more stable at speed (more trail) will tend to weave back and forth more at slow speeds than the bike with less trail. That’s because increasing the fork trail will also increase the “wheel flop,” which is how much the height of the front hub will drop turning into the lean of the bike. The bike gains stability by getting the contact patch of the front tire back under the rider’s mass quicker when that mass moves laterally from the vertical plane of the bike (by leaning). This gives high-speed stability, but at slow speeds, it means that the wheel flops back and forth faster as the rider leans from side to side. While this may not be something that a fast rider ever notices, this can be an issue for slow riders.
― Lennard

Dear Lennard,
I ride tubulars on my road bike. I use Continental Competitions. I injected a dose of Stan’s tubeless sealant through the removable valve stem. I’ve noticed that I’ve had a few punctures in the tires with no ill effects. This makes tubulars more affordable in relation to clinchers.
— Richard

Dear Richard,
I do the same, although I usually wait to put in any sealant until I get my first slow leak. And indeed, it seals the slow leak and allows me to keep riding the same tire for many more months.
― Lennard

Dear Lennard,
Regarding your article on Dynaplugs failing, I have come across two instances of them coming out of a tubeless road tire. Once, I plugged a tire riding up Left Hand Canyon. I put almost 500 miles on that plug. I then went and participated in Ride the Rockies. The plug held for the first two days of riding. However, after the second day of riding, the bike sat in the sun on a very hot day. Over the course of the day, the combination of tire pressure and heat had pushed the plug almost completely out of the tire. I was able to remove that plug and install another without issue. I just made sure to leave the tire pressure low until the next morning.

The second instance came immediately after installing a plug. I caught a small shard of glass coming back from Lyons a few months back. I plugged the gash, and it held pressure fine. I gave the sealant a few minutes to set up at the plug and then started riding back towards Boulder. Within a mile, the plug blew itself back out. I can only assume this was due to using too much tire inflation pressure. I only had CO₂ cartridges that day. I installed a new plug and made sure I didn’t overinflate the tire. That plug held for another 100 miles before I took the tire off and applied a tube patch to the inside of the tire where the plug was. That patch has actually held for several hundred miles.

My only conclusion is tire pressures over 70-80psi can dislodge a plug. A lot of heat can possibly melt the glue on the plug causing it to fail. I run 28mm Schwalbe Pro One tires.

I would personally like to see some data on how well plugs really hold. My personal thoughts are that the plug is not a permanent seal. The seal is constantly breaking as the tire rolls/flexes and hits dirt/rock on the road surface. The only thing that saves the system is the sealant constantly resealing the tire/plug. I have seen sealant marks around my plugs on multiple occasions. I’ve had to top off sealant levels a few times while riding with plugs.

In general, Dynaplugs work great. I can attest with the miles I have put on them.
— Andrew

Dear Lennard,
Regarding plugging tires, I had a lot of experience with tubeless car tires in the 60s working at my dad’s service station and understand the principles and how to deal with them.

Recently, I came across this article in our newspaper wherein the author, a car mechanic, basically says that he won’t use plugs to repair tires. We used them routinely in the old days, but car tires then were less sophisticated than today’s tires, and we were almost always removing a nail and replacing it with a plug.

It did get me thinking about tubeless bike tires and using plugs to repair them. I decided that when (I know the industry will eventually force me to adapt) I start using tubeless bike tires I might use plugs since they really are simple and usually work pretty well. However, given that bike tires are pretty small and thin, I would use the plugged tire like those horrible spares we get in our cars these days — low speed, short distance, and I’d replace the tire as soon as I got home. The consequences of a plug failing on a bike are a lot more severe than you’re likely to experience if a plug fails in your car tire. This is just “two cents worth” from an old guy who’s ridden a lot, dealt with a lot of different tires, and spent 16 days in a hospital as a result of a high-speed crash.
— Jim

Dear Lennard,
Regarding untrue disc brake rotors out of the box, until I started using Shimano road rotors, I always had to do some minor tweaking on almost every rotor I installed. I found I didn’t need any tools. I just used my thumbs, with a paper or cloth rag to keep my oils off the rotor. Using the thumbs ensured a smooth realignment. I would just look at where the rotor was hitting and just push or pull as needed. It usually didn’t take much force. The rotors always seemed to want to be straight.
— Steve

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 on Twitter.