Technical FAQ: Drilling holes in carbon frames and more

In this week's Technical FAQ, one reader asks Lennard Zinn if it's safe to drill holes in the carbon frame of his bike

Drilling holes in a carbon frame

Dear Lennard,
Can I drill holes in the down tube of my carbon mountain bike frame without negatively impacting its structural integrity?

The back story: Rather than using a backpack for my hydration bladder, I place it in a frame bag, which eliminates the weight on my back and is much more comfortable. On a recent ride, I realized that the fairly massive down tube on my bike (a carbon Silverback 29er), offers significant capacity; utilizing it would mitigate the need for even a frame bag. My idea is to drill a 3/8″ inch hole on the upper side of the down tube near the head tube, and another 3/8″ hole on the bottom of the down tube near the BB. I would then sanitize the inside of the tube with food-grade chemicals, plug the bottom hole with a threaded rubber stopper, fill the down tube with water, and insert the appropriate length of 3/8″ OD surgical tubing into the upper hole, affixing the bit valve end on the bars.

Again, I am wondering if drilling holes of this nature would significantly reduce the structural integrity of the frame. (By the way, I hereby indemnify you from any and all ramifications of me drilling holes in my frame.) This may seem like a dumb question (“Of course it’ll screw up the frame!”) but, for example, the fifth picture in the VeloNews photo essay on the new Trek carbon IsoSpeed frame clearly shows a hole for a “rivet or a port” on the bottom side of the down tube. To make a long line of reasoning short, if Trek can do it, why can’t I?
— Jeff

Dear Jeff,
That’s a very poor idea. It’s not a good idea to drill a hole, particularly that big of a hole, even in a metal frame. But at least on a metal frame, you’re not cutting through fibers as you are on a carbon frame.

Below are some responses you should pay attention to.
― Lennard

From Specialized:
Drilling holes in a down tube is a bad idea. The holes will create stress concentrations where cracks could form, compromising the structural integrity of his frame. If we meant for holes to be drilled in those locations we would have reinforced the tube walls in those area. (Needless to say this would void the bike’s warranty). Most of the holes in our frames are drilled, but we have specific reinforcements in those areas to prevent structural issues and the drilling process is very controlled and repeatable.
— Luc Callahan
Engineering Manager- Road
Specialized Bicycle Components

From Trek:
There is no circumstance where a hole can/should be drilled in a carbon frame by anyone other than the manufacturer. The reason we can drill holes in frames is that we specifically design areas with reinforcement for post-molding machining processes. We make intricate and exact fixtures that can take advantage of those areas with the appropriate cutting tools. There is absolutely no circumstance where we would feel comfortable with anyone outside of Trek drilling a hole in a carbon frame.
— Ben Coates
Trek Road Product Manager

More on Trek’s skewer recall

Dear Lennard,
I don’t know if I should be sending this email to you or to “Legally Speaking,” but here we go.

I bet Mr. Tullio Campagnolo is turning in his grave. Between the lawyer lips and this recall, he must be saying, “it’s a simple device, people, but you keep finding ways to screw it up.”

So the video shows the mechanic putting the lever on the side with the disk brake. Is there a law against putting it on the non-brake side?

He describes a scenario were the lever “just opens up.” I’m guessing that this happens because the skewer is not tight enough and/or pointing forwards. So Trek’s “solution” is to replace it with a shorter lever. Sure, it prevents the lever from catching in the disk, but it does not solve the actual problem of not enough tension. In fact a shorter lever will make it harder to tighten.

So here is my question for Professor Zinn: Are there any skewers on the market that let the rider know that skewer is tight enough? Anything that ensures proper alignment, i.e., not pointing forward?

I do realize that the actual solution is to teach new riders the proper way. Each year on a charity ride that I do, I point this out to at least one rider. One guy even said, “That’s how the shop told me to do it.”
— John

Dear John,
I know of no skewers that tell you when they are tight enough, or that they are oriented in an approved direction …

I have always put my lever on any disc-brake front wheel on the non-brake side, but I am amazed at the scorn I occasionally get from people who find it unsightly to have the skewer lever on the drive side.

I believe that having the lever on the non-drive side is absolutely the way to go to prevent getting greasy fingers on the disc and to prevent inadvertently bending the disc by pulling on it with the fingers when pushing the lever over with the heel of the hand. That said, since you can’t prevent people from turning it around the other way. Trek simply training its shops to put the skewer lever on the drive side is probably insufficient to address this issue, which for three people was very serious.

Unlike the front, you can’t reverse the skewer on the rear so the lever is on the drive side, since the lever would be in the way of the derailleur. While a locked-up rear disc would not be as catastrophic as one on the front, it is still not something you’d want to have happen. Trek pretty much had to address this issue in the way that it has.

The recall replaces the skewers with ones that don’t flip open past 180 degrees.

It seems unlikely that other manufacturers of disc-brake bikes aren’t also selling skewers that flip open past 180 degrees; I wonder how many other bicycle brands will follow Trek’s lead.
― Lennard

More on disc-brake hubs on bikes

Dear Lennard,
You stated, “You aren’t apt to find disc-brake hubs with fewer than 32 holes to attach them to.”

Small detail, but I’d like to point out that DT Swiss offers road disc hubs in hole counts of 20, 24, 28, and 32, in Classic flange, plus 24 and 28 in Straight Pull flange hubs. Of course, a low spoke count disc wheel is not for every rider, but we do make the options available to wheelbuilders.
— Steven Sperling
Tech Manager
DT Swiss, Inc.

Dear Lennard,
In a recent column, “Technical FAQ: Disc brakes in road racing,” Mark commented that since kinetic energy is proportional to the square of speed, braking power (and therefore heat generation) was proportional to the square of speed. This isn’t strictly true. If you are braking to come to a stop, you lose all of your kinetic energy, which is proportional to the square of the initial speed, as Mark states. However, when braking on a descent, you aren’t typically losing kinetic energy, but rather preventing gravitational potential energy from increasing it. The rate of loss of potential energy is proportional to road grade times speed, so if you brake to maintain speed on a long descent, the rate of heat generation will be approximately proportional to the speed, not the square of speed. So descending a 20 percent grade at 30 kph would result in twice the rate of heat generation as descending a 20 percent grade at 15 kph. Total heat generation depends only on the potential energy lost, so speed doesn’t matter for total heat, which determines maximum temperature increase on very short descents. Although, if the heat sink (in this case the rotor) is losing heat to the air, then the peak temperature will be roughly proportional to the rate of heat generation on long descents where “steady state” is reached. This all of course ignores power dissipated to wind resistance, rolling resistance, etc. But the key point is significant kinetic energy is not being lost when speed is approximately constant, and there’s no reason to expect heating proportional to the square of speed.
— Dan

Dear Lennard,
Just read your Tech FAQ on disc brakes. Although I agree that it probably doesn’t make sense to convert rim brake rims to disc, folks are not limited to 32-hole hubs. I’ve run two disc brake bikes for over three years now. I live in Seattle and get most of my training in via long commute rides year-round. Prior to going to disc brakes, I went through a rim a winter despite careful and regular cleaning of the brake track. I wanted a go-fast, year-round bike. TiCycles built me a custom road race geometry frame with disc brakes (not a converted gravel or cyclocross). This bike was originally built with Ultegra 10-speed Di2 and Avid BB7s. I upgraded it to Dura-Ace 11-speed and Shimano hydro about a year ago (as an aside, no problems with brake fade even on 95+ degree days on long mountain pass descents). I also have one of their gravel disc frames that got the Ultegra Di2 and BB7s as hand-me-downs.

At any rate, I have four sets of wheels, all disc brake. On my road bike, I run 28 hole, three-cross for both wheelsets. For dry weather, a set of 28-hole DT Swiss 240 centerlock hubs laced with DT Aerolite spokes to Reynolds Competition 46 rims (DT Swiss bearing only make it one winter season for me, so I consider them junk for wet weather riding). For wet weather, I run 28-hole Chris King R45 disc hubs laced with Sapim CX-Ray spokes to Hed Belgium+ disc rims. I weigh about 163 pounds and ride with a messenger bag for about 75 percent of my miles. Both of these wheelsets have seen a lot of beating and are holding up very well. On my gravel bike, I do run 32-hole hubs (both Chris King ISO discs with Sapim CX-Ray spokes to Belgium+ disc rims for my fast tires and Sapim Force spokes to Stan’s Grail rims for my mud tires).
— Joel