Technical FAQ: Sagan wrenches on his bike during Paris-Roubaix

Television cameras caught Peter Sagan tightening the bolts of his stem while pedaling at high speeds during Paris-Roubaix.

Sagan’s on-the-fly bike adjustments

Dear Lennard,
I saw Peter Sagan getting a wrench from the team car and tightening something on his bike while he was in the last 30km or so of Paris-Roubaix. What was he doing? And what was that knob on his stem?
— Brad

Dear Brad,
I watched that video numerous times. He got what looked like a 4mm hex key from the car, and he worked it first with his right hand on his stem and then with his left hand, also on his stem. I think it is fairly clear that he was tightening the stem bolts clamping his stem onto his steering tube. I think the upper bolt on his stem comes in from the right, and he tightened that one first. The lower bolt comes in from the left, and he tightened that bolt next.

Normally, if your stem bolts are only a bit loose, enough for the stem to twist if you pinch the wheel between your knees and twist the handlebar, you won’t notice it while riding (unlike the bolts on the front cap, where your handlebars will flip down when you hit a pothole or slam into some cobblestones). Since the steering tube rotates freely in the headset bearings, allowing the front wheel to turn back and forth with minimal resistance, there is negligible net torque on the stem relative to the steering tube while riding. Thus, your stem will still tend to stay lined up straight with your front wheel, even when the bolts are a bit loose, unless your wheel takes a big side impact while you’re holding the bars (like in a crash).

Sagan didn’t crash, and since it looks like his stem was still pointed straight ahead, I think he was just tightening the stem bolts. I at first thought that his headset might have been loose (which is an easier thing to feel than the stem loose on the steering tube), but, at least in the video I saw, he didn’t try to tighten it. That, of course, could have happened before the video caught him doing it, but I’m not even sure how he would have accomplished that on the bike he was riding, since the top cap on his Specialized Roubaix seems to be a knob — perhaps a lockout knob on his Specialized Future Shock suspension fork. Some suspension travel of his stem moving up and down must have been nice on the cobbles, and a lockout might be a nice feature to have once he got back on pavement. (But if it’s not a lockout knob and is actually a knob attached to the headset compression bolt, I suppose he could have tightened down on it with his hand and taken the play out of his headset, and the cameraman might not have even noticed that and gotten video of it. I doubt it, though; I’ll bet it’s a lockout knob.)

Sagan’s stem bolts were extremely, surprisingly loose. He cranked down multiple quarter turns on each bolt. I believe that they were loose enough that he could feel just the stem loose on the steerer, and Sagan is obviously a guy who is very sensitive to the connection between his handlebar and his fork.

One would not expect to find such loose bolts on a top pro’s bike, especially in such an important event. Certainly, the jarring of racing over the cobbles of Paris-Roubaix could jiggle a lot of bolts loose, but I think they would have to have been loose before he started to be that loose 227km later. Just think if the bolts clamping his handlebar had been that loose; his bars would have been slipping all over. And since they were not, one has to conclude that the bolts securing the bar were tight when he started, and the ones at the steerer clamp were not.

I wonder if Sagan’s mechanics had under-torqued his stem bolts out of fear of damaging the Future Shock mechanism, which is up in the top of the steering tube (or maybe they got distracted while looking up the torque specs and never came back and finish-tightened them). After all, Niki Terpstra (third place this year and winner in 2014) just a year ago crashed when the steering tube of his Specialized Roubaix snapped off on the cobbles during Paris-Roubaix, so a bit of cold feet on the part of Sagan’s mechanics might be warranted. (According to Specialized, Terpstra was apparently the only one of 16 Specialized-Roubaix-riding riders in last year’s Paris-Roubaix riding without a Future Shock cartridge in the fork. He reportedly had requested not to have suspension, and a prototype aluminum plug was fitted into his Roubaix’s steering tube in place of the Future Shock cartridge; due to an apparent miscommunication between the team and Specialized, the prototype plug was not replaced with a fully-engineered one prior to the race, and its overly-thin wall cracked at the stem clamp near the end of the race.)

Of course, Bora-Hansgrohe mechanics (and perhaps engineers at Specialized) might have been cringing watching Sagan reef down on those bolts with the heel of his hand on a pretty long hex key, rather than with a torque wrench. It’s not hard to imagine a guy fully charged with the adrenaline of impending victory in the world’s biggest one-day race over-tightening those bolts and either stripping them or causing damage to the steering tube. However, Peter Sagan is one cool customer. Even with the weight of the hopes, dreams, and expectations of arms thrown up in triumph on the Roubaix velodrome among all of his fans, as well as the Twitter-ready thumbs of the handful of his critics, resting on his shoulders, he is probably the rider you would most want to be tightening his stem bolts in that situation. He solved the problem well enough to keep his stem tight, and ride to victory on the Roubaix velodrome.
― Lennard