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I couldn’t help but noticing the Pinarello TT bikes that both Filippo Ganna and Tao Geoghegan Hart used in the Giro’s last stage time trial had black-boxy things attached just above the bottom bracket where a bottle cage would normally sit. Obviously, in such a short TT, a water bottle is unnecessary, so this space was available for…whatever it was. I’m guessing it’s a Di2 battery. But, if in fact that’s what it was, why in this particular instance? I have seen other photos of the Bolides in action, and the batteries are never in evidence.
First of all, Di2 batteries are small and are easily hidden inside of the frame, usually inside the seat post. I checked with Pinarello, and it turns out that thing is not about improving the performance of the bike. Rather, it is to improve the accessibility of the sport to its fans. I received this response from Andrea Vezzaro of Pinarello:
“I’m in charge of trade and sports marketing inside the company. I received your information request yesterday about the BOLIDE setup during the last stage of the Giro; the black box attached just above the bottom bracket on Filippo and Tao bikes is the Velon tracking device.”
I hope I can contribute to the “greased fastener threads” conversation from your last column.
For a little background, I’ve been a mechanical engineer for the past 15 years, prior to that I was a bike mechanic for a couple of bike shops while I was a student. I don’t pin on a number much anymore, but I’ve raced road, MTB and ‘cross throughout the years.
I’ve designed quite a few fasteners and bolted joints throughout my career, which spans everything from really big bolts when I worked at Caterpillar, to really, really small ones when I worked at Microsoft. Currently, I’m the lead mechanical engineer for the bike and scooter division of Lyft, where we design the bikes and scooters available in many cities as part of publicly available bike-share fleets — we are the largest bike-share operator in the US. But enough about me, let’s talk about screws!
As you pointed out in your column, most bolted joints in automotive and motorsport applications don’t get greased, but it’s standard practice for most bike bolts.
The head mechanic at a shop I worked many years ago always said “grease every bolt when you assemble a new bike, so when that bike comes in for service in 3…4…5 years, we can get it back out”. Anecdotally, he was right, but let me explain a few of the technical reasons why applying grease to bicycle fasteners makes sense, and why it is rarely done in some other settings.
1) Greasing a fastener reduces thread friction (in bolted-joint engineering terms, this is usually referred to as reducing the “K” value), which results in a higher clamp-load for a given installation torque. An ungreased fastener would have a thread friction coefficient, or K value, of 0.18-0.2, whereas a greased fastener would have a K value closer to 0.12-0.14. For example, an ungreased M5 stem mounting bolt, torqued to 4Nm, would result in a clamp load around 6,000N. The same screw, with grease applied, would achieve a clamp load of 9,600N when installed at the same 4Nm of torque.
2) Greasing a fastener helps prohibit galvanic corrosion, which can be an issue even for stainless steel fasteners in painted or anodized aluminum threads. Just like you would always grease an aluminum seat post when installing it into a steel bike frame, the same best practice applies to steel and stainless steel fasteners in aluminum threads. The table below shows the relative risk of galvanic corrosion for various dissimilar metals. These two reasons are as true for a bolted joint on a car or motorcycle as they are on a bicycle, so why grease bicycle threads in particular?
Most fasteners used in automotive and motorsports applications are “grade 8.8” or “grade 10.9” alloy steel bolts. A grade 8.8 bolt has an ultimate breaking strength of 800MPa, and a yield strength around 640MPa, and a grade 10.9 bolt has an ultimate breaking strength of 1,000Mpa and a yield strength around 900MPa. These are the minimum strength values defined in ISO 898. These high-strength steel screws, typically combined with a large, high-torque drive interface, can be installed to relatively high torque values without stripping the drive interface or yielding the bolt shaft. You can really torque ’em down, and if corrosion effects over time cause the bolts to “freeze” (a dramatic increase in thread friction due to corrosion of the threaded interface), you can again apply a relatively large torque to remove them.
Most bicycle fasteners conversely are made from grade A2-70 (or similar) 300 series stainless steels, with an internal hex (“Allen”) drive. An A2-70 stainless steel fastener has an ultimate breaking strength of 700MPa, and a yield strength of 450MPa. These are the minimum strength values defined in ISO 3506. As you can see, most bicycle bolts are only 70 percent the strength of an automotive-grade 8.8 bolt and only half the strength of an automotive-grade 10.9 bolt. Conversely, these bike bolt fastener drives and bolt shafts are capable of considerably less torque than their automotive brethren before stripping occurs. This can be exacerbated if the driving tool (I’m looking at you 10+year old Park Tool 5mm Allen wrench lying on the shop bench) is worn down. The lower friction of a greased thread allows higher clamping forces within the torque range available from the weaker bolt shaft and bolt drive feature. A greased thread also helps ensure that the torque required to remove the fastener after time in the field is not significantly higher than the force required to install it.
So should we all be greasing up our bicycle bolts prior to installation? As I pointed out in the example above, a greased fastener can have significantly higher clamp loads for a given installation torque than an ungreased fastener. For carbon fiber structures that are highly sensitive to clamp loads, it is safest to assume the torque recommendation (e.g. 5Nm) provided by the frame/component manufacturer assumes an ungreased “dry” fastener. Greasing this fastener and then applying the 5Nm installation torque could significantly exceed the clamp load recommended by the manufacturer.
Thanks. This is a great explanation. I think all of us have learned something from you about greasing bolts.
Seeking options for warm cycling shoes (with clips) to wear on my recumbent trike
You’ll want to review the column in which we discuss cold-weather cycling boots.
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.