#6: Pedal installation: a cautionary tale
Plus radial loading of threads, SL7 goes T47, Ronnie O’Sullivan on cyclists
My brother joined a million others in waiting for a global pandemic to get a bicycle in adulthood.
(Is that what it takes?)
He couldn’t find a suitable bicycle near home. JIT inventory assumes a rational world with a strict minimum of pandemics. So after a long search, he ordered a nice Giant from a local bike shop on the other side of the country.
Problem the first: the Giant arrived half built. For example, the pedals came in a plastic bag rather than screwed into the cranks.
Problem the second: Benjamin – let’s call him that to preserve his identity – didn’t own a pedal spanner. So add that, plus shipping, to the advertised price.
I know from experience that fitting pedals can cause problems, as did the guy in the bike shop. So we both cautioned my brother against cross-threading the steel pedal axle in the soft aluminium crank eye. The left pedal screws in the wrong way, we warned. Use grease. Make them tight.
A few days later, Benjamin told me his pedal fell off and sent me this photo:
So what went wrong?
As in aeroplane crashes, probably more than one thing.
First, the bike was shipped in pieces requiring some assembly. This is a common practice, but that doesn’t make it sensible. Cars are not shipped without wheels installed, because customers couldn’t be trusted to fit them correctly. The same could be said about bicycle pedals, but it’s just a bike. They’re just pedals. Just screw them in and just ride!
Second, the pedals were not tight enough. My brother is a sturdy fellow and has restored boats, so I was afraid he would err on the tight side when working on a fragile road bike for the first time. I kept telling him that tight doesn’t mean as tight as you can get it. It’s not a boat engine. Those warnings may have been a mistake. When I later sent him a torque wrench, he realised he hadn’t made the pedals tight enough.
Third, there wasn’t enough grease. My brother didn’t add any but relied on the grease the shop or factory had put on the threads. Grease allows precession to function. Precession helps pedals to self-tighten as described below. Without adequate lubrication, precession may not occur.
Fourth, there were no pedal washers. More on this important detail in a moment.
Radial loads and precession
It was the Wright brothers of flying fame who figured out that pedals work loose by precession because the threads are loaded radially. Those guys were not mere tinkerers but first-rate engineers, and they popularised the left-handed thread on the left pedal as a solution to this problem.
At first glance, the left-handed thread looks like it might encourage the torque of bearing friction to unscrew that pedal. So does the conventional thread of the right pedal. This caused some early bicycle pioneers to use a left-handed thread on only the right pedal instead. But precession torque greatly exceeds bearing friction, and so the Wrights’ arrangement tends to cause both pedals to self-tighten.
In an ideal world, then, Benjamin’s left pedal should have self-tightened – if it wasn’t tight enough in the beginning. Unfortunately, other problems prevented precession from working. These problems are common, which is why stripped crank threads are common.
The first impediment to precession was the lack of new grease and clean threads. Dirty grease isn’t much good, because grit particles penetrate the oil film and hinder lubrication.
The second impediment was an interaction between pedal and crank that often happens with cheap pedals. Notice the shape of the shoulder of the pedal axle where it butts against the crank face:
As you can see, the shape is not a perfect circle but truncated on two sides by the spanner flats. At the intersection of the flats with the circle are corners that gouge into the soft crank face, the gouge ending where these corners come to a stop at pedal installation. The torque of precession isn’t enough to persuade the corners to leap – uphill against the clamping force of axial tension – over the end of those gouges in the crank face, and so no further tightening occurs. However, the pedal readily loosens in the other direction, the corners of the flats travelling smoothly along the furrows they ploughed with their leading edges on the last quarter-turn of the pedal axle at installation.
For this reason, pedal washers are essential with pedals that don’t have full-circle shoulders. The Giant arrived with pedals that needed such washers (one shown above), but there were no washers in the package. Oops.
For comparison, here is a pedal with a good circular shoulder:
Notice the flats for the spanner do not extend all the way to the crank, leaving a fully circular shoulder against the crank face. This pedal does not need a washer.
Pedal washers are made of hard steel. The corners of the flats of cheap pedal axles barely gouge them, and, anyway, the other side of the washer remains a smooth, flat circle offering little resistance to precession against the unharmed crank face.
If my brother had tightened his pedals more, or if the pedals had had washers and the threads had had clean grease for installation, this disaster would have been averted. Instead I shipped him a pair of replacement cranks from my parts bin and a large torque wrench, and I walked him through correct installation in a video call.
Maybe we’ll eventually fix the stripped FSA Tempo crank with a threaded insert, but those inserts are as expensive as a new left crank in many cases. And a new left crank is nearly as expensive as a pair of cranks with chainrings. Such are the follies of the age of shopping.
For now my brother can ride, and that’s the main thing.
Why does this flawed pedal-crank joint persist?
It’s an irony that the pedal-crank joint is one of the few bicycle standards to have survived the onslaught of innovation in recent decades. For although the Wright brothers improved the joint with left-handed threads on the left pedal, it remains unreliable. Benjamin’s story is common, as you can tell from a glance at any cycling forum or Reddit.
The problem is that the threads are intermittently and heavily loaded radially. Loading them like that inevitably causes movement that damages the parts and can unscrew the joint.
Anywhere you see a left-handed thread is proof the designer knew that radial movement (within the necessary thread clearance) would cause problems. But rather than redesign the joint to avoid that, they took the clumsy shortcut of using a left-handed thread instead.
This problem has occurred in other fields. Until about the end of World War II, cars had left-handed lug nuts on their left wheels. The wheels sometimes fell off anyway, just as pedals sometimes fall off bicycles today.
Eventually someone designed the modern lug nut or bolt, which has a deep conical seat that absorbs radial loads with negligible movement, relieving the threads of a duty for which they weren’t designed. Wheels no longer fall off cars.
One of four or five lug bolts that attach a wheel to a car.
Pedal axles could be made with a similar shape, with a matching seat in the crank, if there was an appetite to functionally improve the bicycle at this late stage. Jobst Brandt took these matters into his own hands many years ago to solve a related problem.
On the positive side, even Shimano’s cheapest pedals have circular shoulders on the pedal axles, and the company specifies a high tightening torque of 35–55 Nm for pedal installation. They don’t do that for fun. They understand the problem.
But Shimano may be reluctant to move away from the century-old standard of 9/16" × 20 TPI threads with flat shoulders because they were burned by the failure of their novel Dyna-Drive standard a few decades ago. Those pedals (PDF) had a very large thread diameter (possibly 1", from a quick Google) to allow the pedal bearing to be placed inside the enormous crank eye (PDF), enabling a lower stack height (possibly slightly negative). Dyna-Drive was marketed on dodgy biomechanical grounds but might have incidentally solved pedal-crank joint movement by sheer screw size. The market rejected it for breaking compatibility and Shimano hasn’t touched pedal attachment since.
The joint we have for now
I cringe when I see famous YouTubers tighten pedals to a comically low torque with their soft dextrous iPhone hands and call it good. Doing that relies utterly on precession to slowly tighten the pedal. Meanwhile, the pedal squirms on every downstroke, chewing up the aluminium crank threads and face (and requiring a shake-down sprint to get the data from power-meter pedals to settle down. Whyever might that be?).
If they’re lucky, precession occurs as intended and the pedal gets tight enough before the threads are ruined.
I guess this risk is more tolerable if, like the YouTubers, you get new cranks every ten minutes. But I still object on principle. I drive rental cars with respect, too. I hate wrecking mechanical things without good reason.
Tightening a pedal with 9/16" × 20 TPI threads to Shimano’s upper torque spec of 55 Nm produces an axial tension in the region of 3 tonnes of force. That’s the clamping force between pedal and crank. Achieving that kind of tension is the only hope you have to prevent the lever of the pedal axle from wiggling the screw in the crank eye within the thread clearance when you stand on a pedal. Even so, you’re unlikely to arrest all movement during a forceful sprint, especially if you’re heavy or strong.
Make them tight.
Specialized Tarmac SL7 frame gets threads
Speaking of radially loaded threads, the bottom bracket is another culprit. That’s why the British / ISO bottom bracket standard uses a left-handed thread on the right side.
My bicycle has a British / ISO bottom bracket thread. It’s a good choice in practice even though it’s inelegant, because the alternative options are flawed too and less likely to stick around in the future. It’s not attractive for carbon frames because the carbon stays, being fatter than metal ones, don’t have much room to attach to the bottom bracket.
I guess Specialized got sick of reviewers and customers complaining about creaking press-fit bottom brackets, so instead of fixing that – how about machined metal everywhere? – they went to the threaded T47 standard. That might make creaking less likely by dint of sheer size, like Shimano’s Dyna-Drive pedals. But it’s a crude approach, as shown by the left-handed thread on the right side. I’m certain the Wright brothers would find a better design if they were working on bicycles today.
Frank Berto
The lack of standardization in the bicycle industry is a horror story. It just proves that we’re still basically a cottage industry.
When Berto wrote that in 1988 (in Bicycling Magazine’s Complete Guide to Upgrading Your Bike), the industry had two common wheel sizes (700C and 27"), there were multiple chainring BCDs, companies were moving to a new rear OLN distance, etc.
What were they thinking?
Also
Is this art? It feels like art, or is that the only way I can approach the horror of unreflective British suburbia? I don’t know this guy from Adam, but he’s /u/PaulBarlow113 on Reddit. He’s pretty good for a ventripotent 51-year-old.
Julian Alaphilippe just won a non-TT Tour stage on clincher tyres (with latex tubes that the tyre sponsor, Specialized, doesn’t sell). That’s perhaps the first time that’s been done since the early 1990s Michelin experiment, although clinchers have been tried from time to time since. Miguel Induráin, being heavy, had particular problems with tubulars creeping around the rim under braking heat and torque, so he sometimes used a clincher on the front for mountain descents.
While we’re on tyres and the Tour de France, Stage 1 saw carnage because rain arrived after months of drought, covering the road with a slurry of water, oil, and dirt. But it’s my opinion that the recent Crr obsession accounted for some of those crashes. There is a trade-off between low rolling resistance and good wet traction. You can’t have both, because wet grip arises from hysteresis of the tread compound and that is a major source of rolling resistance. Do we need regulated tyres as in Formula 1 to put an end to this tyre war? I suspect it has caused many broken bones in the last few years.
Six-time snooker world champion and multimillionaire Ronnie O’Sullivan on cyclists: “Running for me is the perfect thing ’cos they are just nice people. It’s not like cycling, where you’ve got to spend £10,000 on a bike. You get a lot of arseholes in that sport because they’ve got money and they think money is the all-important thing. I can’t stand people like that. You don’t get them type of people in the running world.”
Ride bike!
Samuel