24% Stiffer, More Compliant, Able to Leap Tall Buildings

Hi! Welcome back to Killa’s Garage!

Today’s post starts with a few definitions shamelessly pulled off the internet:

Stiffnessthe rigidity of an object — the extent to which it resists deformation in response to an applied force. The complementary concept is flexibility or pliability: the more flexible an object is, the less stiff it is.

Compliance – a property of a material undergoing elastic deformation or (of a gas) change in volume when subjected to an applied force. It is equal to the reciprocal of stiffness.

Compliance and stiffness are opposites!

softrideThe SoftRide people certainly knew about compliance, but that was years ago.

A current manufacturer makes the following (very typical) claim:

“[Their new frame] uses a 27.2mm seatpost, which is designed to both shave weight and increase vertical compliance. Of course stiffness and efficiency remain important, as one can see by the large downtube, tapered headtube, bulbous chainstays, and wide PF86 bottom bracket shell. The net effect is a bike that is 24 percent stiffer…”

Wait a minute. Stiffer and more compliant? It’s one or the other isn’t it?  Not necessarily. Read it again. Maybe the greater compliance is all in the seatpost. I assume the old design used a larger OD seatpost that was stiffer than the new one. Maybe there is nothing more compliant about the new frame at all. The large downtube, tapered headtube, bulbous chainstays, and wide PF86 bottom bracket shell certainly all scream stiffness, not compliance.

The claims actually may both be true – more compliance while seated, achieved with a more flexible seatpost, and greater stiffness when that’s important (sprinting, climbing off the saddle) due to a stiffer frame.

There is another whole discussion around vertical vs lateral stiffness. But for the rest of today’s post I am going to consider only vertical stiffness and compliance.

BTW, the frame is also claimed to be 20% lighter, but that’s also for another day.

Time for a little physics, but don’t run away. It’s all about springs.

A bicycle and rider can be modeled as a system of weights (where the rider is by far the most significant weight) and springs. The seat is a spring. The seatpost is a spring. The frame, the cranks, the wheels, the tires – all springs.

The stiffness of a spring, also known as the spring rate, is defined as the force required to deflect the spring a given amount.

For instance, consider a tire as a spring. Suppose we apply a load of 100 pounds on a tire and it deflects 1/4″. The tire’s spring rate is calculated as 100/0.25 = 400 lbs/inch.

A spring’s compliance is the inverse of the spring rate. So the tire in the example above has a compliance of 1/400 = .0025 inches per lb.

This sounds like a very small number, but it is huge compared to the compliance of other parts of a modern bicycle system.

The neat thing about using compliance instead of spring rate for a series of springs is you can add up the compliances of all the elements to arrive at the compliance of the whole load path. Because the tire has the largest compliance, it dominates the compliance of the system. But still, the compliance of each component contributes to the total compliance.

A rider can be viewed (simplistically) as resting on two stacks, or series, of springs. One series leads down from his/her butt through the rear wheel to the ground. The other leads down from his/her hands through the front wheel to the ground.

What about the series of springs that lead from the rider’s feet through the pedals/cranks/bottom bracket/frame, etc. For now, discussing compliance, let’s ignore that one. When you judge a bicycle’s comfort (compliance), do you think of your feet? I think of my butt and my hands. Later when we consider stiffness and efficiency, that one becomes critical.

I hope this introduction has gotten you interested. In coming posts I will wax esoteric on each load-bearing element of the bicycle system from the rider to the ground. I think I’ll start with wheels.

Until next time,

Killa

 

 

Nails and Tires

My friend Bryan “Doc” Dotson gets around by bicycle – a lot. This is a guy that takes stray cats to the vet in an infant trailer. So, when he makes an esoteric observation on bicycles and cycling, I listen. From Doc:

Flat tires. You would think that they would universally be regarded in the same category as say, fire ants or presidential candidates, but that’s not true. My youngest daughter, when she was about 3 years old, got really excited every time I had a flat tire on the bike. It’s her thing now.

One type of flat intrigues me. I just had my fourth “nail” puncture:

Nail in Tire

I’ve had three on my 2” tire mountain bike; this is the first I’ve had with my 34 mm tires (which, by the way, I ride much more). All have been the rear tire.

This is now more than a fluke.

My best guess is the front tire picks up the nail, which then tumbles in the wheel track. The rear tire arrives when the nail is ideally positioned to drive straight in.

I’m interested in how many others have observed this phenomenon. Anyone ever had a nail in the front [tire]?

An old article by Jobst Brandt describes this phenomenon. So Doc, you are not alone in your observation. I’ve only ever picked up one large nail in a bike tire, and it too was in the rear tire.

Another thing I’ve noticed – at almost any intersection while I am stopped waiting for a light, I can find a nail or screw lying in the street. I usually pick these up, not so much for for fear of flatting my bike tire, but because as a cyclist and a motorist, the car tire that picks up that nail later in the day may be my own.

Video – A Better Way to Remove and Re-install Your Front Wheel

In my most recent post, I described an alternative method to remove and re-install your front wheel that does not involve reaching down to the axle with both hands while trying to balance the bike with your chin. Recall the steps:

1. Stand directly in front of the bike.
2. Hold the handlebars with your left hand.
3. Place your left foot next to the right side of the front wheel.
4. Reach down with your right hand to operate the lever.
5. Press the inside of your left calf against the quick-release nut to hold it still while you turn the lever with your right hand to loosen or tighten.
6. When you are ready to flip the lever closed, release your calf pressure so that the fork can settle down evenly over the axle.

Here’s a video demonstration of the technique for the visual learners among you.

https://youtu.be/naVoZDnj_VM

If you like video and want to see something specific, let me know.

Thanks for watching!

Killa

Not-So-Quick-Release Skewers

I promise to move on to other topics next post, but I want to talk about the relationship between your front quick-release and your fork. And I will show you a trick to make wheel removal and installation easier – and a lot more elegant.

The quick-release mechanism was patented in 1930 by Tullio Campagnolo, then a frustrated bicycle racer. Blah, blah, blah. Read all about it here: Wikipedia Quick release skewers.

Fast forward some sixty years to the introduction of secondary retention methods, AKA lawyer tabs. These tabs on the fork dropouts prevent the wheel from bouncing out of the fork, even if you forget to tighten the quick-release. See Sheldon Brown’s article on quick-releases for some good photos of various prior designs. I think you’ll agree that lawyer tabs are the best of the lot.

Am I the only person that finds it interesting that no-one is concerned about the rear wheel falling out of the dropouts? I guess that would cause a less spectacular crash. Continue reading “Not-So-Quick-Release Skewers”

Skewer Spring Reprise

Amazing! Within 72 hours of my post on quick-release skewer springs, this tri-bike sBackwards springhows up in the garage.

Who can tell us what the problem is here?

That’s right, the spring is installed backwards. I did not stage this photo. I removed the skewer and there it was. Fortunately it didn’t jam the wheel in the dropout and all ended well. One more rider who will never make that mistake again.

 

 

 

 

Thinking About Quick-Release Skewer Springs

Do you have recurring nightmares about quick-release skewer springs? Me neither. But sometimes on a long ride I think about them.

Bicycles are made up of lots of parts. Quick-release skewer springs are among the smallest, right down there with bottle cage bolts. Such simple devices, yet so often misunderstood. I am talking about the small cone-shaped springs that go bouncing off into the grass if you unscrew your quick-release all the way. As you know (which means “You may not know this”), the pointy end of the spring goes towards the middle. It matters. This is how it should look.

FullSizeRender

If you just like to ride your bike without understanding it on a deeper personal level you can stop reading now and go look up a video on how to operate a quick-release. Otherwise, read on.

What do these springs do anyway? They exert a centering force on the quick-release skewer, equalizing the gap on both ends of the axle. This, in theory, makes it easier to insert the wheel into the dropouts. Then when you clamp the lever down, the springs compress to get out of the way.

Are they necessary? No, not really. If you lose one, remSpringWeightove the other one and ride without them. None is better than one. In fact, on bikes with rear-facing dropouts, many riders remove the skewer to remove the rear wheel. If you change your wheel this way, the springs are completely extraneous – and think of the weight you’ll save by leaving them out. Almost a gram!

There are several ways to get quick-release skewer springs right and wrong. Here they are, from best to worst, in my opinion:

  • Two springs, pointy ends inward – Correct!
  • No springs -You may have to jiggle the wheel to get it in place, which you sometimes have to do even with the springs.
  • One spring, pointy end inward – This pushes the skewer to one side, making it more difficult to insert the wheel into the dropouts.
  • Two springs on one side, nested, pointy ends inward – I just came in from the garage and added this ingenious method of getting it wrong after seeing it on a bike I am working on, really!
  • One or two springs, wide end(s) inward – This is bad. The wide part of the spring will ride over the axle. Several undesirable results can occur when you force the axle into the dropout with a spring over it.
    • The axle (plus spring) won’t seat fully into the dropout, causing the wheel to be misaligned. If you’re following someone and you notice their wheel is way off-center, but not wobbly, it is probably because one spring is on backwards.
    • The spring can get mashed and deformed as you jam it into the dropout. Throw that puppy away. Deformed springs will not compress cleanly and may prevent you engaging the dropout firmly the next time.
    • The wheel can get stuck in the frame. I had a friend text me – including pictures – with exactly this situation, wondering how a piece of wire had gotten jammed around his axle. The spring was so mangled it was unrecognizable. It took him an hour or so with needle-nosed pliers and assorted other implements of destruction to remove the culprit and get his wheel off. That’s one rider that will never get it wrong again!

Just remember, pointy ends to the middle.

In my next post, I’ll stay on the quick-release theme and discuss the relationship between your front quick-release and “secondary retention devices”, aka lawyer tabs. I’ll show you a trick to ease wheel removal and installation. And no, I am not going to suggest that you grind off the lawyer tabs.

 

 

Why do they call me Killa?

Why do they call me Killa? That’s a good question. As is common in social circles, especially of cyclists, everyone has a nickname – Cheetah, Harpoon, Patron, Ironman, Smoke, etc. And there is always one guy that is “giver of nicknames”. In our case, it’s Patron. He named me and it stuck. I think it had something to do with our relative speed on our daily commute. You’ll have to ask Patron.

Hi Killa’s Garage Fans!

Hi, my name is Mark. They call me Killa. I’ve been riding and working on bikes ever since I can remember. My friends know me as their mechanic. I’m the guy that fixes everybody’s flats out on the road or performs a quick adjustment in the parking lot before the ride. I work on bikes in my garage (Killa’s Garage, get it?). Anyway, I don’t do this for a living, but wrenching as a pastime has taught me utmost respect for the mechanics and fitters at the local bike shops. I think being a wrench is the coolest thing.

You won’t find videos on how to change a bottom bracket, or step-by-step instructions for taping handlebars. If I see a particularly good video, I’ll link you to it. Go to Park Tools or Art’s Cyclery for excellent straightforward bicycle repair and maintenance videos. I plan to share with you what I hope you find are interesting little details about cycling that I have picked up over the years from racing, riding, working on my friends’ and my own bikes, and simple observation.