Thursday, May 17, 2012

Flat-changing Clinic part 2

This is Part 2 of the 3-part series on Bicycle Maintenance: Tires and Flat-changing.

In Part 1, I talked about what to expect in my flat-changing clinics, held monthly and hosted by B+L Bike and Sport. Here are some of the questions that come up in my flat-changing clinics. Please feel free to add your own questions in the comments. :)


Do I need to take the whole tire off?

No. You only need to take one side of the tire off so that you can remove and replace the tube.

What are the advantages to removing the whole tire?
Facilitates finding the thing that made you flat.
Removing the tire entirely will make it easier to find the glass, thorn, piece of wire, etc. Sometimes the object will remain in the rubber of the tire, slightly poking through the inside, just waiting to flat your new tube. With the tire removed, you can practically turn it inside-out.

Is there any disadvantage to removing the whole tire?
Remounting it in the wrong direction. 
Tires are often designed to roll best in one direction. Unless you know the direction of rotation, which is sometimes stamped into the sidewall of the tire, you need to either
a) pay close attention to decals and logos when you removed the tire, i.e., logos match up to the cassette /skewer nut side or the skewer lever side, or
b) know how to read the directional tread of a tire.

"<= ROTATION" is stamped into the sidewall of many tires.


What do all those numbers on the tube box mean?

Example: 700 x 18-28, 48mm  (road tube)
  • 700 = roughly 700mm rim diameter
  • 18-28   Number of mm wide the tube will comfortably inflate. If the tire is narrower than 18mm, there will be too much flabby tube inside it. If the tire is wider than 28mm, the tube will be stretched too thin.
  • 48mm  length of the presta valve on a road tube. Deeper rims require longer valve stems. A short valve stem inside the rim may not allow you to attach the pump to inflate it.
Example: 26 x 1.9-2.125  (mountain bike tube)
  • 26 = 26" diameter rim
  • 1.9 - 2.125  Number of inches wide that the tube will comfortable inflate.
How do I know I have the right size tube?
Easiest way: take your tire to the bike shop and tell them you need a spare tube. When you remove the tube from the box and put it in a plastic bag, tear off the end of the box with all the numbers and put it inside the bag with the spare tube.

Are bike measurements in inches or metric?
Road bikes are most commonly measured in metric, mountain bikes in inches.

What is the "bead" of the tire?
The bead is the edge that hooks into the rim and holds the tire onto the rim. Tire beads are either wire or kevlar.

What is the difference between wire or kevlar bead tires?

Wire bead
  • often less expensive 
  • harder to mount onto rims 
  • better for 230+ lbs riders (because the bead stays in place and won't blow off the rim) 
  • adds 50-75g in rotational weight, which is fine for flat terrain, but more work in hilly terrain.
Kevlar bead 
  • often more expensive than wire bead 
  • easy to mount into rims
  • fold-able (you can carry one in your back pocket if you need to)
  • reduced rotational weight
How much air do my tires need?
Every tire is stamped with a recommended inflation. Road bike tires are high pressure, low volume, and are generally inflated between 95-125 psi. 
Road bike tire recommended inflation: 115 PSI / 125 PSI

Mountain bike tires are low pressure, high volume, and are generally inflated between 30-50 psi.

Mountain bike tire recommended inflation: 36-65 PSI 

Why are some road tires perfectly slick and others have ridges (tread)?
Perfectly slick road tires are most often used by racers, having the least friction and drag. Tires with more tread provide better grip and are better for directing water or mud away from the center of the tire. Ask at your local bike shop which tire is best for your riding style and goals.
Is the rim and the wheel the same thing?
No. The rim is only the hoop part with the holes in it. The wheel is made up of the rim, spokes, hub, etc.

What is presta and schrader?
Tube valve stems are either presta or schrader (looks like the car tire).

presta valve in a mountain bike wheel
Unscrew the top (little gold piece) to inflate. Be sure to tighten it back down.

How often do I need to pump up my tires?
Check tires before every ride. 

Properly inflated road tires should feel completely solid. If you can make any depression with your fingers in the sidewall at all, you need air. Use a good floor pump and knew for certain how much pressure is in your tires. 

If road tires are to be harder than an apple, mountain tires should be a bit softer than an orange. You should be able to depress the sidewall slightly. 

If you have additional questions or are curious about tubes or tires, please see Sheldon Brown's website. Although Sheldon is no longer with us, his site is maintained by volunteers and remains a great resource for cyclists.

Next week I'll have Part 3: What's in your saddlebag? Items you should have with you when you ride.

1 comment:

Fred Roberts said...

:) this is e very good post. It maybe a bit embarrassing but I haven't learned to change my flat tires until I was 25. So I am showing this to Danny so that he'll start trying it on his own. I have just given him his first set of bicycle torque wrenches since he is fond of cycling.