Designing a bike around a 24″ wheel
Recently I was contacted by a customer who was concerned that my bikes using a 24” front wheel/700c rear wheel design were flawed. Wouldn’t the design put pressure on the neck and shoulders because the 24” bicycle wheel made the front of the bike lower than the rear?
Ah, if were only so simple as throwing a 24” bicycle wheel on a bike designed for a 700c wheel. If one picture is worth a thousand words, then four pictures must be even better.
1). Here’s a bike with 700c front and rear wheels.
2). If the 700c front wheel is replaced with a 24” wheel and no other changes made to the bike, the picture below shows how it would look. Not only is the bike very low in the front, but the brake pads are far above the wheel. The rider would definitely feel not only pressure on her hands and shoulders, but she would feel the thrill of no front brake!
3). If the 700c wheel and fork is replaced with a 24” front wheel and fork, braking is restored, but the bike still “points down.”
4). Here’s the right way to do it. The head tube on this bike is much longer than on the bike using twin 700c wheels. This keeps the front of the bike in the proper position so the top tube is horizontal again and the rider is in a normal position. Compare this to the first picture.
Bottom line: When a bike with a 24” front wheel is build this way, there is no design flaw. If the rider feels as though she is bearing too much force in her arms, the solution lies in tweaking the stem and handlebars.
What Size 24″ bike tire do I need?
I get lots of questions about understanding and sourcing 24″ tires and tubes. We use this size on our smaller bikes. Here’s all you ever wanted to know about the 24″ size. And if it’s not — leave a comment and I’ll be glad to help.
First, let’s talk about tire designations. 24″ is a very broad term. It’s best to refer precisely to the tire/tube size. This is done by referencing the bead seat diameter — the diameter of the tire’s bead, which rests inside the wheel rim.
You can tell what tire size your bike uses by reading the impressions on the sidewall of the tire. Typically, they will look like this: XX-YYY. The first two numbers are the width of the tire in millimeters and the second three numbers are the bead seat diameter in millimeters. This applies to all sizes of tires, not just 24″.
For instance, if your sidewall reads 25-520, your tire is 25mm wide (about an inch) with a 520mm bead seat diameter. Armed with this information, you can order the correct tire/tube for your bike.
This tech page from Schwalbe explains much more about the size markings on tires.
Terry has used two 24″ road sizes — 600A, with a bead seat diameter of 541mm and 24″, with a bead seat diameter of 520mm. The 541 size was used very briefly on only one Terry model — the Precision — for a limited time in 1985 – 1986. Later Precisions and all other Terry road bikes which are built with a 24″ front wheel use the 520 size.
Terry hybrid/mountain bikes using 24″ wheels front and rear have a 507 bead seat diameter.
ROAD BIKE TIRES/TUBES
Most Terry bikes use 24″ wheels with a 520mm bead seat diameter.
Terry Bicycles carries a variety of 24″ (520) tires/tubes which can be found here.
Your local bike dealer can order 520 tires/tubes from Quality Bicycle Products, 800-346-0004 (wholesale only).
Here is a list of some 24″ tires that are available:
Panaracer Pasela 25-250
Terry Tellus 32-520
Schwalbe Durano 23-520
Older Terry Precisions used a tire size popularly called “600A”, which has a 541 bead seat diameter.
Schwalbe RightRun 25-540 (part number 10282387). More information available here.
The Schwalbe RightRun comes closest to the Wolber Rallye 22mm tire originally specced on the Precision. The fit is quite tight (because it’s 1mm smaller than the original Wolber), but it will work. Schwalbe also carries presta valve inner tubes (part number 10519213) which can be found here.
HYBRID BIKE TIRES/TUBES
Schwalbe Kojak 40-507 (1.5″ wide). This tire is a slick, but rides beautifully! Unless you’re on gnarly roads, it’s the best. – Information can be found here.
Schwalbe Marathon Plus 47-507 (1.75″ wide) A very robust tire. Not light, but bomb-proof! – Information can be found here.
Inner Tube Source:
Schwalbe offers a variety of 507 inner tubes with either presta or schrader valves — AV10 and SV10. See here for more information.
REPLACEMENT WHEELS (520 or 507 size)
Do you need a replacement road or hybrid wheel? You or your dealer can obtain these from Velocity USA (800-453-6126). Velocity supplies all the wheels for my hand-built bicycles.