Condensed Answer: In most cases, 24-inch wheels will be too big for a 20-inch fork and frame, especially if the wheel is equipped with a large tire. Thus, it’s best to avoid this conversion as it will more than likely not work.
In a rare scenario, the wheel may fit. However, if the bike uses rim brakes, they will no longer be operational.
Also, the new wheel will change the geometry of the bike and prevent the user from running wider tires and using accessories such as fenders due to the lack of clearance.
Requirements For Installing 24-inch Wheels On a 20-inch Bike
The number one requirement for a 24-inch wheel to fit on a 20-inch bike is clearance.
To learn whether a 24-inch wheel will fit on a 20-inch bike, it’s necessary to do a bit of math.
Below is one of the possible approaches:
1. Measure the radius of the old wheel with the tire pumped to the needed air pressure.
2. Measure the distance between the fork’s crown and the point of the wheel the closest to it.
Combine the radius of the old wheel with the measured distance.
3. Measure the radius of the new wheel with the tire pumped to the needed air pressure.
If the radius of the new wheel isn’t at least 3mm smaller than the combined sum (old radius + fork clearance), then the fork doesn’t have enough clearance to safely accept the 24-inch wheel.
The same procedure can be repeated for the rear wheel, but in that case, the user has to measure two different distances:
a. The distance between the old wheel and the seatstay bridge.
b. The distance between the old wheel and the chainstay bridge.
Of course, in many cases, the user will not have access to a new 24″ inch wheel.
In that case, one can rely on data from a calculator such as this one.
1. The original 20″ wheel is equipped with a 2″ tire and has a 507.60mm diameter and a 253.8mm radius.
2. The new 24″ wheel is equipped with a 1.5″ tire and has a 583.20mm diameter and a 291.6mm radius.
Or in other words, the new wheel is 37.8mm or 3.78cm/1.48in larger.
In order for the new wheel to fit, there should be at least 4cm/1.57in distance between the fork and the original wheel. If the distance is 3.78cm or less, then the new wheel won’t fit unless the user switches to an even skinnier tire.
In the vast majority of the cases, the forks and frames of 20″ bikes come with a significantly smaller clearance (e.g., 2cm). Thus, in most situations, the user will be unable to install a 24″ wheel on a 20″ bike.
Another important aspect of this conversion is the brake system.
If the bike relies on rim brakes, they won’t be operational upon installing the new wheels because the rim will sit higher in relation to the brake shoes. As a result, the brakes won’t be able to grab the brake strip.
There are solutions to this problem but none of them can be described as super convenient. The options are:
- Adapters elevating the brakes bosses
- Re-welding/re-brazing of the brake bosses
- Installation of clamp-on V-brake mounts
If the bike uses disc brakes, the problem described above does not manifest because the rotor is always located at the center regardless of wheel size. Thus, even when you switch to larger tires, the rotor ends up at the right location.
Additional Negative Effects Of The Conversion
1. Altered geometry
The larger tires will raise the bottom bracket of the bicycle and make it more difficult to take sharp corners due to the higher center of gravity.
If only a front tire is installed, then the head tube angle (HTA) of the bike will get notably slacker. The new head tube angle will change the handling of the bike. The effect will be very noticeable at slow speeds.
Installing a 24″ inch wheel only at the back is dangerous because the tire will steepen the HTA and create a real possibility for the rider to go over the handlebars. Therefore, this choice is both illogical and dangerous.
2. Microscopic Tire Clearance
The new wheels will greatly limit the tire clearance of the bike and thus make it impossible to use wide tires and accessories such as full fenders.
Also, if the tire clearance is smaller than 3mm, even a small stone can cause the wheel to jam and trigger an accident.
3. Extra Weight
All things being equal, a larger wheel is a heavier wheel. Thus, by switching to 24″ wheels, the rider may make the bike heavier.
4. Weaker Wheels
When all parameters are equal, a smaller wheel is a stronger wheel. The shorter radius of smaller wheels reduces the leverage exerted by the rim. Thus, it takes more stress for the wheel to buckle.
That said, a quality 24″ inch wheel is incredibly strong and close to unbreakable unless there’s an accident.
5. Slower Acceleration
It’s easier to reach a high RPM (rotation of the cranks per minute) with smaller wheels. Or in other words, larger wheels reduce the bike’s acceleration properties.
However, bigger wheels have the potential to be faster overall. For example, if two bikes have the same high gear, the one with the larger wheels can potentially reach a greater top speed in that gear thanks to the larger circumference.
Or in simpler words, the larger tire covers more ground for the same number of rotations and can therefore be faster.
FAQ: Does a conversion from 20″ to 24″ wheels have a positive side when the bike is built for 20″ wheels?
Conversion to 24″ wheels can theoretically offer the following benefits:
1. Higher top speed (explained above)
2. Greater roll-over-ability (larger tires can roll over bigger obstacles)
3. A larger bike (bigger tires make smaller bikes feel larger).
However, in practice, the negatives of the conversion make the plan a bit shaky. In most cases, it’s better to sell the existing bike and get a dedicated 24″ or even a 26″ model.