Mullet Bike Geometry Tutorial

The Mullet Bike

Mullet bikes are a trending subject in the enduro and downhill mountain bike world. It describes the combination of a large front wheel with a smaller rear wheel, typically 29" up front and 27.5" on the rear. The basic idea is to combine the good roll-over behavior and gyroscopic stability of twenty-nine-inch wheels in the front (where it matters most) with the nimbleness of a 27.5" wheels.

Since the UCI allowed mixed wheel sizes for the 2019 race season, many downhill athletes chose to run mullet setups in their competitions. Naturally, that sparked the interest of the mountain bike media as well as the consumers. As a result, we see many companies starting to sell bikes with mixed-sized wheels. Watch out for MX bikes, as this is the term that bicycle companies often use to market their mullet bikes.

Advantages and Disadvantages of a Mullet

On the plus side we have:

More butt clearance in steep section.
Due to the smaller diameter, a smaller rear wheel has lower gyroscopic forces. An effect which is often cited as the reason for a more nimble ride feel.
More space for increased tire clearance in the tightly spaced bottom bracket area.
A stronger and at the same time lighter rear wheel.

The disadvantages:

Inferior roll over behavior for rocks and roots.
Two different wheel sizes complicate stocking replacement tires, tubes, and rims.
The geometry of the bike may be compromised if the bike was not designed for a mullet setup from the beginning.

Converting an Existing Bike to a Mullet

Chances are that your current bike was designed for equally-sized wheels. If the bike is designed for 29" wheels, a smaller rear wheel will lower the bottom bracket and slacken the seat and head tube angles. The stack grows a little while the reach shortens. It depends highly on the geometry of the bike if the new numbers fall within normal rideable limits.

A similar issue emerges if a 27.5" wheeled bike is fitted with a bigger 29" fork and front wheel. In that case, the larger front wheel diameter has a doubled effect: first for the bigger wheel itself and second for the longer fork to accommodate that bigger wheel. Of course, some of the effect can be adverted by picking a lower travel fork. However, the result will most likely remain a lifted bottom braked, slackened angles, reduced reach, and increased stack.

Assessing the Effect of a Mullet Setup on the Bike’s Geometry

In this section, I walk you through an example calculation in detail. The bike that serves as an example is a Specialized Stumpjumper 2020 in size S4. The bike comes stock with 29" wheels front and rear. We are going to calculate precisely by how much the geometry changes if a smaller 27.5" rear wheel is installed. No rules of thumb and no guessing.

Step 1: copy the manufacturer’s geometry table

I am going to use the geometry of the Specialized Stumpjumper 2020 in size S4 as an example. There we have a reach of 475 mm, stack of 632 mm, headtube length of 120 mm, seat tube length of 425 mm, chainstay length of 432, bottom bracket drop of 42 mm, head angle of 65°, seat angle of 76°, and a fork offset of 44 mm. I enter these values in the geometry calculator and double-check the correctness with the wheelbase (1228 mm provided and 1228.6 mm calculated). Optionally, I adjust the lower headset height to a reasonable 1.5 mm (for an integrated headset it is the gap between the head tube and the fork crown) such that the calculated fork length matches the provided fork length of 550 mm.

Step 2: add the diameter of the stock wheels

I open the Inputs menu, click the Wheels check box, and enter the front and rear wheel diameter. The diameter of the stock wheels can be inferred to be 750 mm from the provided bottom bracket height of 333 mm. That’s the typical diameter for a 29x2.5" tire.

Step 3: modify the wheel diameter

Now that the complete bike geometry is entered into the calculator, I am able to make the mixed-wheel-size modification. To do so, I open the Geometry Modifications menu and enter 712 mm for the New rear wheel diameter. Why 712 mm? The difference in diameter of a 29" rim (622 mm) and a 27.5" rim (584 mm) is 38 mm, and 750 mm minus 38 mm equals 712 mm.

Step 4: check the results

Now we have the results: the angles slacken by 0.9°, the bottom bracket sits now lower at 320.7 mm, reach is shortened by 9.8 mm and the stack increases by 7.3 mm.

What Can You Do to Compensate for the Geometry Changes?

If you are not happy with the geometry changes that come with a mullet setup, then here are some ideas how you can further influence the geometry of your bike:

Does your full suspension bike have a low and a high setting? Use the high setting if you install a smaller rear wheel.
Run different tire widths, e.g., a voluminous 2.8" tire on the rear and a normal 2.5" tire on the front.
Use an angleset headset do modify the head angle.
Use offset shock bushings on a full suspension bike to lift or lower the bottom bracket.

Summary

A mullet setup is a great way to experiment and alter the ride feel of your bike, although a mullet setup might not be superior for any rider or any trail. With the Bicycle Geometry Calculator you have the possibility to check in detail how a mixed-size wheel setup affects your geometry. And that’s a great opportunity to geek out about mountain bike geometry while enjoying your next post-ride beer.