Hang around any mountain bikers that don’t understand bike geometry for very long and you’ll be sure to hear about the magic of mullets. A 29″ (622mm, business) wheel in the front and a 27.5″ (584mm, party) in the back. It’s amazing how this silver bullet has caught the eye of the retail market. The magic go-fast setup that is sure to make a rider better.
Almost everything that I’ve heard or read about this setup online, from friends, or anywhere else is pure hogwash. Wrong or misled. Because they don’t understand fundamental bike geometry, they imagine what could be happening.
Mullets are not faster. They are not magic. They are not better. They are even ‘more fun’.
The easy answer to the question, “Should I run a mullet setup on my bike?” is, NO. You shouldn’t. It’s not an improvement over front and rear 29″ wheels. Mechanically, the full 29″ rig is going to be the highest performance (given the current market) setup that you can use on a bike. That’s why everyone racing on the road or in XC use the 622mm rim exclusively.
Who should look at using a mullet? Mullets solve particular problems that only come up in certain paradigms. If that isn’t something that you (or the bike designer) are facing, then choosing this without a good reason is a significant downgrade.
What are those factors?
- Ass clearance for shorter riders. Riders of shorter stature can often have an issue with tire buzz of their rumps when riding particularly nasty terrain or maneuvering at very high speeds. This is especially true on longer travel full suspension bikes but can still be a factor on hardtails that are used aggressively. The 38mm of increased clearance of the 584mm wheel is really meaningful if you’re currently dragging your ass across tire tread on your ride. Most people that have this issue are shorter than 5’7″ and ride extremely steep terrain or very fast. This is compounded if they are on a suspension bike of any kind.
- Proper effective seat tube angles on longer travel bikes. One problem that a bike designer faces is keeping a slack enough effective seat tube angle on mountain bikes with 622mm wheels and travel in excess of 100mm. The rear wheel and the seat tube fight over the same space on suspension bikes. You can’t get more travel without giving something up, and that price is paid at the saddle. Yes, saddles have moved forward on full suspension bikes in recent years and for good reason. Full suspension bikes are different than hardtails. While I (old fat guy, 5’10”) wouldn’t use an effective seat tube angle over 75 degrees on a hardtail, I’d go up to 76.5 degrees on a full suspension bike. Any more than that and I’m going to be paying dearly in my upper body not long into a big ride. That’s for Marin and NorCal. Pacific Northwest riders can go a little steeper, maybe another degree, Midwest riders might even go a degree slacker. Once 29″ wheel travel exceeds about 100mm, the designer will need to start nudging the seat forward or the axle rearward or both. This is why your seeing very long rear centers and very steep effective seat angles. Far more than work well for a rider or terrain….all for more travel. The question that needs to be asked is, is wheel travel so much more important than these parameters? It’s all a game of compromise, but many bikes have decimated the saddle and axle to serve a fancy travel number. One way to buy yourself 19mm of space is by reducing the rear wheel diameter from 29″ to 27.5″. This doesn’t come without a cost. The wheel snags on edges more, slowing the bike. Space opened up is often eaten up by larger tire widths and doesn’t really fix things, just makes them not as bad
- Sometimes, a designer of a suspension bike will have a linkage or swingarm system that is being implemented that requires space in between the tire and the crank and seat tube that isn’t available with a 622mm wheel. The addition of 19mm of clearance produced by going to the 584mm wheel can make possible what previously wasn’t. This is a problem resolved at the engineering level and shouldn’t effect the retail customer other than chassis specification. Is this a choice that should be made by the designer? It really depends on a variety of conditions and opportunities. Generally, I’d say it’s not but arguments will exist.
None of these solutions have anything to do with going faster, they’re about eliminating problems or opening up space. Here’s the thing:
Are you a shorter rider that’s having a problem with tire buzz?
Do you need the most suspension travel possible, regardless of geometry cost?
Are you designing a complex or novel system that produces gains in excess of the reductions of the 584mm wheel?
If this isn’t you, then you don’t need a mullet.