More MTB Mullets – Comparing Travel, Headtube, Trail & Flop
An interesting question come across my email wondering about MTB Mullet setup. In particular headtube angles and performance. How did the headtube angle compare between the Yeti SB165 in a 27.5″ stock setup with a 180mm fork compared to the Mullet 29″ 170mm concept?
What started as a quick email reply of concept turned into a more thorough review and I thought with more questions on MTB mullets these days we could put it up on a post for everyone.
So, the disclaimers. I couldn’t find Yeti’s published SB165 Mullet geo so I build a model to review. These models aren’t gospel truth on geo, but having been in this game for a lot of years some of the geo’s you’ve seen published I wouldn’t take for gospel either. Point is, these numbers will give you an idea of the concept I wouldn’t bet the farm on precision four points behind the decimal…
Next disclaimer. It could be a thesis trying to explain how these changes directly modify performance so we’re not going there today. We’ll have some basic concept and comparison of how one change directly modifies a whole bunch of the handling.
OK here we go.
So some riders will be familiar with the geo modifications from bikes like the SB130 or SB140 Lunch Ride setups. Headtube angle, trail, flop, reach and stack are all modified when you increase or decrease axle to crown distance.
With a mullet bike you’ve got other factors as well. Front wheel radius increases or rear wheel radius decreases depending on setup. When you’re working with a front wheel modification like the SB165 or SB140 your fork offset is also likely to change. All of these factors make it a bit complex (for me at least) to calculate the geometry and trig needed. So until BikeCo gets a grant to do it we use models to expedite this.
In this case the model utilizes the stock size M SB165 wheelbase, an estimated wheel radius (27.5/2), the published axle to crown of 569.1 as well as the 63.5 degree headtube angle. We build a model and then extend the axle and crown down and change the offset. From that point we rotate the model on the rear wheel axis until the front wheel is raised to the appropriate height.
Alright. So the quick answer to the question: “What would be the head angle of the SB165 in mullet config with 170mm 29″ fork?” It’s about a degree slacker coming in just under 62.5 degrees. That’s around 1.6% difference. So why does less than 2% difference and less than 1 degree headtube angle feel like such a notable change?
Lotta reasons. I decided to build out a couple more options into the model to help highlight where all the changes are.
Fork Travel & Wheel Size
Some considerations if you’re planning a mullet. ALWAYS check that at full compression the front wheel has clearance of the downtube. Remember that in full compression your tire will expand at the top (as the bottom of the tire is forcibly compressed). If at any time the tire can contact the frame you’re gonna get hurt. Period. You don’t compress a fork all the way going slow, in flats, where you can correct this. If it can hit the frame it WILL stop the front wheel turning you into a lawn dart. No good.
So given the 29er has a larger radius it is typical for both geometry considerations as well as keeping the wheel off the downtube to go to a smaller travel option like the 170mm 29er versus the 180mm 27.5 on the SB165 mullet.
Using Yeti’s geo charts we see the SB165 27.5″ 180mm is listed at 569.1mm axle to crown. The 170mm 29″ (from the SB150) is listed at 577.1mm. The fork with less travel, albeit bigger wheel, is 8mm taller axle to crown. Then you add the radius difference between tire sizes, for this we’ve gone with a clean 27.5mm and 29″ diameter but they do vary a bit with tires, the less travel fork again adds 19mm. (more Fox dimensions here)
Well this forces the front end up, pivoting on the rear wheel axis, increasing stack while decreasing reach.
Sidenote: The modification to these dimensions from the fork height needs to have the headtube angle compensated for, but the radius gain is the same no matter what.
Let’s look at numbers we can compare (again don’t use these as Jeopardy answers but you get the point…)
Medium Yeti SB165
I would say most sport level riders would feel the difference between the 180mm stock and 170mm mullet bike. I’d also go on a limb and say I’d be hardpressed to feel a sub 2% change in a lot of things on my bike… So if it’s not the headtube change you’re feeling what is it?I’ve gone down this rabbit hole previously comparing the 160 and 150mm fork options on the SB130 – but again I come back to trail and wheel flop. I think I would feel the 7.7% difference in trail but more notably the nearly 11% change in wheel flop during steering input.Read more on Wheel Flop on Wikipedia HereAlso it’s important to remember in a world of 2.5mm stem spacers and 5mm stem lengths when you mullet that 27.5″ your hands are going up and back. It’s something to consider when dialing in your new setup if you find front wheel traction issues.What Are You Changing?So riders building their own 29 / 27.5″ mullet bikes really are working to define which aspects of the bike they want to modify.Do you want the larger front wheel to monster truck over terrain with the smaller rear wheel to dive into turns more quickly? OK cool. But how do you want to get there?Do you need equal or near equal amounts of travel? (I do like to run out of talent before travel personally) Or do you prefer keeping the bike as close to factory as possible in geometry?Does the additional wheel flop help the bigger wheel feel more lively into turns? I’m not sure. I haven’t done enough research to have an honest opinion here. I love my personal SB130LR but it does have more drift in it than the stock SB130…A rider who solely was looking for the improved front wheel roll over might look at the 150mm option. However they probably are going to have to setup a decent amount of volume spacers as well as compression control to keep the bike behaved with that much less front travel. Are they fast enough / skilled enough to control the differences in suspension setup fore and aft?Frankly it comes down to understanding your riding style, terrain and what you need out of the bike. Good news? Well, kind of like the triangle math used in some of our geo calculations you only need a few data points to extrapolate the rest.Going Mullet?If I was in the market to build a mullet for myself I would sit down with BikeCo owner Joe Binatena and pick his brain on setup. Joe’s been working with Cody Kelley, Brian Lopes and Kevin Aiello for years racing at the highest level. He’s seen what works with his riders as well as other riders on the world level. There is a reason that top pro riders consult with him on setup.And if you’re a BikeCo client you too have access to this expertise. Work with our expert staff to help define what you want and need out of your setup. We welcome questions and if the sales team doesn’t immediately have the answer they have constant access to Joe as well as our racers to help dial you in.Already know what you’re looking for? Well spec your dream bike using the BikeCo Custom Builders Here.Mullet Options:Alchemy Nine7FiveYeti SB165Yeti SB140
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|Headtube Angle||Model Driven||63.5||62.5
|Trail||Wheel Radius*Cos(HT <)-Offset / Sin(HT <)||132.2mm||142.4mm
|Wheel Flop||Trail*Sin(HT <)*Cos(HT<)||52.8mm||58.4mm