In general, 29er wheels are known to offer a smoother ride thanks to improved rollover ability, and they provide better momentum compared to smaller diameter wheels. Each of these solutions introduce their own set of tradeoffs, and with so many possible combinations, you’ll need to focus on solving (or simply living with) the issues that matter most to you. There are other geometry hacks that can be employed as well, including angle- and reach-adjust headsets, fore or aft seat post heads, eccentric BB cups, tire size variations, shorter/longer cranks, and shock link swaps, just to name a few. For example, with a 27.5″ bike conversion, going to a shorter-travel, 29er fork could allow you to maintain much of the original geometry. In the end it may be possible to minimize, or even eliminate, some of the geometry changes listed above through various methods. The Spectral:ON I tested earlier this year comes as a mixed-wheel bike from the factory, no conversion necessary. Going from 27.5″ to mixed wheel increases the BB height by less, at about 8mm. Still, assuming a lower bottom bracket is a good thing, then starting with a 29er and going mixed wheel delivers the goods with about a 14mm decrease. Of course, too low and pedal strikes become an issue, not to mention log and rock hops require more oomph. The other ‘L’ in LLS, lower bottom brackets have been in favor for several seasons now due to the added stability they tend to offer. On bottom bracket height, the outcome is… mixed (get it?). This is one area where you’ll likely be setting your bike back in time. Looking at a the numbers on a couple different bikes, going mixed wheel reduces reach by about 12mm, which can be the difference in geometry from one model year to the next. However, if a bike already feels too large in terms of fit, a shorter reach could make it more comfortable. In both cases - starting with a 29er or 27.5″ wheel - the reach is going to get shorter as the head tube tilts rearward. The “longer” part of the LLS equation generally refers to reach, and on this front, unfortunately, it’s bad news for progressive geometry as well. On the flip side, if it climbs well enough but suffers on the descents, a wheel change could make sense. What does this mean? If your matched wheel bike isn’t great on climbs, mixing the wheels is going to make it even worse. Remember, the frame isn’t changing, it’s only being rotated with one of the axles serving as the pivot point. On this front, the seat tube angle is actually going to become slacker, not steeper, by exactly the same amount as the head tube angle. The latest mountain bikes are seeing ever-steeper seat tube angles, so much so that perhaps we should amend LLS to LLSS. In terms of progressive geometry (longer, lower, slacker ), a slacker head tube angle is moving things in the right direction, and a full degree of change can be significant. I plugged in the geometry for two different hardtails, each with a 66.5° head tube angle, and with a mixed-wheel setup, both HTAs landed just below 65.5°. Starting with either a 27.5″ or 29er bike, converting to mixed wheel will generally slacken the head tube angle by about a degree, give or take. (More on that later.) Head tube angle (HTA) ✅ Also, this analysis assumes the fork’s axle to crown length remains roughly the same between builds. The initial geometry of your bike frame will also make a difference in terms of the size of the changes, though it should still be pretty similar to what I outline here. I decided to use 27.5×2.4″ and 29×2.6″ tires for my calculations so if you’re using something different and want a more accurate picture of what that means in terms of the angles and measurements you’ll want to run the numbers yourself. Whether you’re starting with a 29er or a 27.5″ bike, the geometry changes will be fairly similar and should only vary slightly in magnitude depending on the tire size you select for your build. Taking a matched-wheel bike and mixing things up will change the bike’s geometry, so it’s potentially an easy way to get some of the angles and measurements you’re looking for.įor all the geometry comparisons below, I’ll be using results from the Mad Scientist MTB geometry calculator. Mountain bike geometry has proven to be incredibly progressive over the years, and out-of-favor angles and measurements are often one of the key reasons for an upgrade. Unless your bike frame has a flip chip, its geometry is pretty much set. So is it worthwhile? Well, here are some important things to consider. Mixed-wheel bikes are seemingly all the rage at the moment, and it’s technically possible to convert an existing bike to mixed-wheel with just a couple key component swaps. With 2022 mountain bike models nearly impossible to find at the local bike shop, many riders are looking to get another year or two out of their current bike.
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