A Beginner’s Guide to Mountain Bike Wheel Truing
A Beginner’s Guide to Mountain Bike Wheel Truing
Wheel trimming is one of those daunting tasks for many – often seen as some sort of dark art, mastered only by bearded bike wizards who have spent years learning the craft. Thankfully, that’s not the case, anyone who can turn a wrench on their own bike can really be a mountain bike wheel.
Wheels are very basic at the most basic level. You have a hub in the middle, a rim on the outside, and the spokes that connect the two supporting hubs. The hub is held in place by spoke tension – the bottom spokes hold it in place, and the top spokes prevent over compression of the spokes below. The spokes also control the “trueness” of the wheel, which is how straight the wheel is from side to side, called lateral trueness, and the roundness of the wheel, called radial trueness. The main factor controlling these is spoke tension, which is affected by the nipple. The spoke is essentially a bolt, and the nipple is a nut – screw the nipple further onto the spoke and it will pull tighter, back off and it will loosen.
Aside from a more in-depth look at wheel building techniques and the more whiskered aspects of other things, this is the most basic breakdown of how bicycle wheels work, the fundamentals of which haven’t changed for hundreds of years. Such is the beauty of the humble bicycle wheel. In its simplest form – a regular hub with J-spokes, nipples and rims – there is a depth and breadth of knowledge spanning generations, and spare parts are readily available wherever you go. Happily, it also means that if your approach is methodical, it’s hard to screw up.
How to Check Wheels
Dressing rounds is something that a lot of people find difficult, or at least they think it is. It’s also arguably the most obviously “wrong” wheel element. It’s easy to turn the wheel and see if it wobbles, and it’s one of the quickest ways to check the health of the wheel. When you turn the wheel, it’s very noticeable as it moves from side to side. In my opinion, a millimeter or so is acceptable on a mountain bike wheel. Any more wiggle than that and you may want to consider adjusting your wheels. It’s worth checking that there is no wobble in the carcass, as this is fairly common and tire inserts like Cush Core may exaggerate any tire wobble. It will be fairly obvious whether the wobble is in the wheel or in the tire when the wheel is in the caliper.
Another, and arguably more important, way to check wheel health is to check spoke tension. Since the tension in the spokes is what holds the wheel together and ultimately keeps the bike rolling, this is the more critical thing to pay attention to, and it only takes a few seconds. Starting with the valve, grab each spoke and try to move it to the middle, or pluck it like a guitar string, until you’ve checked each spoke. This isn’t an accurate way to check spoke tension, but if you have one or two completely loose spokes, you can quickly find them this way. Once you’ve found some loose spokes, it’s time to tension and adjust your wheel.
Why is wheel tensioning and trimming important?
As I mentioned earlier, spoke tension is critical because that’s what holds the entire wheel together. If the spokes are not at the correct or even the correct tension, this can put more load on some of the spokes than they should, putting extra stress on them. Imagine a completely loose spoke – that spoke is now unable to support the hub’s load, nor the opposite spoke. Both spokes will bend, stress will concentrate on the threads and J-bends, and eventually break. Riding a wheel with loose or missing spokes will eventually lead to many broken spokes and the wheel needs to be completely rebuilt. That’s why it’s important to replace any damaged spokes immediately to prevent damage to other spokes.
Trimming is also important because it keeps your wheels straight and predictable on the track. A large buckle will be something you can feel on faster trails and roads, and a buckle big enough has the potential to cause your tires to rub against the frame or fork, maybe even all the way through the carbon fiber rear and down cross. Yes, it did happen. Trimming your wheels isn’t as important as it once was. When we all use rim brakes, a slight grommet on the rim means your brakes aren’t working properly or at all, so it’s very important to keep your wheels right. We’re all using disc brakes these days, and for the most part, straight wheels aren’t that important, which is good because it means we can focus on spoke tension, which is more critical.
Wheel Dressing Tool
Now that we’ve decided it’s time to build a wheel, we need some tools. Depending on the level of precision you want to achieve, you may need all or few of these tools. The only thing really 100% needed is a spoke wrench.
Spoke wrench: The most common square nipple uses a 0-gauge spoke key. Check out this parking tool guide for more information.
Spoke tension gauge: Arguably the most important tool after the spoke wrench. This is essential for an accurate reading of your spoke tension and makes wheel trimming easier than ever. This parking tool meter isn’t the most accurate, but it works well.
Trimming Racks: A good trimming rack can make the wheel real by showing where the wheel is misaligned with a gauge. If you don’t have one, you can also attach a zipper to the frame or fork. This Park Tool TS-8 stand is perfect for the home mechanic.
Disc Gauge or Disc Bar: This is the gauge used to check that your rim is centered on the hub of your frame. This is useful, but not necessary for a home mechanic. This Park Tool WAG-4 will be more than most people need.
A simple zipper on the frame/fork is enough for basic household needs.
How to tension and real wheels
Now let’s dive into it and trust me, it’s not as hard as you might think. Once we determined that the wheel was incorrect, it was time to put the wheel in the trim rack. If you don’t have a trim frame, just attach a zip tie to your frame/fork and trim the end to a millimeter or less from your rim. This way you can see any deviations in alignment.
The first thing we need to do is get the wheel tensioned. This is where a tensiometer is very useful. If you don’t have a tensiometer, you can move the spokes with your fingers or tap them with a fingernail or screwdriver to hear the tone, like a guitar. It’s not particularly accurate, but it’s better than nothing. When you notice a loose spoke, tighten the joint with a spoke wrench until it has the same tension as the rest of the surrounding spokes. On a rear wheel with a symmetrical rim, the drive side spokes will always have higher tension than the non-drive side spokes, and on a front wheel your disc side spokes will always be higher. This is because the hubs are offset to account for the flywheel at the rear and the rotor at the front. On an asymmetrical rim, the tension is usually the same on both sides, but it’s important to get all the spokes loaded evenly on each side – it’s normal if one side is higher than the other. Tension meters usually come with conversion tables, and most rim manufacturers specify the ideal tension on their website.
Once the wheels are in tension, we can make adjustments. Trimming wheels can be a more difficult task, but it’s easy enough if you work slowly and methodically. Spin the wheel in your trim frame or frame to see where the rim isn’t real. I like to look for the biggest wiggle, i.e. the rim away from the zipper or the corrective bracket caliper and go from there. Where it moves away, you can tighten the spoke on the side you want to pull it over and loosen the spoke on the other side. I like to work with odd numbers, so starting in the middle of the swing, I move in pairs. For example, for very small wiggles you can simply tighten one spoke, for slightly larger wiggles you can tighten the spokes in the middle of the wiggle and loosen the spokes on either side. For bigger ones, you’d tighten the middle spokes,
The best practice to avoid over-tightening or over-correction is to change the nipple only half a circle at a time. Less is more when it comes to wheel trimming, which allows you to be very precise. After tightening some spokes, continue to rotate the wheel in the same direction and correct any other wobble that may be present in the wheel. Now you can keep going and make small changes every time you turn the wheel, and each turn will slowly but surely get straighter.
Once the wheel is real, it’s worth checking out the wheel’s plate. Disc is the center of the rim above the hub, and for most bikes, the rim should be right in the middle. If the wheel was bumped correctly before and you didn’t have to correct any major buckles, your wheel may still be bumped correctly, but it’s worth checking. You can use a disc bar to measure from the hub end cap, adjust the center of the disc bar to touch the hub end cap on either side. If the end of the plate is on the edge of one side, then you need to tighten the edge towards that side to bring the edge to the center. Alternatively, you can simply place one of the calipers on the trim frame so that it just touches the rim and turn the wheel on the frame. If it’s not touching now or pushing the caliper through, then you need to adjust the rim one way or another. You can also look at the wheels in the frame or fork. If it looks roughly in the center, it’s probably good enough. If it’s on one side, such as the left, you need to tighten all the spokes on the right side of the hub and loosen all the spokes on the left side. Again, only turn half a turn at a time to avoid overcorrecting.
If you want to go a little deeper, you can also try radial trimming your wheels, although this is more difficult with good wheels. Move the trimming caliper or cable tie to the very edge of the rim, turn the wheel, and see if the rim is a little egg-shaped. Where the rim moves inward toward the hub, you can loosen the spokes to move them slightly outward, or tighten the spokes to bring part of the rim closer to the hub. I like to work with all three here for balance, but obviously you either tighten all the spokes or loosen them all. After doing this, you may need to adjust the wheels laterally again.
These can be tricky if you find any flat spots on the rim. If they can’t pull out with a radially trimmed wheel, and if it’s a very noticeable flat spot, they usually don’t, then the rims are almost at the end of their useful life. When I say flat point, I mean the point where the rim is no longer round, usually due to a larger shock, such as a jump or drop. A flat spot moves part of the rim toward the center of the hub, often causing one or more spokes to lose tension completely. For the reasons I mentioned earlier, this usually causes the spokes to fail at one point or another, and it’s usually worth replacing the rim. But if that’s not an option for you, your best bet is to get as much spoke tension as possible and keep an eye on them. If you start breaking spokes or keep losing tension, new rims are the only option.
This covers roughly the most basic parts of wheel trimming and tensioning. The wheel-making rabbit hole is much deeper, and there are a lot of caveats, special cases, and weird spoke/nipple/rim scenes here, but maybe we’ll get to that in the future. Now, feel free to ask any questions you may have in the comments section and I’ll try to answer them.
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