Suspension isn’t limited to undamped leaf spring forks of the more mechanically complex YBB softail frames like we’ve dug deep into over the past weeks. If you have a full-suspension bike, it needs to move smoothly & freely to get the most out of its rear wheel travel. So how does your bike pivot?

How does the suspension move in your bike frame?

Chris Currie mountain bike suspension patent drawings

Breaking it down to the basics, if your bike frame has a suspension system to allow the rear wheel to move and absorb impacts, you are going to end up with some moving parts. No matter what material the frame is made of, we generally want to limit rear wheel movement to the vertical plane, while maintaining horizontal stiffness. To do that frames generally rely on one of three methods to allow for rear wheel movement – controlled material flex, pivoting elements with bushings, or pivoting elements with bearings.

So which is which? How can we identify each and what are their benefits?

Engineered Material Flex

The most basic way to allow for wheel movement is to allow the rear end of a frame to flex. That’s what it going on in the titanium chainstays that act like the spring on the Moots YBB softail design. But this isn’t limited to titanium. We are seeing more and more carbon frames that engineer in flex to the rear end, since the material lends itself well to reinforcing a frame against flex on one direction, while allowing it in another through careful material layup.

The complication in building flex into your bike is to allow vertical movement at the rear axle to absorb impacts, but preventing any horizontal movement (side-to-side flex) or twist in the rear end that would both sap precious energy from your pedaling inputs and make the bike difficult to keep in a straight line, especially when riding rough terrain. Carbon hardtails like the Silverback Superspeed (above) with its flattened seatstays that bypass the seattube are a good example of engineered material flex for limited rear wheel movement.

teamelite-01_XX1_side

BMC’s carbon softail Teamelite takes it a bit further like the Moots, offering 15mm of elastomer controlled travel via engineered carbon flex in the rear end.

2017 Cannondale Scalpel Si full suspension XC mountain bike

But you don’t need to eliminate all of the pivot points to get the benefits of engineered frame flex. The reason to eliminate pivots and replace them with flex is generally weight reduction and simplicity. With fewer components, the frame can be built lighter. With fewer mechanically rotating parts, the frame will require less maintenance to keep running smoothly.

Single pivot or four-bar suspension designs traditionally relied on a pivot either located on the seatstay or chainstay to allow the frame to cycle through its rear wheel travel. But bikes like the Cannondale Scalpel have long used flex in the seatstays to permit rear axle movement. You still need the main pivot, and often some pivots in a linkage driven shock like the Scalpel. But the solution can still often be lighter and easier to keep running smoothly.

What is a bushing, and why are they in my frame.

A bushing is essentially just a sleeve of a low-friction material that is inserted into a frame at a pivot to allow rotation around the pivot’s axle. If you just had a steel or aluminum axle rotating inside of a steel or aluminum hole there would be a ton of friction, and dirt & regular corrosion would make it even worse. But by adding a slippery, low-wear material like some advanced plastics like Delrin, most of the friction goes away.

Ibis Ripmo 29er gets mo' travel, new geometry, and useful features

So why use bushings at all? Bushings are relatively cheap, have few moving parts, and are lightweight. But they do get a bad rap. There has been a general trend to move away from bushings to replace them with bearings. But in situations where you only have limited rotation – like a short-link suspension bike – and relatively heavy loading, bushings can be hard to beat. The newest Ibis Ripmo specifically has bucked the bearing conversion trend and returned entirely to igus self-lubricating plastic bushings in their lower links. They say it saves 80g, is a more appropriate solution and they stand behind them with free lifetime replacement on the bushings.

Why does my suspension have (sealed cartridge ball) bearings?

A ball bearing is the most common bearing we find in bike frames. It generally works by having tiny steel (or ceramic) balls rotating inside a pair of hardened races that contact your bike. In suspension frames they usually get pressed into a specific hole/recess in the frame and pivot around an axle. Bearings are ideal for elements that rotate a large degree (like a full 360° in a wheel or bottom bracket) and can be well protected from the elements with a series of seals.

The next generation Instinct from Rocky Mountain has more travel than before.

At the same time that Ibis chose bushings, the new Rocky Mountain Instinct has dropped the ABD bushings of its previous generation in favor of sealed cartridge bearings. It’s true that the bike’s main pivot and rocker linkage pivots may move through a wider range of rotation than the Ripmo, the driving factor in the move to bearings here is probably more about sealing out the elements. With sometimes double, labyrinth seals, and packed with grease, bearings can be more resistant to riding in the wettest & muddiest climates.

2016 Alchemy Arktos full suspension mountain bike with Sine suspension design

So what does all this mean for the end user? Now that you have an idea of what makes your bike’s suspension pivot, we can start to talk more about how to keep it running smoothly for years of riding enjoyment, besides just keeping your bike clean. Each one – flex, bushings & bearings – requires its own maintenance methods to keep your rear wheel travel moving smoothly. The good news is that it isn’t too hard to keep each pivoting as it was designed.

 


The fun never ends. Stay tuned for a new post each week that explores one small suspension tech, tuning or product topic. Check out past posts here. Got a question you want answered? Email us. Want your brand or product featured? We can do that too.

7 comments

      • Cory Benson on

        You both are correct. We are referring here to Ibis’ idea of “‘bushings where you need them, bearings where you don’t” that works best with their high-load, low-rotation lower short link design. The upper link which has more rotation and bears less load sticks with sealed bearings.

        Reply
  1. Leo Butters on

    Years ago I thought bushings made sense but in reality they only ADD friction the the swingarm movement and don’t last very long. Even if greased regularly. Most companies know this, and offering free bearings (SC) or bushings (Ibis, like they’re in the same “free ___ for life” building as SC or something) isn’t a real solution to something that wears out prematurely when compared to the rest of the industry. It’s almost an afterthought. At least Ibis KNOWS they won’t last long and are proactively promoting their availability. Hilarious! Can’t wait to see how this really unfolds on the trails and shops. I really hope it works, but in my experience this sounds like a step backwards.

    Reply
  2. Mark Visser on

    Bearings vs bushings – both are a compromise.

    Take an SKF 6802 bearing for example. They’re rated for 36,000 RPM. In bike suspension, they rotate 0 RPM. They actually never make a full rotation (which is a problem because bearings require rotation to spread their grease around and keep themselves adequately lubricated). This same bearings is rated for 1.1kN static loading (or ~250 lbs). This is an issue because the limit of most bearings is the surface hardness of the bearing race. If you exceed that 250 lbs, it will put tiny little dents in the bearing race. This means that the balls will fall into these dents, and want to stay there.
    Another issue with bearings is the tiny contact patches of the ball and the race. Let’s say a bearing has 20 balls in it. When it’s loaded, only half the balls are supporting the load. So now you have 10 balls supporting this load. But the balls closest to the 3 o’clock and 9 o’clock position are taking a fraction of the load that the balls at 6 o’clock are taking. And each ball really only contacts the race in two spots. (If you did line contact, there would be too much friction and wear). So it’s very very easy for the balls to dent the races, which then cause the bearings to be notchy.

    Bushings – from an engineering perspective, they are the perfect choice for these applications. Your load is spread over a much larger surface area, so you don’t have to worry about indentation.
    However, there are two downsides to bushings that drive most manufacturers back to bearings.
    1 – stiction. The suspension isn’t supple enough because of the stiction in the bushings.
    2 – sealing. Bearings are easy to seal – bushings, not so much. (Or at least no one takes the effort to seal them well).

    Ultimately, I think the designers need to look at the application. If it’s a low rotation angle joint (ie, most shock frame mounts) bushings do well. (Actually, Fox shocks have only used bushings for their shock mounting for the last how many years. And no one complains about them, unless it’s a specific design that sees a lot of rotation at the shock. Looking at you Giant Anthem). But even then – bushings are cheap. Hence Ibis offering lifetime supply of them for free.

    I’m not saying one is better than the other. In every single bike out there is a mix of both (because I don’t know of any shock manufacturers that spec bearings rather than bushings for shock eye-lets).
    If both are properly maintained / replaced at regular / appropriate intervals, they’ll both do well for years.

    PS > enduro max bearings are an interesting attempt to solve the issue with bearings in bikes. They forego the bearing cage so that they can fit more bearings & grease in the same space.

    Reply
    • Carl on

      I’m really liking that addition at the very end. I noticed the Enduro Max bearings didn’t have the retainer like others such as DTs stock bearings but thought it was just a cost thing. Is the ability to hold more grease of greater advantage than a retainer that keeps them appropriately separated for more even loading?

      Reply
      • Graz on

        Actually the absence of the cage means the load can be spread across more balls (because you can cram more in!) for a given bearing. The flip side is for actual rotation the balls themselves can rub against each other – but as Mark points out there’s pretty minimal rotation anyway. Incidentally this is where the exploded view in this article comes from, and plenty of other manufacturers do ‘full complement bearings’.

        Reply

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