Santa Cruz Blur - how it works - Bike Magic

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Santa Cruz Blur – how it works

Suspension bob has always been seen as the enemy. Which is fair enough. The last thing you want is for the big dollops of oomph you thrap through the pedals to contribute as much to squishing a pivoting back end around as they do to driving the bike forward. We’ve lost count of the number of suspension systems that promise “bob-free” performance. Have any of them delivered? Er, generally not. Or at least, if they have, there’s some compromise somewhere else.

Four-bar linkage bikes are generally set up to move the rear wheel up and down instead of in an arc. The theory is that if the wheel can only move perpendicular to the drive forces, those forces won’t have any effect on the suspension. And so they don’t, but that’s not necessarily a good thing.

Chain tension isn’t the only thing going on. The mere act of turning the pedals involves your legs moving up and down. There’s a fair bit of mass in most people’s legs, and moving them up and down puts vertical forces into the bike. The bike can’t tell what vertical forces are bumps and which ones are the rider’s legs so the whole thing tends to compress and rebound. If chain tension has been isolated, there’s nothing to stop it.

This is one of the strengths of single pivot bikes. The most successful designs, like Santa Cruz’s own Superlight or the Marin FRS system, have pivots set quite a way forward and middling-high, making the rear wheel travel through a large radius arc that almost approximates to vertical if there’s little enough travel. Almost, but not quite – pedalling forces extend the suspension but not by all that much. Generally enough, in fact, to counteract the vertical forces generated by your legs. The act of pushing down on a pedal tends to make the bike squish, but the chain tension generated by pedalling tends to make it sit up again.

That’s the theory, anyway. In practise a single pivot can’t get it right all the time. In certain gears at certain speeds it works a treat. But most single pivots have rather too much chain tension effect in very low gears and under full travel situations. This has the twin unwelcome effects of jacking the back of the bike up under power and of the pedals kicking back at the rider up steppy climbs or on big hits.

So, too little chain tension effect is bad, and so is too much. The ideal would be to have chain tension effects where they’re needed and not where they’re not. And this is the thinking behind the Blur’s VPP design. The twin linkages control the path of the rear axle, making it follow a compound curve. The idea is to combine the benefits of a straight axle path with those of a curved one.

Most suspension bikes are designed to sag under the rider’s weight. This allows the suspension to move up and down from the rest position so it can drop into holes, swallow bumps and generally make a better job of keeping the back wheel following the ground. The Blur is designed to have a small degree of chain tension effect at that sagged position. Push down on the pedals and the bike lifts just enough to counteract the vertical forces imparted by your weight moving up and down. Unlike some single pivot designs, though, the effect isn’t so great that bumps have trouble moving the suspension – hit a bump and the suspension moves freely. As the wheel moves towards the extremes of travel, though, the chain tension effects are lessened so that pedalling doesn’t hinder suspension movement and big hits don’t kick the pedals back at you.

Now carry on and read the rest of the review…


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