What Difference Does Wheel Size Really Make?

Joe wades into the wheel size debate and reckons it doesn’t make too much of a difference bar one key element.

Bike wheel size has caused much debate. We go back to the science it’s a fairly simple topic. I’ll discuss the differences between 29″ and 27.5″ wheels.

I’ll consider one parameter at a time. Tyre contact patch, rollover, dropping into holes, rotational energy, gyroscopic stability, relationship to BB drop.

Tyre contact patch. OK, let’s do the easiest one first. Bigger wheels do not have a bigger contact patch. Contact patch is purely a function of air pressure in your tyres. It is the air pressure that support your weight. Force (your weight) = Tyre pressures x contact patch area. Simple.

Different size wheels may have a slightly different shape contact patches, longer and thinner on bigger diameter wheels, but the area is the same. On a side note, the same is true of fatter tyres, a 2.2″ tyre with 20 psi has same contact patch as a 4″ tyre with 20psi. It’s just that the bigger volume allows you to run lower pressure without damaging your rims.

Rollover. Bigger wheels roll over bumps better, right? Well, no, not really. Realistically, I would consider a bump of 2” (50mm) in the realm of rollover. Anything much bigger and you need to start lifting the bike up and it’s not really rollover. If you consider where a 2” bump hits a wheel on both 29″ and 27.5″ wheels, you can see the difference in angle of attack is negligible, 1.14°.

Bigger wheels don’t drop into holes as much. Realistically, when the bike is moving forward at a speed any faster than a crawl, it’s unlikely you’ll touch both side of a hole. The forward motion means you’ll hit the backside of the hole. In this case, wheel size then becomes irrelevant and we revert to a rollover case.

Rotational Energy. Bigger wheels carry more rotational energy. This is true, but only as a function of the difference in wheel weight. A bigger wheel is heavier by 29/27.5=5% (probably about 0.5% of bike plus rider system).

If this heavier wheel was spinning at the same speed as a smaller wheel, there would be more angular momentum, but bigger wheels spin slower. Think of the amount of ground the circumference of the wheel covers in one rotation. It is proportionally less for a bigger wheel, so it spins slower. This affects acceleration too; other than the tiny bit of extra weight it requires no extra energy to accelerate bigger wheels.

Gyroscopic stability. OK, this is the important one. Although there’s no change in angular momentum, there is an impact on the gyroscopic stability of the bigger wheels. This is because it is proportional to the diameter squared. The gyroscopic stability is the tendency for you wheel to stay ‘in-plane’ when rotating. Like the child’s toy, that stays upright when spinning, it doesn’t want lean over. Take your front wheel out of the bike and spin it up to speed holding it at the axle. Feel the forces to try and move the wheel out of plane.

Now imagine riding along on your bike and trying to lean it over, bigger wheels will make it harder to lean. But also, it means the bigger wheel will not be knocked off line as much by bumps. Anyone who has moved from smaller wheels to big 29″ wheels will have felt this affect. The big wheeled bike is harder to lean over, you need to put more effort in. But once it’s leant over, it’s more stable. People who are capable, will be able to tell you a 29″” is harder to whip off a jump.

Relationship to BB drop. The offset from the wheel axles and the bottom bracket is called the BB drop. For bigger wheels, to maintain a similar BB height from the ground, the drop is more. This drop in relation to the gyroscopic forces acting in the wheels will affect how the bike rides. To be honest, I’m yet to fully understand this topic and have plans to partner up with a final year Engineering student to try and understand this effect.

So, what does this mean for my bike designs?

The Murmur 29″ bike will track and carry speed better (due to gyroscopic stability not rollover). The Swoop 27.5″ bike will be more manoeuvrable, allowing you to hop and pop between line choices. The Twist mullet bike, aims to give the stability on the steering front wheel, and manoeuvrability on the rear wheel. But, it’s not the golden ticket, rather a mid-point between the other bikes.

Joe Mcewan

Ex-aerospace engineer Joe Mcewan is the founder and chief engineer of Starling Cycles. Passionate, outspoken and fond of a cuppa and a debate, Joe loves to challenge the established thinking of the industry.

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