Contact Points and Marginal Gains

While we have just 5 contact points with the bike, each of those contact points are made of whole systems of contact points. And your tires are the ONLY contact points between the bicycle and the earth. Think about it: all the forces, traction, braking, cornering, all have to be transferred through tiny patches of rubber, dynamically moving on the surface of the earth. Clearly, contact points aren’t as obvious as they seem, which makes them a perfect topic for…Marginal Gains!

Got a question you’d like to ask Josh? Text or leave a voicemail at the Marginal Gains Hotline: +1-317-343-4506 or just leave a comment in this post!

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7 thoughts on “Contact Points and Marginal Gains

  1. Nice, as always.

    Speaking of converting from Crr to mass, I did a little translation of Crr difference to mass difference (for two relatively low Crr tires: the Conti 4000S and Conti 5000) at different slopes:
    http://anonymous.coward.free.fr/conti-4000-5000.png

    Re: the Velonews 1x vs. 2x drivetrain efficiency piece, the losses are in the chain, not the rings or cogs per se — the problem is that the chain has to 1) wrap around the cog and ring, and 2) chain speed is determined entirely by the ring size and the rider’s cadence — the gear ratio and cog don’t affect chain speed. Since power transmitted along the chain is the product of chain speed and chain tension, smaller rings (at the same cadence) mean slower chain speed so at the same power the chain tension must be higher. So when you have high tension and the chain has to wrap around a small cog, you’re doubly screwed (I believe that’s the technical term). You can see this in the super small rings used by MTBs: even at the same gear ratio, their losses are higher since chain tension is so much higher and the chain articulates around tiny cogs.

  2. Interested to hear what Josh thinks about why aero cranksets have not taken off yet, some 11 years after Zipp did the vumachrono?

    Also are you going to make aerobar pads out of the high efficiency foam?

  3. Hi,

    I have read and heard every blog post and podcast with Josh regards of tire pressure. With that I have learned a lot and bought the Viaggio Travel pump for high precision pressure measurement.

    Something I did take from the hours reading and listening is that you need to test out tire pressure to find the optimal one. Any advice how to do a proper test? How long should the test course be and how to actually analyze the difference? It’s very difficult to ride at exactly same watt and identical cda. Maybe testing uphill to reduce the cda factor?

    I also have a question about crr in regards of tarmac. How do you roughly find that value? Especially when competing in IRONMAN races abroad there is limited possibilities to ride the course in advance so you just might only ride a small part of the course and estimate the crr.

    Would love any clarification and advice.

  4. Hi Josh. Thanks for being so friendly and taking time to chat at the Melbourne Handmade Bike Show.
    In addition to what Robert Chung said about smaller rings: the chain tension goes up, and the number of teeth in contact goes down. So your stress at each tooth goes up as an inverse square of the tooth count. Cue more friction and faster wear. In addition to the increased articulation angle, I would guess it’s triply worse, not doubly.

    Given the ever changing world of standards, I’m surprised we are still with 1/2 inch chain; maybe a smaller pitch and increasing the tooth count would reap some benefits. (I recall Shimano unsuccessfully tried 10mm pitch a while back). Thoughts?

    On the Ceramic Speed drivetrain, that thing is awful awful awful. Single point of load through each roller bearing (not shared across many sprocket teeth) – hello massive hysteresis. Roller bearings used as wheels with no housing. Open bearings with no ingress protection. The outside face of the rollers shearing across the chainring and “cassette” (definitely not frictioness rolling). No flex restraint in the cassette. Limited flex resistance in the frame. And no functional shift system. Please don’t ever mention it again.

    OK, here’s some questions:
    Shoes: Does stiffness really matter? Manufacturers would have you believe so, but there is never any supporting data. Given your foot is pushing through a rotating pedal spindle, is there any benefit to stiffness anywhere other than under the ball of the foot?

    Second: Tyres: Are 23s dead? If wheelset budget was limited to say some alloys, they don’t come in wider than 24mm. With a depth around 30-35mm max, would 23s or 25s be faster, or is that road surface dependent?

    1. Regarding the 23s I thought aero was best when your tyre width matches the rim width? On my TT bike I have HED S2 wheels and a 23mm GP4000 seems to be the right match for the rim. Would a bulging 25 be worse?

    2. On stiffness: flex is a loss of kinetic energy, usually as heat. So a stiffer bike will have less loss. But, with rougher surfaces, the stiffer bike will transmit more vibration to the rider, which is wasted energy. So too stiff is bad too. So that is where laterally stiff, vertically compliant is good, and can be engineered with carbon layups. But the tire is the dominant spring in the system.

      On 23c tires. I think they are alive and well with aero wheels (and many clinchers measure 24-25mm anyway). For lighter riders, and smother surfaces, the extra tire width is not always needed, so the aero benefits of narrower tires can be realized. All else the same, wider tires are not more aero, or even just as aero… wider rims just take some of the aero hit off of the wider tires, such that you can minimize aero losses vs a more narrow tire. The wider tire/rim will always have a larger surface area, which hurts aero.

      So the ideal tire width will depend on rider weight, average speed, and the course surface. Wider tires expand the range of effective use of the tire, so for the average rider, this is a good thing. But i do not think they will go away, because there are places of benefit… like high speed tt on smooth surfaces, or for smaller riders.

  5. Hi
    Only recently discovered your podcast and am really enjoying it.

    A question for you that is not really a marginal gain but I haven’t been able to resolve:
    Is there an advantage to being heavier when going downhill?
    Everyone says heavier is better and faster but with gravity’s acceleration being constant doesn’t it all come down to aero?
    In a vacuum objects accelerate at the same rate ith only wind resistance affecting the speed.

    Thanks

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