# 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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• Allan Hovda

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? Itâ€™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.

• Romney Phillips

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

• Nick

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?

• Robert Chung

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.