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Optimal Gearing


I wrote this 2 years ago and distributed to many of the parents of kids I coach. There has been many scientific explanations posted lately so some of the information is online but this might be a basic read for some. Enjoy !

WHAT IS OPTIMAL GEARING?

By Brendan Arnold

I have had many discussions in the past 3 weeks on gearing so I thought it might be important to put some info together for everyone.

I want to start this piece by giving some definitions to some of the terms located in this document.

Cadence or RPMs - this is basically how fast a person can pedal. It is measured in pedal strokes per minute. So if someone pedals at a cadence of 100 it is 100 Revolutions per Minute.

Roll out - this is measure by how far one RPM will take you. So put the pedal of the bike parallel with the ground. Mark on the ground where the starting point is. Apply force and rotate in one full revolution back to its original position. Then mark the finish position. Then measure that distance and three is your roll out.

Let's start with optimal gearing. 90% of the top riders run the same gear 44-16 with. 1.75 or 1.85 rear tire. This equals a roll out of anywhere from 53.5 to 54.3 inches in roll out. So why is this the optimal number? Good question. We leave that stuff to the scientists but from the data that has been produced using power meters/timers etc the optimal mix of speed and power comes from a gear in this range.

In my observations it is common that BMX athletes reach top cadence and max power output anywhere from 2.8 to 4 seconds. This all depends on hill size, surface, conditions etc but it is a good guide. In Supercross the Men reach peak at around 2 seconds. The top riders push over 2000 watts of power with RPMS well above 180.. fast fast!!

A combination of pedal speed and power is the formula to getting out front. Now to get into how to get the speed that's another topic..today we discuss gearing.

Many kids start BMX with a bigger gear on. (many bikes come with a larger ratio) Why? With small legs and small cranks it's takes a lot of work for a kid to even complete the track let alone worry about any speed. Many parents leave these big gears on even as the child grows and is able to pedal faster with greater force. Young kids tend to push rather than spin. It’s easy for them and they can control the increases in pedal speed down the hill and out of corners..

Pedaling with resistance is much easier as you can apply force against the cranks. Remember riding the exercise bike and taking all the resistance off and the cranks spun so fast you couldn't keep up.. well that's how it feels going from a huge gear to a spinning gear.

The issue is some try to change the gears from big to small in one go. This is a huge change as it changes pedal stroke, body position and tempo of the bike rocking side to side. Some people often refer to this as pedaling squares or chopping wood.. pedal stroke is jerky and not usually very efficient.

Now onto gearing. Wheel size and crank length play a major role in determining roll out. For example a 44- 16 on 1.75 tires is a 53.63 inch roll out. To get the same roll out with1/3/8 (skinny) wheels you would need a 41 -16 or to go a little harder a 39-15 which is a 54.5 rollout.

Kids often say and I hear it all the time.. my gear is to easy. In many cases the gear is fine but it's the change in cadence /tempo etc that makes them feel uncomfortable. Also doesn't help that at every BMX Race people have opinions on kids gears and they don't take into account all the factors like what I have mentioned above.

Remember it's not always about winning now! (although that's nice)It's about solid development and sound fundamentals. Spinning is a skill that needs a huge amount of work. Sprints will help with this.

How do you know when a gear is to small? This is tough, once an athlete has mastered high cadence it's an observation to see how quick they are topping out - one way is to look at the bottom of the pedal stroke. If it's jerky and they are not able to apply force in a 360 degree movement it might be time to change it up. Also if they are super quick out of the gate but are losing top end half way down the first straight it's worth looking at.

I will leave you with this thought. If a rider is being beaten out of the gate and down the hill and they are saying there gear is to easy they are telling you a good story. If in fact a gear is to easy the only place it would be good is for the first 30- 50 meters. But the point to this is wouldn't you prefer to be faster in the first 50 meters? People say we'll my kid makes up ground in the last part of the race... so what is my answer.. the race is usually determined by the first corner so why spend time focusing on the last 2 straights if you are in last place...RACE OVER!

The best gear chart I have found us

http://bmxultra.com/gear-calculator/

It allows you to customize ratios using tire sizes per brand.

Happy Riding

BA


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