Power differences and weight

Lately, I have been training with someone who is 115 lbs and I am 160 lbs. We both have powermeters, both ride in the TT position/aerobars, etc. They consistently put a lot of time on me in the hills, especially the steep ones but on the flats and downhills, the difference is unbelievable with myself, the heavier rider going a lot faster. Obviously, weight is a huge factor when you get into the up hill sections. Does anyone have experience riding with someone that is almost 50 lbs. lighter?

115 lb rider avg. power is 190w.
160 lb rider avg. power is 250-260w.
15 min. intervals.

If using constant power, should the heavier rider gain 2 min. during a 15 min. flat interval? On uphill sections, the 160 lb rider loses almost 1 min. on the 115 lb rider. Can someone point me to locations on the internet to read up on this and to learn more about weight, power, elevation and speed or perhaps provide info. here?

Thanks,

Your experience is pretty typical. On steady and especially steeper climbing it's all about power to weight ratio or watts per kg.

Your 115 pound rider is putting out a little over 3.6 w/kg while the 160 pound rider is putting out a bit less at 260 watts. The example you gave is actually very close from a power to weight standpoint, but the lighter rider has a slight edge. The heavier rider also has a bit more rolling resistance, but that's a small factor in the overall picture.

For fast flat riding your speed is dictated primarily by power to drag ratio or watts per CdA. The larger rider will typically punch a larger "hole" in the air and have a higher CdA, but not always depending on how aggressive the rider's positions are. A small rider in a comfortable and upright aero bar position could have a higher CdA than a larger rider who's really worked on good time trial positioning.

But even with very aggressive positions for both riders the larger rider will generally have a CdA that's only higher by IIRC the ratio of their weights raised to the 2/3 power. I'm drawing that from memory and the exact relationship will be different based on specific positioning and specific body types but the point is that CdA goes up with body weight but not as rapidly as power typically goes up for two comparably trained riders. IOW, bigger riders have an advantage in flat time trials, especially if they've taken time to develop an aggressive aerodynamic position.

There's a lot of stuff on line on this subject, including some very good original research on PubMed. An easy way to get a feel for the relative importance of weight, CdA, and rolling resistance for various grades, speeds, and power outputs is to plug them into the on line calculators you can find here: http://www.analyticcycling.com/

Try the "Static Forces on Riders >" Power given Speed or Speed given Power calculators to get an idea of how much these parameters effect each other. There's a lot of other good info on that site as well such as an analysis that shows why an aero wheel rim is a better deal than a lighter box section rim even in the first 100 meters of accelerating out of a crit corner even though the aero wheel is quite a bit heavier. That kind of stuff is counter intuitive and very interesting.

Good luck,
-Dave