what is the relative or expected difference between running and cycling FTP in watt/kg?

im talking about the relationship between cycling ftp and running ftp in watts/kg

I've given this some thought during a slow day at work...

Considering just the aerobic engine, what pace should you be able to run at if you can produce a certain power on the bike? If you make a couple assumptions, you can estimate this.

Assumptions:
1) You burn 1 kcal / kg body weight / km run - I know, can be way off.
2) Your efficiency running is the same as cycling (24 %) -- Again, I know, WAG

Then, you take a certain pace, say 4:00 min / km, your body weight, say 70 kg, and preform the following calculations:

4:00 min/km = 15 kph
15 (km) * 70 (kg) = 1050 kcals energy required per hour
Assuming a 24 % efficiency, Eout = Ein *0.24
1050 * 24 % = 252 kcal energy generated
252 kcal * 4.18 Joules / calorie ~= 1050 kJ -- isn't that convenient?
1 W = 1 j/s therefore 1050 kJ / 3600 s (in one hour) = 290 W

You can work out the power for other paces. It seems kind-a close for me, at least judging by my PER riding at set wattage and running at set pace.

At 70kg
3:45 min/km = 310 W
4:15 min/km = 275 W
4:30 min/km = 260 W
4:45 min/km = 245 W
5:00 min/km = 230 W

What say you?

How do you plan on measuring watts while you're running?

When I am doing around 4W per kilo ftp (252W) at 63 kilos, I am usually running right around 38 min 10K's. I can an almost predict how fast I will run at races, based on some bike hillclimbs that I have locally. The faster I climb, the faster I run (no rocket science).

Wow, that's a lot simpler that what I've been doing, thanks! At least there's some agreement to my calculation.

As another poster pointed out, there are individual variations

That's why the scatterplots are useful: they remind us that these are simplifications and although the relationship between running and cycling is pretty strong, it's not perfect.

But, performance on the bike and run on the same day will have an element of inverse correlation, as the more you put into the bike leg, the less you have left for the run. So the correlation between potential performance if each were done in isolation may well be better than the scatterplots suggest.

I've wondered about those treadmills too. I bet the calculation is just a simple weight to speed with elevation calculation.

When I am doing around 4W per kilo ftp (252W) at 63 kilos, I am usually running right around 38 min 10K's. I can an almost predict how fast I will run at races, based on some bike hillclimbs that I have locally. The faster I climb, the faster I run (no rocket science). To Seasonschange, I'll let the smart guys provide actual conversions....this is what applies to my N=1

Dev

The faster I climb, the faster I run (no rocket science).

It would be great if someone smart took a stab at equating watts per kg to Vdot ranges!!!

(Directed to no one in particular in this thread.)

The notion of "running power" is antithetical to many classically-trained biomechanists - at most, they will admit it exists, but is small. I can see their point, but if you define it as "the rate of performing work against the external environment", then clearly it exists. Defined as such, however, mechanical efficiency is significantly higher than observed during cycling - in fact, it can be "impossibly high", at least compared to the efficiency of muscle contraction. This is because the environment also does work on the runner, with this energy being recycled via the series-elastic elements of muscle. In essence, this results in a form of double-counting, and hence an elevated efficiency.

Anyway, my point is that although there may be a modest correlation between cycling power and running speed, you can't accurately estimate running power from speed by assuming a comparable efficiency.

what is the relative or expected difference between running and cycling FTP in watt/kg?

Not sure what is expected but I got a Stryd last year and was using it for a while. I found my running FTP was about 5-10% higher than my cycling FTP. I didn't do many hardcore tests and they're all stored as cycling activities in my Garmin so difficult to pull up exact numbers.