Watts and W/kg help


Fairly new to Zwift and have mostly been doing workouts for the past few months. In the past week or so I have rode a few rides to try them out and a few questions have come up in my mind.

First, some details … I am about 150 pounds, am riding a road bike on a Cycleops Fluid2 trainer (not smart sadly … out of my price range at the moment) with Garmin sensors and a Wahoo chest strap heart rate monitor. Using an iPad to Zwift and my iPhone as companion to give plenty of Ride Ons.

The rides I joined were C category and I had to work really hard to stay in the pack, most times getting dropped fast … maybe this is expected? I am just not sure how to translate say 175 watts of power on a ride to the w/kg rating? Does doing an FTP actually change or readjust my rating or speed in the game or is that for my own info?

I have done a lot of reading but I’m still confused. Sorry if this has (most likely) been asked and answered many times.

Either way, great workouts but I would also like to learn and obviously improve.

Joel :slightly_smiling_face:

Hi @Joel_Barrette

Welcome to the forum.

So you weight is 150lb (68kg).

If you ride comfortable at 175watts then. 175 devided by 68 = 2.5w/kg. So a C or D ride will be good.

Races are based on your FTP. If your ftp is 200w then 200/68=2.94w/kg that’s a C.

No a FTP test only give you a number to use to pick a racing category.

If a C group ride is to fast then pick a D group. Group rides is supposed to be fun.

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This helps a lot, thank you!! :grin::+1:

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Exactly the reason I’m searching on the forum. Only a week new to zwift and riding a road bike on a Fluid2 equipped with garmin speed sensor and garmin hrm strap. All the Power readings I get are about 170-200 watts and I’m 52kg (you can see the stats on my activities probably), but I can’t believe it’s realistic. I have mounted a garmin edge 520 plus on the bike and on outdoor ride, read an FTP 200/52 Watts. I did a group c race yesterday and got 26th out of 57 riders but how can my avg watt output of 149 be true? I almost feel embarrassed towards riders with accurate power meters

Hi Gerrie,
I’m having a similar, but slightly different problem. I have a Tacx blue matic set up with a barmaid speed sensor. All connected and working fine. Resistance set to 3 as requested. The problem is that when I ride it won’t let me get above 80 watts. I’m a reasonably fit 55 year old Whois used to cycling around 15 km in 35 mins. On my ftp test, I covered 8 km in 50 mins averaging 49kw. Any help would be appreciated… Many thanks. P.s. I’m using a Bianci road bike. Derek

Excuse the predictive text that changed Garmin to barmaid!! :joy:

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It’s hard to discuss the “best” course of action if you feel your data is inaccurate.
The hard answer is “try to improve the accuracy of your data”.
If I interpret your post correctly, your outside FTP is 200-250?
That’s actually a very wide range.
With your wt of 52 kg that means your at 3.8 - 4.8 w/kg.
I would say that data is incorrect.
The range of 3.8 - 4.8 is just too great to provide an accurate description of your ability.
Even if your FTP is 170, that’s still 3.2 w/kg and that is very good.

My ftp as calculated by garmin is 3.8=200/52 (after outdoor rides), but I cant believe it’s even close to accurate since others I know struggle on the same effort. That’s why I refered to the yesterday zwift race I attended where my avg watt output was 149. I know that without a powermeter nothing can be accurate, but can’t spend 700 euros on covid season, so I try to find the next best solution!

With the wheel-on trainers they’re very sensitive to exactly how much you push the tire/tyre into the drive wheel. When I started using one (Kickr Snap) I would occasionally see the same thing and discovered it was because I had pushed the tire too far, which causes such an increase in friction (because the rubber has to deform so much to get past the roller) that you are having to use tremendous extra power to turn the drive. I found the best method was to use the minimal contact that would produce no slip on steep climbs, or during sudden accelerations when starting a sprint (note, both of these cases are different to slipping when getting things up to speed when you begin riding, when you’ll get slip due to inertia of the roller).

If this is a new problem there could be something else going on, but it would be the first things I’d check. Other possible sources are inflation of the tire (make sure its properly inflated), or hardware problems with the sensors.

Thanks Justin, I tried rolling back the pressure on the drive wheel and straight away noticed a difference in wattage/output. I will keep tweeting it to see where I get to. But for the moment I can ride at almost double the power to yesterday! When setting up I had noted that the tyre should be pushed firmly into the drive wheel. This is clearly not the case for Zwift. Many thanks, Derek

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Derek - glad to help you solve it! Note that the tyre positioning isn’t specific to Zwift, it should be the same positioning for this trainer no matter which app you’re using it with.

Also, another reason to not overdo the contact is that your tyre will last a lot longer - if you check your tyre now I wouldn’t be surprised if the 50 minute ride with it pressed hard on the drive wheel has put a pretty good flat spot on it already…