Zwift and Tacx


(Bertram Redmeijer (B)) #1

Zwift and Tacx

In November 2014 I bought a Tacx Genius. I rode a number of Real Life Video’s on it. I became stronger, and about a year ago I had a FTP of about 240 (according to Strava). My maximum Wattage was about 450.

Since this fall I ride mainly on Zwift. I upgraded the Tacx Genius to a Smart version. I like Zwift for the ease of use, and the fact that you can just jump on and off – with a RLV I always have the idea that I have to finish it. And the other riders make it more fun as well. I ended 2015 with a FTP of 290, and a maximum Wattage of 680.

In 2016 I already rode 600 km in 21 rides – more then I usually do. And my FTP went up to 300 (according to Zwift) or 320 (according to Strava). Max wattage is now 780.

So yesterday I decided to ride one of the RLV’s I rode last year (Classic Liege-Bastogne-Liege, part 1/3). I expected to go quite a bit faster – after all, I made good progress…… Last year I did this ride in 1h, 12 min 10 sec. Now I did it in a blistering….1h, 11 min 55 sec. Yes, that is a 15 second gain over more than an hour of riding…. Average wattage over the hour was 250, with me pretty much total loss. So my FTP would be close to this 250….not 300 (and certainly not 320).

Now this Tacx RLV is of course quite different from the average Zwift ride. The hills are resp 3, 2 and 4 km long, and the % runs up to 17% (the second one is actually 12% average). And yes, I tackled the first one way to aggressive – I thought I could keep that 320 watt for an hour. And I did ride for a couple of days in succession before the RLV (got to get that achievement!).

Still, the difference between what I expected and what I actually could do is pretty big, and the advance from last year pretty small…

I do of course use the same equipment (bike and brake), so the difference must lie in the way Zwift and Strava use the numbers they get back from the brake. What I noticed is that it feels like the RLV has a bit more resistance at higher speeds. Tacx maxed out at 60 km/h, but that is also about the max speed I could get – (at 105 rpm and 290 Watt). In Zwift I could get to 76.3 km/h (was going for the 80 km/h achievement). This was at 120 pm and 780 Watt. I simply cannot get that high in Tacx, even though their slopes are steeper and longer.
Zwift also felt smoother, which makes some sense, as the virtual roads probably have less micro changes then the real ones. It also felt like I kept rolling longer in Zwift.
And of course Zwift uses the open ANT+ protocol to communicate with the brake, while Tacx (I think) still uses their own protocol. No idea if this matters….

Now I am curious if others have noticed similar things with their brakes, be they Tacx or of another brand. Or maybe something similar between their progress on Zwift, and their real world speed… (yes, I am afraid my real world times will be more like the Tacx ones, that is not improved much, instead of like the Zwift ones. But it is too cold and to wet outside to try it out).


(Stef Levolger) #2

It sounds rather like you were merely overzealous on the RLV ride. With an FTP of 300 in Zwift, and Strava guesstimate of 320 (both set for a 20 min value), expecting to ride at 320 watt for an hour may quickly wear you down.

For a 1 hour value you may expect at best 0.95*FTP, being 285 - 304 in your case depending on which FTP were to be considered correct. For cyclists who don’t frequently ride that close to their FTP over an hour a 0.90*FTP is more reasonable to aim for. In that case a 270 - 288 avg watt may be more reasonable.