Something ain't right

[Michel_van_Dijk (Michel)]

Hello everyone, i’m kinda new to Zwift and borrowed my sisters Wahoo Kickr V5 to have some fun.

I own a Canyon Endurace with a 52/36 chainring and a 11-34 12 speed (i have mounted this 11-34 on the kickr). Now i was doing a group ride on Mountain route with a friend checkmarking “keep group together”. But my buddy is definitely the faster/more powerful rider, so i was chasing almost entire ride. (definitely on the climbs)

From IRL experience the cadence speed chart [website calculator](https://www.bikecalc.com/speed_at_cadence) is pretty accurate with my rides on the real roads and speeds and on the flats in Zwift also within considerable margin. Now we hit the radio tower part and as you can see i was mostly doing 80-90rpm and between 5 and 8kph. I even maxed at 100RPM at a speed of 7.5kph. But the cadence chart tells me that in my lightest gear (which i remember selecting because i can’t downshift furter i should do a minimum of 10.9kph in that gear on 80RPM and on 100RPM even 13.63kph.

• 36x34 is a gear ratio of 1.06
• 700Cx30mm tire has a circumference of 2180mm, 2180/1000=2.18m
• (2.18x80(rpm)x60)/1.06=9871 meters in 1 hour aka 9.87kph minimum
• Even a 20mm tire would have a circumference of 2120mm what comes to a result of 9.6kph minimum where zwift registered 5-7kph

Is this correct that the speed is 30-50% off of real life or where would this difference be created?

My Weight 59kg and Length 1.80m are correct. The indicated power seems within considerable margin. My FTP was set to 140 and after this 1 hour ride corrected upwards to 147 what all seems fine and pretty accurate. So i’m satisfied and pretty confident with everything registered except the indicated / registered speed.

I could give another example of a somewhat similar ride a couple of days ago on Road to Sky when we didn’t check the ride together function. I recall mostly riding in 1-3 and 1-2 gears… so 36x 27 or 30 cogs on 80RPM pretty constant which would result in 12-14kph but Zwift registered.

Are my speed calculations with the website tool wrong or is it essentially impossible for me to ride with 80-100RPM at these speeds in the lightest gears or maybe the Kickr isn’t translating something right from what is happening? What am i overlooking or what is going wrong? or should i not pay attention to the speed at all?

[gloscherrybomb (James)]

What is your trainer difficulty? For those speed charts to be roughly accurate, trainer difficulty would need to be 100%. Otherwise it would be like using an easier gear than you actually have.

The default is 50% BTW, if you have never touched it.

[Gerrie_Delport_ODZ (Gerrie)]

Hi @Michel_van_Dijk

Welcome to the forum.

Zwift calculate Speed based on the power number not on wheel speed. So 200w can be from doing 60rpm or 120rpm, it depend on the amount of resistance of the trainer.

You can play with the trainer difficulty to tune the resistance a bit if you like, it won’t change the speed at set power but will change the RPM at the same power on the same climb.

[Michel_van_Dijk (Michel)]

@gloscherrybomb @Gerrie_Delport_ODZ

Thanks for the quick replies. I’ll try and play with the trainer difficulty upcoming days.

[Mark_Napier (Mark)]

If “keep group together” feature was in use, it should be impossible to ride off the front and to get dropped. So all your calculations would go right out the window.
Riding alone, you’d have to be doing WELL over your FTP number to maintain 7.5 on the radio tower climb.
Try some solo rides or group events without the “rubber band” and see what your numbers look like.

[RC_Ivany]

Yep, agree with Mark above, the “keep group together” option in the group ride would have changed your speeds and pulled you along behind with an invisible rope. Similarly, I believe it also slows down the faster rider but other more knowledgeable zwifters would have to confirm that.