Computrainer vs. Dumb Trainer - Something isn't right

(T acoma Cyclist (LVE)) #1

Hi guys - 

First off - thanks for everything.  Zwift is the single most awesome thing I’ve seen all year.

On to the questions.  I have a side-by-side setup right now with a Computrainer and a supported Elite trainer with variable manual resistance set to level 3 as recommended.  I ride the computrainer, and my son (an excellent juniors racer) rides the dumb trainer for the time being.

On both trainers, the power calculations seem off.  He and I will start at the same time.  I’ll put down 250-300 watts in my 53x15 on a straight away section and show that I’m going 21 MPH (when, on level ground, in normal conditions, I’d be putting down around 25-27 in the gearing that I’m in).  However, he’ll be right beside me in a 46x25 spinning along going 26-27 MPH.

Second, I’ll be in my 53x12 sprinting at 100+ cadence and the most I can hit is 28-29MPH.  That normally nets me around 34-35MPH on the road.  He’ll pop down to the 46x14 and whip out an astonishing 32MPH.

Third, on climbs, he zips up showing 10+ w/kg.  He can sustain 6 w/kg in a 10K TT in real life, but 10w/kg seems a bit out of whack.  On the same hill, I’ll be spinning 80 RPM in a 53x25 and going 7MPH.  Also, if I get out of my saddle and really hammer it, my wattage drops to like 150 or below.  On the road using my road rear wheel PM, I can peak above 1,000w.  On Zwift with the Computrainer, the most I’ve ever mustered out was a lowly 550w.  

I’ve done calibration on my CT.  Is there something I’m missing?

The next step is to get the kid a power trainer, but in the mean time, I’d like for it to be at least a little realistic for him.  


(Eric C. (Zwift HQ)) #2
  1. I’d re-evaluate your Computrainer setup. We read the wattage directly from the CT and translate that into speed within Zwift. If it’s sending low wattage readings then perhaps your tire pressure isn’t set correctly? Or that the resistance unit is on too tightly with the wheel?

  2. What size bicycle does your son ride? Is the tire 700c or smaller?

(T acoma Cyclist (LVE)) #3

Hi Eric - 

I’ll recheck the computrainer setup.  The tire pressure is set at 120 for a clincher.  The rear wheel is also tight enough that it doesn’t slip but not much tighter.  However, I’ll recalibrate it and try it side by side with another PM to make sure it’s not reading wrong.  


The kid’s bike is a 700c wheel using a Mavic Ksyrium SLS with 25mm clincher.  Right now, he’s just getting a false sense of confidence in his own ability.  :-)  Once he gets the smart trainer for himself, I’ll let reality come crashing down for him.


(T acoma Cyclist (LVE)) #4

So, I’ve re-evaluated my setup with the computrainer and things still aren’t right.  I did a pull down a -4% grade at 300 watts.  I hit a whopping 23MPH.  That was after repeated calibrations of the computrainer.  I then did the same thing in Racermate and was doing 34MPH according to their software. 


(Ron Skinner) #5

when you calibrate your computrainer what number do you get.  It should be close to 2.00 to 2.50 range lower and you do not have the resistance unit turned on tight enough and higher and you need to back off the resistance pressure on your tire.  I find my computrainer works pretty well with the algorithm.

(Eric C. (Zwift HQ)) #6

Additionally, I believe speed in the Racermate software does not account for CdA, road resistance, and additional virtual factors on Zwift.

(T acoma Cyclist (LVE)) #7

Hey guys - following up - I think there might actually be an issue with the CT.  I’ve picked up a pair of Tacx Bushido Smart trainers and find them to be far more accurate.  I may have to send the CT back for a bit of rework.  

To Ron’s comment - I typically shoot for around 2.5.  However, on some of the steeper hills, that causes major tire slippage.  If I’m on Richmond, I’ll shoot for around 2.8.