General Discussion Triathlon Talk » Accuracy of bike computer on a trainer? Rss Feed  
Moderators: k9car363, alicefoeller Reply
2005-03-08 5:36 PM

User image

Coach
9167
5000200020001002525
Stairway to Seven
Subject: Accuracy of bike computer on a trainer?
I have a Sigma 1600 bike computer with front wheel mount. Last night, i moved the mount to the rear wheel (same diameter), and climbed aboard to see how lightening fast I've become with several weeks of training. I was stunned to see that in my middle chainring and big cog on the back, pedaling at 90rpm, I'm only going 8-9 mph!!! Can that really be possible? I'm doing everything right, keeping a high cadence and everything, but boy, that's even slower than I was riding outside last fall! How can this be??? I'm so bummed...I was hoping to see speeds of like 16 mph.

My bike is an old Bridgestone MB-4 mountain bike with 3 chainrings on front and a brand new Shimano 9 speed set of cogs on the back (not sure what model, or how many teeth on each cog).

Help!

Edited by AdventureBear 2005-03-08 5:56 PM


2005-03-08 5:39 PM
in reply to: #126982

User image

Resident Curmudgeon
25290
50005000500050005000100100252525
The Road Back
Gold member
Subject: RE: Accuracy of bike computer on a trainer?
I wouldn't trust the computer. Get yourself a GPS to verify the speed.
2005-03-08 5:43 PM
in reply to: #126982

User image

Master
1661
10005001002525
Newbury Park, CA
Subject: RE: Accuracy of bike computer on a trainer?
Yeah, I wore my Garmin on the treadmill the other day.  I am one slow dude
2005-03-08 5:47 PM
in reply to: #126984

User image

Resident Curmudgeon
25290
50005000500050005000100100252525
The Road Back
Gold member
Subject: RE: Accuracy of bike computer on a trainer?
Yeah, talk about a reality check!
2005-03-08 5:57 PM
in reply to: #126982

User image

Coach
9167
5000200020001002525
Stairway to Seven
Subject: RE: Accuracy of bike computer on a trainer?
You boys are no help at all!!!
2005-03-08 6:08 PM
in reply to: #126982

User image

Master
1661
10005001002525
Newbury Park, CA
Subject: RE: Accuracy of bike computer on a trainer?
Darn Boys!  I truly hope the compputer is way off.  I bust my butt to get to 18 when I'm on the trainer.  I can avg in the 20's much easier outside.  All I do is worry about my HR and cadence on the trainer.


2005-03-08 6:19 PM
in reply to: #126993

Master
1597
1000500252525
Colorado
Subject: RE: Accuracy of bike computer on a trainer?
How many teeth on the middle chainring and large cog?  With that info and your RPM's you can figure out what your speed should be and know for sure...
2005-03-08 6:47 PM
in reply to: #126982

User image

Elite
3498
20001000100100100100252525
Chicago
Subject: RE: Accuracy of bike computer on a trainer?

bike computers on trainers are not accurate because the resistance isn't the same as the resistance on the road, which takes into account your weight.

the only "trainer" the bike computer would be accurate on would be a computrainer that's calibrated to your weight.

also, you can't calulate it using the # of teeth on your cog/cassette and your rpm because your trainer doesn't have accurate resistance as I mentioned above.

Example:  I do a z1 ride and my computer tells me its 13mph on my cyclops fluid trainer when in fact on the same bike on the road on a z1 ride i'm doing 17mph.



Edited by Steve- 2005-03-08 6:48 PM
2005-03-08 7:08 PM
in reply to: #127016

Master
1597
1000500252525
Colorado
Subject: RE: Accuracy of bike computer on a trainer?

I think one of us is missing the point.  I am assuming she wants to know what MPH her wheel is spinning at, which you can calculate from a gear ratio and cadence.  Translating that into a HR zone on the road is another story...

Steve- - 2005-03-08 6:47 PM

also, you can't calulate it using the # of teeth on your cog/cassette and your rpm because your trainer doesn't have accurate resistance as I mentioned above.

2005-03-08 7:17 PM
in reply to: #126982

User image

Expert
1180
1000100252525
Iowa
Subject: RE: Accuracy of bike computer on a trainer?
I trust my computer. It is a CatEye, and I am definitely slower, even at high RPM. That's because you have no inertia to carry you along on the trainer, no downhills, no tail wind, etc. Don't worry about your speed on the trainer. Follow a good training routine consistently and it will translate to a better "real world" bike experience when you're outside.
2005-03-08 7:24 PM
in reply to: #126982

User image

Coach
9167
5000200020001002525
Stairway to Seven
Subject: RE: Accuracy of bike computer on a trainer?
I think I'll just move the sensor back to the front wheel and not worry about it! I felt so good about my biking until last night! I know my legs are stronger, so I'll just keep plugging away and hope that springtime comes soon. Thanks!


2005-03-08 7:28 PM
in reply to: #127028

User image

Resident Curmudgeon
25290
50005000500050005000100100252525
The Road Back
Gold member
Subject: RE: Accuracy of bike computer on a trainer?

Move it to the fromt wheel and it will REALLY read slow.

Seriously, on the trainer, focus on time and intensity (heart rate). Don't try to compare trainer speed with real speed.

2005-03-08 8:49 PM
in reply to: #127030

User image

Coach
9167
5000200020001002525
Stairway to Seven
Subject: RE: Accuracy of bike computer on a trainer?
You're right. I guess that I should be happy my speed increased to an infinate degree overnight when I moved it from the front to the back!
2005-03-09 8:23 AM
in reply to: #127023

User image

Coach
10487
50005000100100100100252525
Boston, MA
Subject: RE: Accuracy of bike computer on a trainer?

How do you calculate your real speed using the "gear ratio" and cadence?

Stake - 2005-03-08 7:08 PM

I think one of us is missing the point.  I am assuming she wants to know what MPH her wheel is spinning at, which you can calculate from a gear ratio and cadence.  Translating that into a HR zone on the road is another story...

2005-03-09 3:07 PM
in reply to: #127174

Master
1597
1000500252525
Colorado
Subject: RE: Accuracy of bike computer on a trainer?

Go here:

http://www.hostelshoppe.com/tech_gearcalc.php

Or if you want a real program (free I think) go here: http://www.machinehead-software.co.uk/bike/gears/gear_calculator.html



Edited by Stake 2005-03-09 3:09 PM

New Thread
General Discussion Triathlon Talk » Accuracy of bike computer on a trainer? Rss Feed