BB flex on trainer

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mdusink
Posts: 47
Joined: Fri Aug 23, 2013 7:39 pm

by mdusink

It's getting colder outside, so I've put the bike on the trainer again and noticed about 2mm flex at the top of the chainrings relative to the front derailleur. This happend at an easy 150 watts output. No flex at zero watts.

Total setup is a 56 Tarmac SL4 pro, BB30 S-works cranks, power2max s-type spider, SRAM Red chainrings (50/34).

I'm guessing the flex comes from the bb-area, because the smaller chainring is moving in sync with the bigger ring. I find it strange that this flex can happen even with such a small distance between the bb and the front derailleur. Even with all the reviews saying the SL4 was a very non-flex frame.

The trainer is a kurtkinetic (holds on to the rear axle)

Any thought, are others experiencing the same thing?

thanks!

by Weenie


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dmulligan
Posts: 314
Joined: Wed Jul 23, 2014 6:16 pm

by dmulligan

It happens to all bikes and it happens on the road too. It is just easier to notice on the trainer. The amount of flex varies by frame and crankset. My old aluminium 2009 Kona Jake with a 105 crankset flexed a lot more than my new 2015 Cervelo R3 with an Ultegra crankset.
I've always imagined it would happen less if I had kept the Rotor crankset with it's 30mm axle, but I wanted to keep using my Stages power meter.
Recently there was a long and interesting discussion here on whether BB flex has a detrimental effect or not. I'm positive it had a psychological effect on me and I know I ride, and climb, faster on my R3. Either way ignore it as there is nothing wrong with your setup. The flex looks magnified when your bike is immobilised and you are either bored or suffering on the trainer.

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mdusink
Posts: 47
Joined: Fri Aug 23, 2013 7:39 pm

by mdusink

Ok, so I shouldn't look down while on the trainer :) . Do you perhaps got the link or key words for the other discussion?. I'd like to read up.

(I searched before this post, couldn't find much...)

Thanks for your reply.

tinozee
Posts: 764
Joined: Wed Jun 01, 2011 7:53 am

by tinozee

I use a Kurt as well and notice a huge difference between S-Works SL3 Tarmac and Evo HiMod. Sl3 does not flex at all while Evo sways lateral 5mm or so. I am looking at the BB cup itself on the nds not the chain rings. I attribute that to the Evo being flexier. The Sl3 is notoriously stiff in the BB, no clue about the SL4. On the road they both feel great so I don't care, but it is definitely happening.

bm0p700f
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by bm0p700f

You ride and climb faster on your r3 because it is a more aerodynamic frame than the Kona Jake not because it is stiffer. Aerodynamic effects work at all speeds .
No has proved one way or another the influence of bb flex on power transfer. Probably means there is a weak or no link that is swamped by other factors.

dmulligan
Posts: 314
Joined: Wed Jul 23, 2014 6:16 pm

by dmulligan

mdusink wrote:Ok, so I shouldn't look down while on the trainer :) . Do you perhaps got the link or key words for the other discussion?. I'd like to read up.

(I searched before this post, couldn't find much...)

Thanks for your reply.


http://weightweenies.starbike.com/forum/viewtopic.php?f=3&t=133901

dmulligan
Posts: 314
Joined: Wed Jul 23, 2014 6:16 pm

by dmulligan

bm0p700f wrote:You ride and climb faster on your r3 because it is a more aerodynamic frame than the Kona Jake not because it is stiffer. Aerodynamic effects work at all speeds .
No has proved one way or another the influence of bb flex on power transfer. Probably means there is a weak or no link that is swamped by other factors.


I agree that aerodynamics is a factor when descending and on a flat, especially in the windy foothills of the Rocky Mountains. However I disagree that stiffness is not a factor although I concede that it may be more psychological than I feel. I am fully OK with psychological gains because gains are gains and I was very demoralized plodding up 8-12% grades while watching my BB sway back and forth by at least 1CM. This post in the flex thread I mentioned earlier expresses what I've felt ever since I first test rode my R3.

I love my R3, it makes me smile every time I ride it outside. Now I only hate my trainer rather than my bike and my trainer when riding indoors.
D

rbrtwyn
Posts: 252
Joined: Thu Dec 09, 2004 1:57 pm

by rbrtwyn

It's that time of year. Next will be the "is cold weather bad for my carbon frame?" threads ;)

fignonsbarber
Posts: 146
Joined: Fri Jun 25, 2010 10:24 pm

by fignonsbarber

atmo, racing bikes aren't designed to be tourqued upon while fixed in a trainer. Throw in the corrosive effects of heavy sweat, and you have a less than ideal environment for your prized possession. I purchased a used Lemond Revmaster from a local gym equipment company ( they constantly take them back from gyms as the lease expires) for $300. You can get your road position close, it's solid, and great for power workouts. I use rollers to mix in supless training.

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wheelsONfire
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Location: NorthEU

by wheelsONfire

There is other things one really should consider riding on a trainer!

For one, the stress it put on the frame is tremendous.

No frame is really built for this type of riding, even though no manufacturer says so in the open.

All those riding super light weight frames probably should void all sorts of sprinting and all out of saddle pedaling.

I am not sure, but i doubt our warranty would be agreed for if the frame cracks due to this.

I know a guy who cracked his BB due to this type of riding.

The company who got the frame back could not understand what had happened.

But during our discussion, it was more or less pin pointed the frame cracked during out of saddle pedaling in his trainer.

His frame was not a light weight frame, so i guess the light frames are even more prone to damage.
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Ex bike; Vial EVO D, Vial EVO Ultra, Scott Foil, Paduano ti bike.

srshaw
Posts: 175
Joined: Sun Apr 05, 2015 6:06 pm

by srshaw

rbrtwyn wrote:It's that time of year. Next will be the "is cold weather bad for my carbon frame?" threads ;)


Or is it ok to put my carbon frame on a trainer.....

goodboyr
Posts: 1487
Joined: Wed Jan 10, 2007 10:56 pm
Location: Canada

by goodboyr

wheelsONfire wrote:There is other things one really should consider riding on a trainer!

For one, the stress it put on the frame is tremendous.

No frame is really built for this type of riding, even though no manufacturer says so in the open.

All those riding super light weight frames probably should void all sorts of sprinting and all out of saddle pedaling.

I am not sure, but i doubt our warranty would be agreed for if the frame cracks due to this.

I know a guy who cracked his BB due to this type of riding.

The company who got the frame back could not understand what had happened.

But during our discussion, it was more or less pin pointed the frame cracked during out of saddle pedaling in his trainer.

His frame was not a light weight frame, so i guess the light frames are even more prone to damage.

Like everyone is saying....its that time of year again for all this bs. No way your bike will break ( unless you drop it or it falls out of the trainer). And if you are worried about a trainer, then don't be descending a mountain on your fragile machine. Rant over.

AZR3
Posts: 998
Joined: Wed Sep 12, 2012 9:00 pm
Location: Az USA

by AZR3

The first time I used a trainer I was surprised by the amount of movement I could see while pedaling but if you watch YouTube you'll see some fatigue testing of carbon frames and they flex the crap out of them for a long long time without issue. Everything is fine, it's just a little unnerving to see the first time.

by Weenie


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