Is Slight Trainer 'Rocking' Fine?

noonievut said:

I noticed today that when I'm pedalling hard on my fluid trainer the bike rocks / sways slightly side to side. Is this normal?

The bike seems securely attached to the trainer and I have the front tire on a riser block.

I have the bike and trainer on a mat on my basement floor.

My trainer (a Tacx) doesn't move around at all. Have you checked that the feet are all meeting the floor on a level base? Perhaps one of the feet needs packed up to get it to sit completely level on each.

My Kurt Kinetic Road Machine and my wife's Cycleops Fluid2 trainers do not rock. Is your saddle so high that it would cause you to rock on the bike?

does it rock as in it clunks? or does your bike simply flex/sway with your pedaling?

clunking is bad - verify you have the correct skewer (the steel one that came with the trainer), it's tight, and your wheels are tight.

if the frame is swaying a bit, well, that's normal.

the two other suggestions are very good too - checking if the feet are level, seeing if your saddle is so high that it forces you to rock.

good luck
cdr

noonievut said:

I noticed today that when I'm pedalling hard on my fluid trainer the bike rocks / sways slightly side to side. Is this normal?

The bike seems securely attached to the trainer and I have the front tire on a riser block.

I have the bike and trainer on a mat on my basement floor.

Some slight flexing under heavy load is normal. That actually happens on the road too, but you just can't see it and it is accentuated on the trainer because the bike is fixed to the trainer.

KellyT said:

My trainer (a Tacx) doesn't move around at all. Have you checked that the feet are all meeting the floor on a level base? Perhaps one of the feet needs packed up to get it to sit completely level on each.

My Tacx Sirius rocks (in both interpretations), but its made to. If you can see your BB flexing it is just frame flex, with a bit of trainer flex. If it is not too much, I wouldn't worry.

carpediemracing said:

does it rock as in it clunks? or does your bike simply flex/sway with your pedaling?

clunking is bad - verify you have the correct skewer (the steel one that came with the trainer), it's tight, and your wheels are tight.

if the frame is swaying a bit, well, that's normal.

the two other suggestions are very good too - checking if the feet are level, seeing if your saddle is so high that it forces you to rock.

good luck
cdr

No clunking (the skewer seems to be securely mounted), just a bit of swaying, and only when I'm pushing a big gear. I will see if the feet are level though. My saddle is at the same height it's always been, however, would be interesting to drop it a bit and see if the same thing happens (would mean it's always been too high?).

Thanks.

noonievut said:

just a bit of swaying, and only when I'm pushing a big gear.

Sounds like its just your frame flexing, which is normal. No way to stop that.

John M said:

Some slight flexing under heavy load is normal. That actually happens on the road too,

Actually, no, this does not happen on the road at all, at least not in this way. If you use your bike on a trainer, it will be subject to bending stresses at the mount points caused by the inevitable motion of the rider on the bike. Such moments never occur on the road (where you have some bending because of the moments applied at the bottom bracket only).

Enough reason for me not to torture my good bike on a trainer. Given enough time on the trainer, fatigue failures may occur, depending on the frame and weight of the rider. Road frames are not designed for this kind of load. YMMV, as they say...

Dietmar said:

Actually, no, this does not happen on the road at all, at least not in this way. If you use your bike on a trainer, it will be subject to bending stresses at the mount points caused by the inevitable motion of the rider on the bike. Such moments never occur on the road (where you have some bending because of the moments applied at the bottom bracket only).

Enough reason for me not to torture my good bike on a trainer. Given enough time on the trainer, fatigue failures may occur, depending on the frame and weight of the rider. Road frames are not designed for this kind of load. YMMV, as they say...

I agree that the amount of flexion that occurs on the road is not exactly the same as on a trainer, but the direction of forces delivered to the frame due to pedaling would be same regardless of whether the bike in on a trainer or on the road. A rear axle mount trainer (the most common design these days) would fix the bike in the same way that the rear axle QR anchors the bike to the wheels. While riding the road, the wheels would also flex and absorb some of the forces, but it seems to me that the frame flex due to pedaling forces on the BB would still be referenced to the frame's fixed points: the front and rear dropouts. Now I agree that if the rider is throwing their weight around alot and directing twisting forces through the saddle, these forces would markedly differ on the trainer as compared to the road, since the point of contact would be the bike-trainer interface rather than the tire-road interface that occurs in road riding when the bike/rider rock side-to-side on the road.

Whereas you may be correct that the amount of load that would be subjected to a bicycle frame in a trainer would differ from what they are intended for, the vast experience that is out there with these trainers and the persistence of the mounting design would indicate that the design is probably safe for the majority of bicycle frame materials and manufacturing methods and their riders. Perhaps I have been falsely reassured by my experiences since I only weigh about 68kg.