Adding inertia to an indoor trainer wheel



grahamspringett

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Feb 26, 2004
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I'd love a Kurt Kinetic with the huge flywheels but I can't afford one so I shall stick with my Elite.

I want more inertia (coastdown from 40kmh is about 2-3 seconds) and was wondering if I could add weights to the inside of the tyre. Adding weights to the rim or spokes is obviously dangerous but if the weight is inside a tyre they should be happily tucked away.

I was thinking of adding weights all along the tyre's circumference and tucking the inner tube back in as best I can.

I won't be using the wheel on the road.

Can anybody see any flaws in my great plan for DIY inertia adding?
 
A couple of winters ago I tried just that. I bought an old PT Pro wheel off ebay and a box of adhesive wheel balancing weights. I sandwiched each spoke between four 1 ounce weights and secured the whole mess from spinning freely about the spokes with duct tape. It added 9 pounds to the wheel and all out by the rim for a dramatic increase in MOI. Bottom line, Tempo workouts felt like silk with a very smooth roadlike feel but at similar power levels to Tempo workouts without the added weight. Sustainable power for Threshold and VO2 Max workouts didn't budge at all nor did RPE diminish for those efforts when compared to a standard wheel.

I eventually stripped the weights off and sold the wheel as it didn't really change my indoor workouts in a meaningful way.

Basically short of a huge flywheel like Alex has on his Thunderbird or perhaps on a Velodyne or gym stationary bike it's really hard to increase MOI enough to emulate the linear momentum you get from rolling your body mass down the road on an outdoor ride. I run a KK trainer and a Computrainer and they both have their advantages but the KK isn't so much smoother or easier to ride that my power numbers are noticeably different when I swap trainers.

So based on that experiment I wouldn't bother adding just a bit of weight to the rim.

-Dave
 
Thanks for the comprehensive reply - sounds like you've been where I was heading and found it was a dead end.

Is the KK a worthwhile purchase? How do long intervals compared to on a trainer with less inertia? My SST intervals are real grinders and feel like threshold efforts. Has a Velodyne got more inertia than a KK with both flywheels attached?

I do a fair amount of indoor work (working dad!) and am looking for a more realistic feel.
 
grahamspringett said:
...Is the KK a worthwhile purchase? ...
The KK is a very well built and reliable trainer but my indoor power still drops by 5-7% relative to what I can do outdoors and like you say SST work feels like maximal Threshold work or looked at the other way indoor Threshold efforts are performed at outdoor SST/Tempo levels. It kinda sucks but I never seem to quite close the gap between indoor and outdoor power numbers by the time spring rolls around. My first winter of indoor training I didn't see such a big gap, but as my FTP increased the gap widened.

I have no trouble recommending the KK trainer, but don't expect the power numbers to suddenly jump. If it helps over the past four seasons I've seen an almost immediate jump in power as soon as I get back outside in the spring and it's always been to a higher level than where I left off in the fall so that indoor work is doing something good even if the recorded power is a bit low.

Good luck,
-Dave
 
grahamspringett said:
Can anybody see any flaws in my great plan for DIY inertia adding?

Mainly the surface of the tyre would no longer be even and you'd get stuttering contact with the roller. There is also the distinct possiblity of rotational imbalance...

To do it right, you need to be able to store the same kinetic energy in the rotating rear wheel and flywheel as you would in your moving mass out on the road.
Generally a flywheel is better because it can be made to spin at many times the speed of the wheel, so that the same energy can be stored in far less rotating mass. The difficulty with this is that a flywheel would then need to be tailored to each rider's/bike's individual mass.

In general the flywheels on most trainers are still too light.
 
tafi said:
Generally a flywheel is better because it can be made to spin at many times the speed of the wheel, so that the same energy can be stored in far less rotating mass.

Yes.... this. You'd be much better off crafting some kind of extra flywheel for the trainer rather than adding weight to the wheel.

Something else you might try if your trainer has an adjustable resistance setting, is running your trainer at a lower resistance setting and compensating for that by running your bike wheel at a higher speed (higher gear). By spinning the trainer faster it will have more inertia without modification, and by reducing the resistance setting you can be at the same power as before.

In either case, expect the difference to be small.
 
frenchyge said:
By spinning the trainer faster it will have more inertia without modification.

Be careful! Inertia does not vary with speed. Only the energy stored in the flywheel does.

I think you might run out of gears with current trainers before you reach "realism".

Check out the "BT Training System" Has a nice big fan in it which should double as something of a flywheel (though how much I have no idea).
 
tafi said:
Be careful! Inertia does not vary with speed. Only the energy stored in the flywheel does.

Touche`. Probably not the right word, although the principle still fits. Having a higher speed (more energy) and lower frictional force (less energy drain) does reduce the tendency for the wheel to slow down and produces an inertial effect. Inertial loading is a term I've heard to describe the difference in pedalling feel associated with either greater flywheel mass *or* use of higher gearing/lower resistance to increase wheel speed at constant power.




tafi said:
I think you might run out of gears with current trainers before you reach "realism".

Definitely, but it's worth pushing as far as possible before buying a new trainer or adding lead to the wheel. :)
 
You could try pumping the tube / tire full of water to add a bit of weight. If you keep the valve stem at the highest point you can vent out the air as you add water. That could add almost 1 KG of weight depending on your tire size.

The challenge would be filling up the tube.
 
Having now looked at the Kurt website, the separable flywheels on the "PRO" resistance unit look interesting.

Can anyone tell me what the dimensions of the flywheel parts are?

It looks like the small "6lb" one is a solid disk (I'd need the thickness and diameter of this) and the larger "12lb" flywheel has a recess to fit the smaller one inside (I'd need the width and diamter of the flywheel and the depth and diameter of the recess).

Then it's easy to determine the moment of inertia.
 

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