Rollers: tire pressure <-> rolling resistance?

[sandyrs]

I've been riding the rollers more lately and have noticed that on certain days, it takes much more power to push the same gear- as in one day, it'll take a 270w constant effort to push the 53x11, and another day it will take well over 300w. It would help me greatly to know how to keep this number constant, as I've had problems with not being able to do workouts due to the resistance of the rollers being too low to hit necessary numbers.

I'm always riding the same bike on the same rollers with the same wheels- the only variable I can think of is tire pressure dropping from one ride to the next. Has anyone else experienced this, or can anyone point me to a definitive source discussing what I seem to be seeing? Thanks!

[ultraman6970]

I have to be honest with you, IMO (i might be wrong) the problem is that you are putting watts to the equation and rollers IMO aren't built to take watts as a factor unless you have one of those rollers with built in rolling resistance. Thats the reason you have trainers with rolling resistance and who knows what else. Rollers main function is to warm up, cadence, agility work outs, keep your legs moving if you cant go outside and pretty much that's it.

BTW never seen anybody doing 53x11 in rollers, i have tried doing rollers in a road bike and the work out you can get out of a set of rollers is totally different than what you can get out of rollers plus a track/fix gear bike, which bike is the ideal bike to work out with rollers. Personally I never liked rollers in a road bike at all.

THe other detail is that if one day you are hitting 60rpms and the next day you hit 70 rpms because you are less tired obviously you will see a change in the watts.

If you lose pressure using rollers the rolling resistance will increase but the riding over the rollers will suck and actually you will bounce like too much and even the handling of the bike gets weird if you ride with the pressure too low over the rollers.

BTW I was a tracker and we did not have watts machines back in the day so the way I see this it might be (it is) totally different to what an actual racer will say you know. I was you I would get a trainer and use the rollers with fix gear (increase pedaling technique, agility) but thats just my opinion.

[sandyrs]

I have to be honest with you, IMO (i might be wrong) the problem is that you are putting watts to the equation and rollers IMO aren't built to take watts as a factor unless you have one of those rollers with built in rolling resistance. Thats the reason you have trainers with rolling resistance and who knows what else. Rollers main function is to warm up, cadence, agility work outs, keep your legs moving if you cant go outside and pretty much that's it.

BTW never seen anybody doing 53x11 in rollers, i have tried doing rollers in a road bike and the work out you can get out of a set of rollers is totally different than what you can get out of rollers plus a track/fix gear bike, which bike is the ideal bike to work out with rollers. Personally I never liked rollers in a road bike at all.

THe other detail is that if one day you are hitting 60rpms and the next day you hit 70 rpms because you are less tired obviously you will see a change in the watts.

If you lose pressure using rollers the rolling resistance will increase but the riding over the rollers will suck and actually you will bounce like too much and even the handling of the bike gets weird if you ride with the pressure too low over the rollers.

BTW I was a tracker and we did not have watts machines back in the day so the way I see this it might be (it is) totally different to what an actual racer will say you know. I was you I would get a trainer and use the rollers with fix gear (increase pedaling technique, agility) but thats just my opinion.

Thanks, I'd like to get a smart trainer but don't have the money to spend on one at the moment and hate riding my regular old turbo trainer so I'm making do with what I have. I'm not trying to do workouts with big spikes in power, just trying to do maybe some 2x20's at threshold or some sweet spot and even that sometimes outpaces the resistance the rollers provide. I'm actually planning to do cross-country mountain bikes races this summer so right now the goal is just to work on steady longer efforts.

Mostly just interested in knowing what's causing the variation, and it sounds like you're agreeing that tire pressure is the cause. Cheers.

[MattTuck]

Don't have time to find it right now, but yes, tire pressure is 99% sure the culprit. There was a chart posted here or somewhere else in the last 2 months that showed the impact of different tire pressures and weights. on roller wattage.

That said, I just started riding rollers this year and don't know enough about them to answer the following question. Does the power curve change during the ride like it can on a fluid trainer? If that is the case, just make sure you're comparing apples to apples.

[sandyrs]

Don't have time to find it right now, but yes, tire pressure is 99% sure the culprit. There was a chart posted here or somewhere else in the last 2 months that showed the impact of different tire pressures and weights. on roller wattage.

That said, I just started riding rollers this year and don't know enough about them to answer the following question. Does the power curve change during the ride like it can on a fluid trainer? If that is the case, just make sure you're comparing apples to apples.

Matt, what do you mean by the "power curve" in this scenario?

[phutterman]

Matt, what do you mean by the "power curve" in this scenario?

The graph of resistance (power) by speed; it's typically a set line (or curve), so whatever speed you're moving when say, you're doing 90rpm in your 53x11 would require a particular amount of power to maintain (and there'd be similar set values for other speeds; it's what you're seeing vary). With fluid, it'll change over the course of a workout as the fluid warms up (though I think typically it'll settle in once it gets up to 'operating temperature'), whereas with most other kinds of trainers (rollers included) it really shouldn't.

See https://www.cycleops.com/post/blog-15-cycleops-science-resistance-curves for instance, for theirs

Edit: here's one for rollers, actually, with the roughly expected high end being around 300w at 40mph (with no resistance unit) similar to what you're seeing around ~300w in your tallest gear. https://www.sportcrafters.com/blog/virtual-power-sportcrafters-rollers

Edit again: I agree tire pressure is the likely variable. Another question would be over what period of time you're looking at power (instantaneous, 3sec avg, lap avg) because there's of course also a lot of movement in what you see with the more instant values. But if the same length of effort, same gear/speed/cadence, is actually averaging ±10% then it's probably tire pressure.

[Mark McM]

I agree with the others, the most likely culprit is tire pressure.

Riding on rollers eliminates the largest sources of drag (aerodynamic resistance, and gravity force if climbing), leaving rolling resistance as the majority of the drag. Also consider: Tire rolling resistance is caused by hysteresis losses as the tire flexes in the contact zone. The small diameter of the drums increases the flexing of the tire in the contact zone, amplifying the magnitude (and importance) of rolling resistance. Finally, tire rolling resistance changes with inflation pressure - lower pressure has higher rolling resistance.

In other words, differences in tire pressure can have a dramatic effect on the drag when riding rollers, and will have a large effect on the power required to ride at a given speed.

[thwart]

It would help me greatly to know how to keep this number constant, as I've had problems with not being able to do workouts due to the resistance of the rollers being too low to hit necessary numbers.


With simple rollers (those without magnetic or other methods to vary resistance) besides the obvious option of increasing resistance by playing with tire pressure, some folks have also had luck with a piece of rolled up carpet pressed firmly up against the rear roller.

My own experience is that with tire pressure under 100 psi or so (25 mm tire, rider weight 150 lbs) the front tire is just too mushy feeling if I decide to get out of the saddle to mix things up a bit.

[kunfuzion]

I wouldn't doubt the issue being the variation in tire pressures. Best way would be to check is to intentionally set the tire pressures differently and see how it feels.

[MattTuck]

Matt, what do you mean by the "power curve" in this scenario?

Answered well above. But there is another effect in fluid trainers (I think, in particular) in which the resistance at a given wheel speed is lower at the start of the ride. I seem to remember in some cases that it takes 5-10 minutes to warm the device up, and then it is consistent for the rest of the ride.

Not sure if this occurs on rollers as well, and honestly, I don't recall that happens on every fluid trainer. But it is something that has been discussed here before.

[Mark McM]

Answered well above. But there is another effect in fluid trainers (I think, in particular) in which the resistance at a given wheel speed is lower at the start of the ride. I seem to remember in some cases that it takes 5-10 minutes to warm the device up, and then it is consistent for the rest of the ride.

Not sure if this occurs on rollers as well, and honestly, I don't recall that happens on every fluid trainer. But it is something that has been discussed here before.

Actually this was mentioned, or at least alluded to, but phutterman above. Any (passive) resistance trainer creates a drag resistance by dissipating kinetic energy as heat (that kinetic energy being provided by the rider's pedaling). In a fluid trainer, the drag resistance is provided by the viscosity of the fluid (viscosity being a fluid's reluctance to flow). This in turn heats up the fluid (which is why you'll usually see cooling fins on the fluid resistance unit). The viscosity of most fluids decreases with temperature (i.e. the fluid gets "thinner"), reducing the drag resistance. The result is that typical fluid trainers start out cold with maximum resistance, and then the resistance decreases while it warms up, until it reaches a maximum steady state temperature, at which point it also reaches a steady state resistance.

[phutterman]

Oh also aside from the carpet/towel/etc shoved against the drum, you can add resistance to rollers with metal drums (need not be magnetic, either, but metal) by placing strong magnets very close to the drum (like this: http://girogeek.blogspot.com/2009/11/so-i-decided-that-in-order-to-gain-some.html).

I experimented with doing that a little bit but never bothered to build a precise/solid enough magnet holder. It definitely changed the spin down time of the roller I put a strip of magnets (as in the link above) near. In any case I decided to keep the rollers for warming up/down at races and such and got a trainer for resistance workouts.

Rollers definitely feel harder to ride (and harder to control) with squishier tires. That the drum diameter also effects their resistance so much (smaller = harder) also speaks to how much of the resistance comes from the amount the tires are deflecting.

[stephenmarklay]

What did you say you are using to measure power?

[Mark McM]

Oh also aside from the carpet/towel/etc shoved against the drum, you can add resistance to rollers with metal drums (need not be magnetic, either, but metal) by placing strong magnets very close to the drum (like this: http://girogeek.blogspot.com/2009/11/so-i-decided-that-in-order-to-gain-some.html).

I experimented with doing that a little bit but never bothered to build a precise/solid enough magnet holder. It definitely changed the spin down time of the roller I put a strip of magnets (as in the link above) near. In any case I decided to keep the rollers for warming up/down at races and such and got a trainer for resistance workouts.

Rollers definitely feel harder to ride (and harder to control) with squishier tires. That the drum diameter also effects their resistance so much (smaller = harder) also speaks to how much of the resistance comes from the amount the tires are deflecting.

This is the same principle (induced eddy currents) as used by Mag Resistance trainers. Any resistance unit disipates energy as heat, and in your case, the heat from the eddy currents will go into the (metal) roller drums. Be careful that you don't heat up the drums so much that you melt the grease out of the bearings.

Also, be careful if using carpet or towel to rub against drum - this drag also generates a lot of heat. I've seen carpets melted, and even start smoking, when this has been done (I've also heard of fires being started, although I haven't actually seen it).

[MattTuck]

Actually this was mentioned, or at least alluded to, but phutterman above. Any (passive) resistance trainer creates a drag resistance by dissipating kinetic energy as heat (that kinetic energy being provided by the rider's pedaling). In a fluid trainer, the drag resistance is provided by the viscosity of the fluid (viscosity being a fluid's reluctance to flow). This in turn heats up the fluid (which is why you'll usually see cooling fins on the fluid resistance unit). The viscosity of most fluids decreases with temperature (i.e. the fluid gets "thinner"), reducing the drag resistance. The result is that typical fluid trainers start out cold with maximum resistance, and then the resistance decreases while it warms up, until it reaches a maximum steady state temperature, at which point it also reaches a steady state resistance.

Ahh, yes, I missed that. Here's the thread I was mentioning from a few years ago. The general consensus of some users is the opposite of what you are describing, namely, that the resistance increases as it warms up, not decreases as you note. It is sort of a mystery, because I agree the science suggests it should get easier. thread from 2014 (http://forums.thepaceline.net/showthread.php?t=159692&highlight=trainer+resistance+increase)

Anyway, this is what I was referring to with regards to Sandy's original question. Namely that if the resistance of the system changes over the course of a workout, given heat or whatever other factors, that could mess with perception. I still think it is his tire pressure.

[Peter P.]

Oh, tire pressure is DEFINITELY an issue!

I normally ride my 700x25 tires at 80psi. on the road.

On the rollers they feel positively spongy.

100psi. feels better, especially out of the saddle as the bike won't "squish" on every pedal stroke.

I'd say you're going to have to inflate your tires before any workout where you're interested in accurate comparisons to previous stats. Otherwise, lower tire pressure is less of an issue.

[11.4]

With rollers there isn't any appreciable heat accumulation in the roller drums. The issue with tires on rollers is that they start out stiff and because your full body weight is flexing the tire against the roller and doing it to both tires, there's quite a bit of resistance when you first start, then they warm up and briefly seem to ride faster, but then with continued riding they deflect further due to softening up and the extra tire displacement causes extra resistance. If you ride tires that are slightly low in pressure, this is all exacerbated.

Setting a precise pressure is imperative on rollers every time you use them, and the pressure you choose can make quite a difference. Some people lower tire pressure to increase resistance, but it makes resistance more unpredictable and you run the risk of burping a tire or otherwise having a failure.

I wouldn't say there's anything wrong with using a road bike on rollers, though I principally use a track bike on mine. And if you pump the tires up to 125-130 psi, a 53-11 can get spun out quite easily. I generally do suggest a slightly higher pressure than on the road simply because the roller drums have the effect of softening the ride a bit (the contact patch is smaller than on the road, at least in profile) and a squishy tire on rollers simply doesn't feel good and can occasionally have sidewall failures.

And I've never seen a towel even get particularly hot while used to build resistance under a roller drum, but that's not my problem with that practice. It's more that it's very unpredictable -- a lot of resistance one day, only a little the next -- plus the towel can occasionally get snapped up and thrown into your wheels or chain.

[chiasticon]

some awesome insights in this thread as to how rollers and trainers work. simple devices, I thought. not entirely so. love this place. :beer:

I have no real insight other than to say that I'd entirely agree it's tire pressure. I'm pretty particular about making sure I'm at 110psi before riding rollers (ride 80psi outside). one because the bike is easier to control on the rollers (especially standing up) and two because resistance is then the same from ride to ride.

I'm for sure in the minority, but I don't agree that rollers can't be used for intense workouts. I wouldn't go too deep into the pain cave on them, to where you run a serious risk of falling, but 2x20's at threshold and similar workouts are entirely doable. assuming your rollers offer enough resistance to hit your target.