wheel weight and acceleration?

[Adam]

Hi all, hoping to get some thoughts on the role of wheels in acceleration and climbing.

I recently picked up my first Riv (Hillborne) and am running Dyads and Barlow Pass tires. Among other things, I'm amazed at the difference in acceleration, speed, and particularly climbing vs my other bike, which is a pretty heavy Salsa Marrakesh with stock wheels (WTB sx19 and Shimano m475 hubs) and Cazaderos.

I'm not necessarily jumping to replace the Marrakesh's wheels ATM, but I am curious whether anyone has thoughts on whether or not that's likely the difference I'm feeling in acceleration and speed?

There's definitely a substantial weight difference between the two builds, but I've loaded up the Sam a few times and it's still way quicker. I'd just swap the wheels, but the Marrakesh's are disc.

thoughts?

[Patrick Moore]

I've used all sorts of wheel sizes and weights, from just shy of 20" to just over 30", and tires ranging from 175 grams to over 900 grams. IME, wheel diameter and tire weight affect the ride far less than tire quality (supple, light casing) except on hills, when at least in the extreme -- in my case, 800 gram rim and 800 gram tire + 200-250 gram tube; the rim and the tire were the "lite" models! -- you certainly can feel a difference climbing over a sub 175 gram tire, 360 gram rim, and 70 gram tube, especially if the overall wheel diameters differ by about 5".

One of my "plane-y-est" bikes is that Matthews dirt road bike with 700C X 60 tires. Granted that the wheels are quite light for their size (29 1/2" tall, 60 mm wide, Velocity Blunt SSs, Big Ones), but the bike just feels easier to pedal in a given gear in given conditions; at least as good as my bikes with much lighter wheels. On the flats. Climbing, the light-wheel bikes feel faster.

All that said, good quality light wheels and especially top quality tires really do affect the pleasure of the ride.

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Patrick Moore

Alburquerque, Nuevo Mexico, Etats Unis d'Amerique, Orbis Terrarum

[Patrick Moore]

Also, the Cazaderos have small knobs; this will certainly affect rolling resistance.

[Philip Barrett]

Car & motorcycle racers talk about unsprung weight, in other words the weight of the components that the suspension system does not act upon (rims, tires, brake calipers etc.). Keeping this number to a minimum is critical to building a competitive vehicle as this directly effects acceleration & braking performance and cannot be manipulated by suspension design. This is consistent with Newton's 2nd law which states that Acceleration = Mass/Force.

To my mind, in hard tail/front bicycle design the tire & tube are that suspended component, everything after them is "sprung" by the sidewall stiffness & tire pressure (in racing design, tires parameters take priority over the rest of the unsprung parts too) so as Patrick Moore says, they are the most critical part of the acceleration equation (not the only by any means, but the foundation).

Ignoring the friction component for now and looking at the numbers, a Rene Hearse tire set is 1/2 pound lighter than the Cazaderos (125 grams per wheel). This doesn't sound like too significant difference on a 30lb bike EXCEPT as above, this is unsprung weight which most significantly affects acceleration. Put friction back into the equation (the coefficient of that between the tire and the pavement is your grip) and consider a tires viscosity (how sticky it is), deformability (how much the sidewalls absorb your energy input versus transmitting it directly to the road) and hysteresis (the speed at which the deformation of the tire returns to it's normal shape after deformation) and I think you have most of your answer in tire science? The Rene Hearse is lighter, most likely less susceptible to deformation, has faster hysteresis and a tread pattern that offers more grip.

As a note, another 1/3rd or so of a pound can also be saved in switching to lightweight tubes (which are also susceptible to all the same parameters as the tires).

Now this sounds like a lot of semantics, bicycles are light and the forces involved (our legs) are weak compared to ICE technology. But it's exactly because those forces are so weak that small gains in tire performance become significant. Your tires are actually fairly closely matched, quality brands, if you move into the word of low end Kendas or similar, the savings can be in the 2 to 4lb range for a set!

Bear in mind, I'm merely a dilettante at this, the physicists amongst you may well be correcting me on the above. But the theory can be tested at least in perception by swapping the tires & tubes between your bikes?

[Patrick Moore]

I think I understand the concept of unsprung weight, but I thought that it affected handling, ie lateral forces and their control, and not acceleration; wouldn't a vehicle with a very great unsprung weight accelerate as fast on a horizontal and straight surface as one with very little unsprung weight? I am thinking for example of a bike and rider with suspension seatpost and stem, compared to a bike and rider with suspended wheels. Would the latter system have an advantage in simple straight line acceleration?

Not that this is not (double negative!!) an interesting conversation!

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[Philip Barrett]

On Wednesday, April 7, 2021 at 11:11:44 AM UTC-5 Patrick Moore wrote:

I think I understand the concept of unsprung weight, but I thought that it affected handling, ie lateral forces and their control, and not acceleration; wouldn't a vehicle with a very great unsprung weight accelerate as fast on a horizontal and straight surface as one with very little unsprung weight? I am thinking for example of a bike and rider with suspension seatpost and stem, compared to a bike and rider with suspended wheels. Would the latter system have an advantage in simple straight line acceleration?

Not that this is not (double negative!!) an interesting conversation!

Yes, lots more to unsprung weight than just acceleration & my theory in bicycles is loose as presumably part of the tire would have to be considered as sprung depending on the construction? Also, the fork, chainstays & the frame overall will flex (that lateral stiffness all the bike reviews mention) so we should take that into consideration too. Especially as Rivendell's penchant for steel (a fairly flexible material) and long chainstays may impact that more than other designs. The math gets PhD hard at this point but in the case of the OP the Salsa looks to be of a similar geometry to the SH so we might be closer to apples to apples there at least.

 For sprung weight the suspension (in whatever form) will absorb some of the input force as deflection, binding/friction, shock aeration, heat etc. so there is a disadvantage. The lighter the components acted upon by that remaining force, the faster they can transmit that into acceleration. Many modern race series allow some form of suspension lockout to aid quicker starts ("holeshot" devices), a little Googling has informed me that some MTB systems offer a similar process. 

The overarching conclusion here may also be that no suspension is better than cheap or badly set up suspension.

[lconley]

It is rotating weight that matters for acceleration. I could bore you with a bunch of physics equations or you could Google "rotational inertia" if you are interested - the rotational inertia is a function of the square of the distance from the weight to the axis of rotation (lightweight derailleur jockey wheels don't count for much more that non-rotating weight because of the small radius). Drag racers know it (look at the teeny tiny front wheels and tires on all manner of rear drive drag cars), autocrossers know it (serious autocrossers use the lightest wheels they can find), automobile manufacturers know it (the one commonality of the stock tires that come as standard equipment on most cars is that they are lightweight - better mpg on the EPA test results). Look at a Formula 1 or other racing car's wheels - are they relatively big or small diameter compared to the wheels you see on street cars? - they are small  - they use the smallest wheel they can for the brakes that they use and that the rules allow   - for a given tire diameter, the wheel-tire combination that gives the lightest weight uses the smallest wheel (talking automobile tires here). If you have ever ridden a Bike Friday or similar small wheeled bike, you know how "zippy" they are - smaller, lighter weight wheels.

Taking 1 lb. off your wheels helps more than taking 1 lb. off your frame (or dumping 1 lb. out of your water bottle).

Laing

Delray Beach FL

[Philip Barrett]

Adam, have you weighed the wheelsets?

[ascpgh]

The comparison to motor vehicle unsprung weight may parallel bicyclists' focus on wheel and tire weight but available horsepower of even weak engines makes all but the most competitive motor vehicles seem sloppy with their unsprung, wheel and tire weights comparatively. 

You on your bike won't make enough horsepower to overcome much weight increases by anything distant from the hub. The greatest improvement to angular acceleration (increasing a wheels rotating speed) will be at the greatest distance from the axle; tires/tubes. Smaller changes in weight net larger feelings of better response at that distant component, tubes come in second, rim choice third. Spokes are next and the hubs may be eclipsed by having lighter pedals. Light wheels, on the unsprung weight topic, do seem to ride nicer because they don't require such impact to move up and down over surface irregularities. Heavy rimmed wheels cancel benefits of light tires in my very subjective experiments. 

Total weight of a bike can affect what you've felt as a difference between your two bikes but the fastest way to improve the ride of a bike to me is a wheel made of a light rim/spokes and a light tire. I ruined the young woman we mentor by loaning her my Rambouillet's PJW-made wheels for a long ride. Velocity Synergy rims with 36°, straight gauge spokes, XT hubs and RH Stampede Pass ELs and light tubes. She is having me work up a price for a set of responsibly lighter wheels with a dyno hub front as a result. 

Some strategies in automotive design to reduce unsprung weight are elegant. The Alfa Romeo Giulia GT 1600 Junior De Dion rear suspension removed the brake disc from the unsprung weight by putting them on the other ends of the half shafts, on either side of the differential. that carried on in their designs for decades. 

Andy Cheatham

Pittsburgh

[Philip Barrett]

So we're back to #1 - light tires & tubes. Followed by #2 - light rims & spokes.

No one else want to chime in on friction? How much slip does say, a semi-knobby tire like the Cazadero allow compared to the RH & is that noticeable when accelerating?

Inboard brakes are a thing of beauty & danger, I believe the Lotus 72 F1 car may have been the 1st to implement. The problem has been the load on the half shaft which, according to the inquest, the associated failure of that was the cause of Rindt's fatal crash in that car.

[David Person]

Adam, good choice on the Barlow Pass tires.  I ran a set on my Hillborne for 5 years till the rear wore out this past fall.  I switched to Snoqualmie Pass tires for a bit more cush.  I have them mounted on Dyad rims and they measure 41mm wide, where the BPs measured 35mm, if I remember.  Tried a pair of Oracle Ridge but they were a bit on the large size.  They would fit but hard to get the rear wheel on and off.  In my opinion, the Snoqualmies ride/handle/respond very much like the Barlows with a bit more tire volume to soak up the bumps.  Not an answer to the exact question you were asking, but wanted to congratulate you on your tire choice and give my two cents.

[Adam]

Thanks for all the thoughts.

Someone asked if I'd weighed the wheels, unfortunately I don't have a scale.

Maybe I'll swap the tires sometime. Not really my end goal, but would help me understand what's going on.

Just curious if anyone on here has ever swapped from a heavier wheelset to a lighter one on one of the burlier Rivs?

There's a somewhat absurd aspect to my curiosity, since I bought the Hillborne to have something livelier than the Marrakesh, but now the ride of the Hillborne makes it hard to use the Marrakesh. I intended to keep the Hillborne on road and use the Marrakesh for gravel and light singletrack but have been taking the Hillborne for those rides too. The only real "problem" is that I've banged my fenders up a bit and would rather be using the bosco bar and knobby tires of the Marrakesh off-road.

Anyway, regardless of practical stuff, it's good to understand the effects of different wheels.

[Ben Miller]

Wheel weight vis a vis moment of inertia matters almost nothing to the modern bicycle. Based upon normal human power outputs (100 W), cruising speeds (6 m/s), and wheel sizes (.35 m) , a bicycle's wheel based upon moment of interia alone comes up to speed in fractions of a second. The difference of a 2 kg wheelset and 1 kg wheelset is a factor of 2 difference in this time, but it happens so quickly this is not the phenomenon determining the overall acceleration of the bike  (.025 s for a 1 kg  vs .05 s for 2 kg)*. This is opposed to the many seconds it will take the full bike plus rider to get up to that speed (84.6 s with a 80 kg rider and 10 kg bike). Rolling resistance will play a much larger roll than the moment of inertia of wheel in determining acceleration (which I have left out).

Go ahead and put you bike in the bike stand. A quarter turn with moderate arm strength will get the will up to speed in a fraction of a second. And the mechanics of your arms is producing a lot less power than you can sitting on a bike. Doubling you wheel weight will double this time, but because it is so small to begin with the overall effect on the bicycle is negligible. 

If it were true that it effected the bike, than as Laing pointed out, bike racers would want the smallest possible wheels (as the radius of wheel effects the moment of inertia much more than the weight). The TdF riders would be looking to get the smallest wheels possible, which they are not. 

Much like how angular momentum is not really effecting the handling of the bike, it is also not effecting the acceleration of the bike. This is because the weight of the wheelset is so small compared to the weight of the bike plus rider. 

Ben

*If this seems too fast, it's because it is. But I didn't want to solve the PDE for non-constant power output, and the human body definitely can not produce peak power from a dead start. But even with power curve it will happen very fast. If I recall my math correctly a linear curve will double the time, putting it the range of a tenth of a second which is starting to seem reasonable. 

[Thomas Lawn]

The tires are definitely going to be the biggest factor here for sure. Rotational inertia or whatever people wan to call it matters a lot less than previously believed, but there are still advantages to nice and/or lightweight wheels. I'd hate to recommend buying a $1000 wheelset when changing the tires would have the desired effect, so my recommendation would be to put those Barlow Pass tires on the Marrakesh (assuming both are the same diameter) and see if it feels better. You can equalize the weight with water bottles if you want to make your test more controlled as well. There will still be geometry and likely fit differences that cannot be controlled for, but that's okay—having two bikes that are exactly the same kind of defeats the purpose of having two bikes.

I'd be extremely surprised if the Barlow Pass tires aren't significantly faster than the Cazaderos given that they're smaller, lighter, and a good deal more supple. You might be able to get a similar experience with a larger Rene Herse tire (maybe try their new knobbies) on the Marrakesh to keep the tire volume/knobby capability without making it feel like a dog in comparison. After all, even expensive tires are way more affordable than new wheels.

Cheers,

Thomas in Portland

[Thomas Lawn]

Ignoring the friction component for now and looking at the numbers, a Rene Hearse tire set is 1/2 pound lighter than the Cazaderos (125 grams per wheel). This doesn't sound like too significant difference on a 30lb bike EXCEPT as above, this is unsprung weight which most significantly affects acceleration. Put friction back into the equation (the coefficient of that between the tire and the pavement is your grip) and consider a tires viscosity (how sticky it is), deformability (how much the sidewalls absorb your energy input versus transmitting it directly to the road) and hysteresis (the speed at which the deformation of the tire returns to it's normal shape after deformation) and I think you have most of your answer in tire science? The Rene Hearse is lighter, most likely less susceptible to deformation, has faster hysteresis and a tread pattern that offers more grip.

...

But the theory can be tested at least in perception by swapping the tires & tubes between your bikes?

 

I'm neither a physicist nor an engineer, but I'd suggest the tires are, in fact, the only significant source of suspension on a rigid bicycle and shouldn't count as unspung weight. The Rene Herse tires, at least in theory, should be more susceptible to deformation and therefore require less energy to deflect over road texture.

Rene Herse does do some fairly good real-world tire testing and they've essentially reached the conclusion that softer tire that deforms more easily, coupled with a higher air volume, is faster until the drag created by a wider tire equalizes that effect (this will happen at a certain speed depending on tire width...). Even if we're all wrong, I'm pretty sure the "swap tires to neutralize the easiest/cheapest variable" path is the best one.

[Philip Barrett]

I read through much of RH's tire research and was so impressed I ordered a piar of their 650b Loup Loup Pass.