Mountain Bike Reviews Forum banner
1 - 20 of 83 Posts

·
Registered
Joined
·
102 Posts
Discussion Starter · #1 ·
Hi all,

I read on a site that wheels are the first place to look to reduce weight. This site mentioned that the rotational weight of wheels make a big difference on acceleration, turns etc. Any thoughts why this is? I am trying to get as light a bike as reasonably possible for my wife so if this is the best place to do achieve that I would love to know what kind of wheels to get for her to help.

Regards,

YD
 

·
Trail rider and racer
Joined
·
4,691 Posts
Hey,

Here is my simple view of it:

Spinning the cranks drives the rear wheel which in turn gets the front wheel moving too. When you pedal a bike you have to use energy in getting the wheels to turn.

You use more energy to move a heavier wheelset (Tyres, Tubes, Rim tape, Rim, nipples, spokes, hubs) per revolution then you would with a lighter wheelset.

I would rather have to exert energy upon a wheelset that weighed 1600g then on one that weighed 2600g.

Very very simple, non scientific answer. Hope that helps.

Trevor!
 

·
Registered
Joined
·
102 Posts
Discussion Starter · #3 ·
i guess i was thinking wheels were pretty close in weight. i am going to make a bold statement that i am pretty sure lighter wheels cost more money! ah, this is going to hurt trying to lighten up my wife's bike. maybe the extra weight would help her get in better shape should be my selling point?!
 

·
Trail rider and racer
Joined
·
4,691 Posts
yellowwducky said:
i guess i was thinking wheels were pretty close in weight. i am going to make a bold statement that i am pretty sure lighter wheels cost more money! ah, this is going to hurt trying to lighten up my wife's bike. maybe the extra weight would help her get in better shape should be my selling point?!
Your average 1650g wheelset like am classic hubs mavic rims supercomp spokes are very realistically priced!

Wheelsets around the 1350g wheelset are over priced and not worth it IMHO for MTB unless your racing elite and need every little saving...

Cheapest way to save weight:
Tubes
Tyres
Rims
Spokes
Rim Tape
Grips
Saddle (Off ebay)
Handle bars (Off ebay) You can get carbon Hyperlight flat bars for $40 from jenson!

Trevor!
 

·
Registered
Joined
·
102 Posts
Discussion Starter · #5 ·
so looks like you gotta go tubeless tire as well then. oh the budget is getting creamed! now that i look at it though, you are right, lot of weight to be saved on wheels - the diff between 2600g and 1600g is 2.2 pounds! that is almost 10% of the total weight. thanks for the advice Trevor!!

yd
 

·
Registered
Joined
·
601 Posts
It is a fact that rotational weight on wheels influences acceleration of the bike but in case of MTB this is greatly exaggarated within this community. Commonly quoted is that gram saved on wheels is worth 2 grams. Here are the actual numbers where a gram saved is worth this much on a particular part (and only for cases of acceleration):

hub 1.0007
spokes 1.2043
rim 1.7006
tube 1.8723
tire 1.8723

wheel overall

front 1.563
rear 1.467

Most important thing to note is that this is only the case for acceleration which is not an important performance parameter in MTB as rarely you are accelerating and most of the time going at constant speed grinding up a hill or on a flat. My point is that saving rotational as opposed to static weight has only very small theoretical and no actual real world advantages. In line with that I'd advise you to save weight where it gives you the best gram saved/$$ ratio regardless of weather its rotational or static mass.
 

·
Registered
Joined
·
102 Posts
Discussion Starter · #7 ·
Thanks for that BOJ.

Looks like there is some physics at work relating to distance from the axis of rotation. Cool. Regardless, I am going to try and shave as much off as reasonably possible from my wife's bike so that she can have a good experience and will continue to want to ride.

yd
 
G

·
acceleration

to the person who claims you are rarely accelerating in mtb then he should be able
to ride a single speed, he doesnt need gears. when you slow down or speed up at all
that is an acceleration!
 

·
Registered
Joined
·
192 Posts
"constant speed" ?!?

You must do your "Mountain Biking" in a stadium. Out on the trail your speed is constantly changing, not remaining constant. I'm in FLORIDA, one of the flatest places on earth, and I'll see speeds of anywhere from 5 to 22 mph when riding mixed single-track/fireroad/pavement routes. For people with serious elevation changes, it's even worse. You never slow down for corners? Your trails are always perfectly flat? You never sprint to pass someone? What planet is this that you're talking about?

It sounds like your analysis would be more suited to someone setting the World Hour Record in a Velodrome, and some, but not all, Triathalon events. Even normal road Crits involve huge amounts of acceleration and braking on a tight course.

There's a reason why top pro riders dis their sponsors and spend $3000+ of their own cash on custom-built ADA wheels, but put up with "off-the-shelf" sponsored hardware on the rest of their bike.
 

·
Registered
Joined
·
192 Posts
Wheels rotate at twice the ground speed of the bike.

The mass at the outer edge of a wheel travels at twice the speed of the bike over the ground. The reason for this is that it is traveling through two directions. The bike is only moving forward in a (theoretically) straight path. The mass of the wheel is rotating, so it's moving not only forward but also must travel an equal amount up-n-down as it travels through a circular path.

So to accelerate the mass of your frame to 20 mph, you have to accelerate the wheel to 40mph at the outer edge. Obviously, it takes more energy to accelerate the mass of your tire or rim to nearly 40mph than it does to accelerate the same amount of mass in or on the frame to only 20 mph.

Tires are the best place to look for weight savings for this reason.
 

·
Registered
Joined
·
601 Posts
I was simply stating that case for rotational weight is WAY overrated in case of MTB.

I am aware of acceleration in MTB being present (and what acceleration is to the other poster) at tight twisty singletrack or any place that offers sudden possibility of pedalling hard, however in vast vast majority of races I've raced and places I've ridden it is a matter of grinding up a hill or flats with no MEANINGFUL accelerating taking place. What I mean by meaningful is acceleration that would justify distinction between rotational and static weight. Human motor does not supply enough power to give huge accelerations and hence the effect of rotational mass is really not significant to be worshiped as it is.

The fact that you ride in Florida where it is all flat makes for a more twisty/tight singletrack makes for more pronounced acceleration when riding. However even in Florida if you have 500g lighter wheelset than your buddy and he has a 500g lighter frame than you I assure you the difference in acceleration (about 250g worth) between you two thanks to the rotational effect is not worth consideration. Across the distance of a 1.5 hour race, even in Florida, the roatational effect of your lighter wheels would be worth 1-3 seconds advantage maybe. Far more important parameters would be your power output, overall mass (bike + rider), rolling resistance of the tires etc.

On the other hand if you had a 500g lighter wheelset and your buddy had 750 g lighter frame, your bikes would accelerate the same however he would have actual weight advantage of 250 g (as opposed to percieved with rotational mass) which would be easily worth more than 1-3 seconds in a race in saved rolling resistance or any hill climbing.

My point is simply that acceleration is a negligable factor in MTB and hence rotational effect of wheels shouldn't be given such great publiity as it is given. When it comes to saving weight save it where ever your $$ best allow you to, wheels or frame alike.
 

·
Tonight we ride.
Joined
·
769 Posts
I'm not a weight weenie...

...But based on my own experience I completely disagree. I notice a huge difference switching between two tires of the same model/tread/durometer, one kevlar, the other steel bead with heavier casing. That's a difference of 800g or so, and I feel it for the entire ride. Yet I don't discern any difference in how fast I'm going between a full water bottle and an empty one, or my 100oz Camelbak when half full or at maximum capacity.

I don't have lap times to back that up, but I'm sure someone has done similar experiments. I'm betting a bike with a two pound weight attached to the frame but light tires will be considerably faster than a bike with no weight on the frame and a pound of weight on each wheel, at least on typical singletrack. I'm not sure what the results would be were the times recorded on a perfectly flat paved road, but I bet that even the smoothest spinners in the world still have slight deceleration and acceleration with each pedal stroke.

Every pedal stroke is an acceleration, followed by a slight deceleration, over and over. I can easily notice a difference of just 100g when I swap tires, but I can't tell whether I'm running my DH or XC BB or not.

I don't think rotational weight is overrated at all.
 

·
Registered
Joined
·
102 Posts
Discussion Starter · #13 ·
I think i am going to just err on the side of shaving off as much as possible - esp for my wife. 2 pounds can be saved on wheels so that is a big start. another pound on shock maybe if i can find something that will work for her yet be lighter and still reliable. given her light weight, I am sure everything will support her; especially with her conservative riding style.

as per the discussion at hand, I can see both sides. it would seem to me that acceleration would definitely be impacted in a big way when you start up but I am not that convinced that once you are pedalling away on the flats that it would really matter - yes each stroke is an accel/decel but since you are only maintaining a speed rather than trying to build a speed up I would think the effect would be minimal then. so for a mostly flats rider the accel/decel thing not that important but for someone with say a straight into technical and then accel over and over again would matter. around here our trails are ups and downs which wouldnt be too tough to maintain speed over - except for fact that the bottoms are always technical so you always are at the slowest speed in the dip and have to get that damn bike up the other side so weight in general and the spinning effect i would think would be very important for my wife and i.

my 2 cents - i am not a physics major!
 

·
Trail rider and racer
Joined
·
4,691 Posts
fonseca said:
...But based on my own experience I completely disagree. I notice a huge difference switching between two tires of the same model/tread/durometer, one kevlar, the other steel bead with heavier casing. That's a difference of 800g or so, and I feel it for the entire ride. Yet I don't discern any difference in how fast I'm going between a full water bottle and an empty one, or my 100oz Camelbak when half full or at maximum capacity.[...]

Every pedal stroke is an acceleration, followed by a slight deceleration, over and over. I can easily notice a difference of just 100g when I swap tires, but I can't tell whether I'm running my DH or XC BB or not.

I don't think rotational weight is overrated at all.
Rotational weight is not overrated at all - I agree 100%

I totally understand what you mean by:
Yet I don't discern any difference in how fast I'm going between a full water bottle and an empty one, or my 100oz Camelbak when half full or at maximum capacity.

A water bottle adds around 770-800g of weight to your bike as the water weighs 750g if it is 750mL an the bottle is some more yet I would never really notice a big difference in how my bike rides. It still feels flickable etc.

I think one of the reasons why we don't notice the added or lost weight when we have consumed the water is because that weight is nearer the center of our gravity. Yet if it was further away from the bike we would.

So it seems rotational weight is not overrated at all.

Interesting about a water bottle because the only way I can feel a difference is if I lift the bike.

Trevor!
 

·
Registered
Joined
·
601 Posts
While I would question anybody's ability to 'feel' 800g difference on tires or anywhere, rolling resistance is more significant reason why your tires may feel faster.

Rolling resistance is the greatest opposing force in MTB and when changing from one tire to another differing rolling resistance properties (which change from steel to kevlar belted tires) would be the most significant reson a tires would feel faster or not. Lets not forget that you are carrying 800 g less mass and whatever difference in body weight between the runs. To make comparison fairer you should fill your bottle full when switching to lighter tires (though that still leaves difference in rolling resistance of the tires to compensate for). Either way the rotational mass (ie acceleration) would play a part, but not a significant one. Other effects would overshadow it by far.

It would take a lot of dedication to set up a proper experiment. You would need to have same tires and different wheelset, but thats not enough. It would be impossible to compensate for riding technique as no 2 runs in the singletrack even of the same rider are the same. Having just a straight line race would probably be better though that wouldn't prove that there is little acceleration in actual MTB riding. Whats more important is that there is no need to measure it, as the effect of rotational mass is simply proportional to how much a bike accelerates. Show me how much acceleration there is in a MTB race and I'll tell you how much faster you go because of rotational mass. The truth is not much.

Every cyclist does indeed have power on and power off in the pedalling stroke. When the pedal stroke is on, you accelerate and when the stroke is off you decelarate. However while reducing mass helps accelerate faster when the stroke is on, it is actually slowing you down when the stroke is off because the bike loses momentum faster with less mass (ie decelarates faster just as it accelerates faster). That is why there is little dynamic effect at constant bike speed (effect of just slight acceleration/deceleration due to pedalling forces) for change in rotational mass. To judge the effect of how rotational mass changes you have to have the whole bike system accelerating like from 0-20 kph, 10--20 and such. The effect from just pedalling mostly cancels itself out.
 

·
Tonight we ride.
Joined
·
769 Posts
Boj said:
While I would question anybody's ability to 'feel' 800g difference on tires or anywhere, rolling resistance is more significant reason why your tires may feel faster.
I can feel just a 100g difference in tires. Heck, I can easily feel the difference between my DH tubes and my Specialized thin tubes. When I rebuilt my spare wheelset last week I was reaching into a bag of brass and al nipples and selecting the brass ones by weight in my palm. You don't have to believe me. I'm not a gram counter at all, my Titus Switchblade is pushing 30lbs, and that's my primary XC bike.

Most of my riding has lots of short, steep hills, where I am out of the saddle hammering all the way up. That's acceleration/deceleration at it's most apparent. Light tires and rims make a big difference in how easy it is to get to the top.

I don't think rolling resistance is the reason I notice such a difference, as I mentioned above I have two sets of tires with identical tread and durometer. How does a steel bead and thicker sidewalls affect rolling resistance when I'm running them both at the same pressure?

The plus of DH tires and tubes is the gyroscopic effect can carry me over rock gardens and obstacles in a way that light setups don't. Is it all my imagination? I wouldn't be swapping tires all the time if I didn't feel a substantial difference.
 

·
Registered
Joined
·
601 Posts
I'm sorry, no disrespect to you (there are numerous people here who say stuff like that) but you can't feel 100 g difference. If the question was would you be able to feel 100g difference in a blind test I'm sure you'd fail to prove this ability. When you know the bike is 100g lighter its human nature to feel it faster but in a blind setting where you don't know what bike is lighter you can't really tell the difference. If you don't believe me consider this:

Your body weight in a course of a day can vary by up to 2 kg depending on how much you eat/drink and how much you leave behind you. Around 1 kg total change up and down is about standard variation that I was able to find pretty much every day when I poped on the scales (and I only weigh about 65 kg). Across long term this is even more as your body accumulates and loses fat.

So your body weight fluctuates by up to 1000g every day, which easily dwarfs 100g change on the bike. My point is that your body weight is far greater source of mass fluctuation and nobody ever picks a difference in ride there every time they throw a leg over saddle. If you are contending that decisive difference is that weight is rotational, it is still apparent that 1000g would have far geater effect on your acceleration than about 200g worth of apparent rotational weight.

Its not my intention to completely discount feel as a means of assessing performance but it has to be used in conjunction and under thorough critical examination of speedo, stopwatch etc. Often this is not possible (eg stiffness) and even if this is done, it doesn't guarantee accuracy - what power you put for example varies readily but is rarely measured in any experiments on bikes. It is SO difficult to achieve similarity between different runs on a bike yet so many people on this forum regularly claim performance difference using simply their assomometers without considering anything else. ;)

Lighter tires/rims will always climb faster than heavier ones and heavier tires/rims will always be more stable than lighter ones. That's not in contention. I'm simply argueing that inertia effect of the saved rotational mass (the infamous each gram saved is worth 2) does not translate into any actual gains as compared to simply saving equivalent static weight.
 

·
Registered
Joined
·
1,066 Posts
here is some maths for you

simplified :
You're going to do some work going from zero to 30kph
Assume that you + bike weigh 80kg & each wheel weigh 2kg

Linear kinetic energy = 1/2mv^2 = 0.5 *80kg* (8.333m/s)^2
= 2777.8 joules

rotational energy = 1/2(moment of inertia) * angular speed^2
= 1/2(m*r^2) * 22.62 radians/s
= 69 joules (per wheel)
= 138 for 2 wheels
So rotational energy contributes about 5%

What happens if you stick on tyres which weigh 150 grams more your total weight goes up by 0.375% but :
linear kinetic goes up to 2788 (0.375%)
rotational kinetic goes up to 149.3 (8%)
total energy increases by 0.74%

i.e you do disproportinately more work....

i've simplified the moment of inertia assuming all the mass is on the rim.
 

·
Registered
Joined
·
601 Posts
And here's some more...

a = M/[r*(2*m1+2*m2+m3)]

where:

a - acceleration (m/s^2)
M - torque at the back wheel (N*m)
r - radius of the back wheel (m)
m1 - mass of the front wheel (kg)
m2 - mass of the back wheel (kg)
m3 - static mass (rider + rest of the bike, kg)

Also can be expressed as

a =[1/(2*m1+2*m2+m3)]*(M/r)

For Bike 1, 80 kg bike + rider and 2 kg each wheel thats:

a1 = 0.01190476*(M/r)

For bike 2, the same 80 kg bike plus rider (ie frame heavier 300g) but 150g lighter each wheel:

a2 = 0.01194743*(M/r)

Which means that if we use the same input power (torque, M) and same wheel radius (r):

a1/a2 = 0.996428 (meaning that bike 2 accelerates about 0.36% faster, YAY).

Now how much faster is that in a 0-30 kph sprint:

t1/t2 = a2/a1 = 0.996428^-1 = 1.003585

Now 0 - 30 kph takes about, say, 4 seconds in a good sprint on bike 1 (insert any number you find reasonable here):

So thats t2 = 4/1.003585 = 3.9857 seconds

t2 - t1 = 4 - 3.9857 = 0.0143 seconds

So in conclusion, by having 150 g lighter each wheel and 300 g heavier frame you go faster in a 0 - 30 kph sprint by a grand total of (...carry the y, square root, plus grand total equals...) whopping 0.0143 seconds. At 30 kph that means bike B will be ahead by all of 11.9 cms :)
 
1 - 20 of 83 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top