Large-platform SPDs vs. small ones
I’ve been riding large platform clipless pedals (DX resin cage etc) since I started riding clipless way back when. I quite like them, however push is coming to shove and I’ve had enough sand kicked in my face etc etc, to cut a long story short, I have found myself needing to shed some weight from my bike. The weight difference between DX clip ins and say XT pedals is pretty enormous in the scheme of things.
Pedals must be rotating weight. Should this be as noticeable as lighter wheels and tyres?
With stiff soled race shoes, do you think I’ll miss the extra platform? From memory running regular spd size pedals doesn’t make you any less aggressive on the trail.
Chris is a strong, skilled rider who will actually benefit from optimizing his equipment. We he visited from Australiea this year, we did a few clinics together, then he stomped me on an XC ride — on my heavy bike, at altitude.
What an awesomely nerdy question. I’d love to rock some diagrams, but it’s late, and I want to get some dinner.
Benefit of smaller pedals #1 – Less rotating weight
Pedals do rotate, and rotating weight has a much greater impact than non-rotating weight.
Simplified, but fun:
– Pedal radius = 175 millimeters = 7 inches
– Wheel radius = 330 millimeters = 13 inches
So … reducing wheel weight has almost twice (1.85x) the benefit of reducing pedal weight.
– DX platform pedals = 568 grams per pair
– Deore XT pedals = 350 grams per pair
Taking 218 grams off your pedals is like taking 118 grams off your wheels. That’s four ounces, or 1/4 pound, for you Americans. Not earth shattering, but noticeable.
But! This 1:1.85 ratio only applies when your cranks and wheels are rotating at the same rate, say when you’re buzzing your 34×34 up a long climb. If you’re in a lower gear, light pedals make more of a difference. If you’re in a higher gear, lighter pedals make less of a difference.
Braaap! is still spelled Braaap! in Australia.
Benefit of smaller pedals #2 – More clearance
Missing a rock by a millimeter is still a miss!
I used to drive a forklift in a warehouse that was laid out with one inch of extra room for the forklift and a pallette. Talk about stress for a 17-year-old. My boss used to say, “An inch is as good as a mile.”
Benefit of larger pedals #1 – More support
If your shoes are soft or worn out, small pedals will make your feet sore. Good shoes nullify this.
Benefit of larger pedals #2 – Easier to get your feet back on
Good skills nullify this.
The bottom line
If you want relatively high-impact weight savings at a relatively low cost, try the smaller pedals.
Chris, since I know you’re a strong, skilled XC racer, I say rock the XTs.
PS: A few years ago I wrote and illustrated an article for VeloNews about the effect of heavier, grippier tires over the course of a trail ride. This piece was way over the top, even for that magazine, and it never got published. I’ll dig it up for you guys.
Definately out of topic here, but any news about the BMX book
It looks so good I can’t wait.
Would the pedal weight being non-suspended and at the low center of the bike mean it wold be even less of a factor?
Speaking of rotating weight, is there any truth to more weight on the outside of the wheel benefits stability in the air due to a gyroscopic type effect?
Being quite the tire tramp, I would love to nerd over the heavier/grippier tire article. Lately I’ve been of the mind that a better rolling tire (less rolling resistance) is of a far greater benefit than a lighter weight tire. The challenge is finding the magic elixir of grip/rolling resistance/toughness/weight, all for my riding style and riding conditions. Not too much to ask 😉
This needs to be a series of posts. Totally awesomely nerdy. For now:
Pedals being low is good.
Your pedals are sprung weight, which is also good. The more weight you have on your un-sprung components — wheels and swingarms — the less responsive your suspension.
Heavier rims/tires make your bike more stable in the air, as long as you keep your bars straight. When you turn the bars, greater gyroscopic force act on the frame, making it more likely to rotate to the side (that’s what laying a flat tabletop is all about). I guess, heavier wheels give you more control in the air?
Dude: I found the nerdy heavy/light tire article. Whoa. Even I have to concentrate to follow it … No wonder VeloNews pulled the plug!
One thing to keep in mind if you are going to worrying about pedal weight is you might want to check and see if you have heavy shoes. Your shoes rotate just as much as your pedals.
Actually, the effect of rotating mass is proportional to relative radius and gear ratio squared. In your example, taking 218 grams off of your pedals would have the same effect on rotating mass as removing 218/1.85^2 = 64 grams from your tires or rims (with 1:1 gearing). This is because kinetic energy is proportional to the square of velocity, and rotating mass shows up as a result of the additional kinetic energy stored in rotating parts.
Of course, those extra 218 grams still need to be lifted to the top of every hill and accelerated out of every corner.
Lee, you are awesome! How cool is it to be too nerdy for VeloNews?!
If you like to stick a foot out in a corner then a larger grippier platform is a benefit because you can slam your foot back on and start pedaling without having to feel for the clip in point.
Oops I forgot.
If you want to save weight for racing don’t go for the XT pedals, get some Eggbeaters.
Much lighter, 266g for Eggbeater SL vs 355g for the XT’s at the same price, if you are lightweight and have moneey to burn there are the Ti options. Also four sided clip in, better mud shedding, and an easier release tension together with no upward release (so you don’t pull out sprinting).
You sent me scurrying to http://hyperphysics.phy-astr.gsu.edu/hbase/hph.html
It looks like you took the kinetic energy approach:
*** Rotational Kinetic Energy = 1/2 x rotational inertia x angular velocity squared
Kinetic energy seems important when it comes to a bike that is already hauling mail.
But to determine the amount of power it takes to accelerate a wheel or crankset, don’t we just look at rotational inertia? Rotational inertia is mass x rotating radius squared. In the post above, I forgot to square the radius. That square means mass on wheels has a MUCH larger effect than mass on cranks.
Re-doing the math correctly (I think) says taking 218 grams off your pedals has the same effect as taking 61 grams off your wheels.
Almost the same result you got.
Learning is fun. Am I on track?
OK guys, get your geek on:
Never knew anyone else thought about those two sets of pedals. I’ve always used the DX platforms for general light freeride use and the XT’s for the Downieville Classic (mostly for clearance). The weight savings is just psychological for me on my 36lb Reign.
Will had one correction, I’m going to take it a step further (at least).
Moment of Intertia (or so-called “rotating mass” is the integral of each bit of mass times each respective radius squared. For a theoretical zero-width hoop spinning about it’s center axis, this works out to
MOI = m*r^2
for a disc of uniform material, it’s
MOI = 1/2 m*r^2
So, if one were to estimate the MOI of a wheel, you might estimate the hub as just a point (i.e. effectively no MOI) since the radius is so small. (I guess this logic fails if anyone remembers Seismic hubs ;-D ). The spokes might be estimated to contribute using that disc formula (a spinning bar has the same MOI as if you shaped said bar into a disc), and the rim/tire is sorta close to a hoop.
Point being, a lighter rim or moreso a tire) helps waaay more with MOI than a lighter hub. That said, most people won’t suffer a performance decrease by switching to a lighter up (‘cept maybe wallet performance), but switching to a lighter tire will likely hurt.
I think the aforementioned case that pedals are usually spinning 2-3x slower than the wheel is also an important one.
OTOH, this is also a good argument to switch to shorter cranks!
Right on. Thanks Scott.
On some trail, somewhere: 130mm cranks, bare ti pedal spindles drilled directly into your midsoles, and 20″ 20-spoke wheels with 18C tires are the hot tip.
If the high school cheerleaders could only see us now …