When it comes to cycling, we all want to find every possible way to maximise performance. Aero gains, short cranks, 120g/hr of carbs, all of these have been pursued with the aim of boosting performance by professionals and amateurs alike. But in recent years, a new performance gain has come into focus again for the supposed performance gains to be found by taking advantage of it. I’m talking about stack height. In your pedal system.
What is stack height and why does it matter?
Zero stack height is theoretical, as you have to have a pedal spindle on which to stand, but minimising stack height to get as close as possible to nowt, by making everything between it and your foot closer to the point you’re standing on, is the game we’re discussing today, and it’s got some relatively unknown upsides, if you’re willing to play it.
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The reason it’s come to the forefront is because of its link to performance and power transfer and you’ll have heard it referred to if you’ve checked the reviews on any competitive pedal or shoe system.
The Italian clothing brand Q36.5 is a big proponent of low stack height in pedals. They make shoes, among other items, and states that the ‘latest studies of cycling biomechanics’ found that a 1mm reduction in stack height results in up to a 1% increase in power. Now, this study is not actually referenced, and I could not find it after trawling various research article sites and journals. It may be a proprietary study.
But the principle is well researched in general and has some solid foundations. Power is not created, only transferred, to summarise Einstein. When we put power through the pedals, depending on where we measure it, the reading changes. For example, hub based measurements are often lower than those that are pedal based, as power is lost via moving the chainring, chain, cassette and such. Some powers goes into noise, even heat in some systems, and other transfers of energy.
On a far smaller scale, like in this example, a higher stack height means a fraction more material that experiences load and minute deformation. So in theory, reducing that stack height means less material to absorb your energy and power.
Are there other benefits?
Power transfer is not the only benefit that can come from reduced stack height. There is a very small benefit in that less material means less weight. An added bonus here is that since the pedal is a rotating component of the bike, it has a greater effect on weight due to the rotational mass at the end of a crank. Again, this is minor, but it is still an advantage.
The other benefit is one that actually has far more significance with the developments and trends of riders switching to shorter cranks. As riders switch from 170mm to 165mm or less, this has an impact on rider biomechanics. Since the distance from the saddle to the bottom of the pedal stroke is decreased, the saddle height needs to be increased so as to maintain the same maximum knee extension angle. In upping the saddle height, you push the rider up into the air, increasing the total height of the rider on the bike, and increasing the drag experienced by a small amount. Only a marginal increase in CdA, but an increase nonetheless.
So why does a low stack height help? Well it has the opposite effect on saddle height, as in a lower shoe and pedal stack height means that the saddle can be dropped down so as to maintain the knee extension angle.
There is an additional aerodynamic benefit as well, in that by reducing the total stack height of the pedal, cleat and shoe you also reduce the total frontal area by a small amount. Often these low stack options are also slightly more aerodynamically efficient by virtue of providing less disruption across the bottom of the shoe. Speedplay and Ekoï in particular offer aerodynamic savings from the actual cleat interface as a selling point. More on them later.
How can we reduce our stack height?
At present, the most common platform for road cycling pedals are three-bolt cleat pattern systems covering Look Keo, Shimano SPD-SL, and Time pedals. These all have varying stack heights. Shimano claims the Dura Ace PD-R9100 pedal and cleat system has a total stack height of 14.6mm, while the new Look Keo Blade is 14.8mm, made up of 8.5mm pedal and 6.3mm cleat.
Speedplay offers another option, with some shoe brands like Nimbl coming in Speedplay specific 4-bolt options which lower the stack height to 8.5mm. However, many shoes only come with 3-bolt fitting options meaning that Speedplay pedals require an adaptor which brings the stack back up to 11.5mm.
New to the market though is the Q36.5 x SRM pedal system, which we reported on earlier. This is a new clip-in pedal similar in engagement to 3-bolt systems, but the stack height has been dramatically decreased. This system uses a 2-bolt cleat fitting system, and everything has been slimmed down massively so that the pedal and cleat make up just 9.7mm of stack height from the pedal axle. Add to that the 4.4mm cleat mounting platform, and sole of the Q36.5 shoes and you have a total stack (without insole) of just 14.1mm, less than a full Dura-Ace pedal without the shoe.
It’s hard to compare this directly to Nimbl, as the brand claims a sub-2mm sole height, but that does not necessarily correlate with where the cleats are mounted due to mounting hardware. Q36.5 for example acknowledges that the shoe sole is 2.2mm for most of the carbon, but 4.4mm where the mounting hardware for cleats are. You can also get aftermarket bolt holes drilled into your shoes from companies like Wattshop who have partnered with Detail Bike Tech to perform the drilling and fitting. This will drop the Speedplay stack from 11.5mm to 8.5mm.
An honourable mention also goes to Ekoï as well for the PW8 pedal shoe system with 8mm of stack height, available to the pros before it was banned by the UCI. Although replacing cleats is not really an option here as the shoe is partially the cleat.
Low stack height doesn’t have to mean you can’t have proper foot support. An insole insert can be really important for foot stability and power transfer, and this will of course take up some space, adding to stack height. Still, ideally you want a specialist, custom fitted type that is as thin as possible under the forefoot, but provides ample support for the foot and any arch that you might have.
Are there any downsides?
When it comes to lowering stack height in general, there are not any direct negatives associated with doing this. As long as you adjust your saddle position accordingly you will keep everything else the same biomechanically. Some brands claim that lower stack height improves stability, but this is a bit of a over reach unless you go from pedalling in platform boots to thin carbon soled shoes.
There are some limiting considerations of course in terms of how you achieve that desirable low stack. Thin carbon soles that are stiff are also expensive. Stiff, light, cheap, choose two. So there is an outlay to be had from purchasing low stack height shoes. Then there are the prices of the pedal systems. At present, Speedplay and SRM are the lowest stack options, but both are more expensive than Look and Shimano’s ranges. Speedplay starts at £99.99, while Shimano starts at £59.99 and Look at £50. SRM, based on the system pricing with Q36.5 are £550, but you need to purchase the shoes as well so perhaps £1,100 in total. That’s a lot of money for a few millimetres. Keep in mind that for the low stack Speedplay option you need the Nimbl Pro shoes at £399 starting price in the UK for 4-bolt. Wattshop’s 4-bolt adjustment less, but that is also a £160 cost.
You should also consider biomechanics. Personally for me, I like Speedplay with the adjustable and customisable float, but the cleats are more expensive than Shimano or Look options and need replacing a similar amount. For some users Shimano or Look may provide a better option biomechanically, which is really the consideration that should come first when it comes to your key contact points, and available performance gains. Speedplay and SRM also have a large contact surface area, which can give a more stable sensation when pedalling as pressure is spread over a wider area.
CW says
Although lowering stack height does have definitive advantages, including improved aerodynamics and reduced weight, as well as the potential power improvement, it does tend to come at a substantial financial cost. The question remains then, is reducing it worthwhile for the majority of us? Probably not in all honestly.
Testing super low stack heights versus higher ones offers only a small discernible difference for me, while the error margin for power meters is frankly large enough to make measuring any available power differences nigh on impossible anyway. For those seeking every gain there is to be had, in the pro ranks, then of course it may make sense, but it is certainly not one of the most cost effective savings, with even the lowest cost options probably standing you at £1000 in expenditure, all in.