Sunday, August 09, 2020

Assessment of the Stack Heights of 2020 Road Racing "Super" Shoes

Article by Derek Li

Assessment of Stack Heights of Running Shoes

After recently reviewing the Nike Air Zoom Tempo Next% Flyknit, which does not have any published stack numbers from Nike, I did some manual measurements of the midsole/outsole stack from the outside of the shoe. Bearing in mind that the platform is scalloped in both the forefoot and the heel, I measured the stacks from the outside as follows:

Tempo Forefoot 36mm, Heel 46mm for a 10mm drop

Alphafly Forefoot 41mm, Heel 50mm for a 9mm drop

This is of course a considerable deviation from the published stack heights for the Alphafly. Advertised stacks sometimes deviate quite substantially from the measured stack, and ultimately it only matters how the shoe feels when you run in it. 

Due to the scalloped designs of both shoes, it is difficult to get an accurate measurement of what the actual stack is, especially with the glued in insole of both shoes. You can see from the pictures, that the Tempo does have a lower apparent stack than the Alphafly.

With that in mind, I decided to explore a more accurate way of measuring the stack height, one as close to the IAAF official rule standards for the measurements, short of sawing the shoe in half to obtain the numbers. I also took the opportunity to measure the stack heights of a few other shoes I had, partly to use as a reference point, and partly out of curiosity about the veracity of the stack heights of shoes, as declared by manufacturers.

We should at this point bear in mind that for the purposes of legality in World Athletic rules, the stack heights of shoes include the thickness of the insole (aka sockliner), something which I feel has never been universally adopted as standard practice in the declaration of stacks by manufacturers, but which I think is a reasonable approach considering World Athletics’ goal is to limit stack underfoot, rather than to know the correct stack of each shoe.

The shoes I measured were as follows, all in men’s US9.5 / EUR43

Nike Alphafly (0 km)

Nike Tempo Next% (56km)

Nike Vaporfly Next% (97km)

Saucony Endorphin Pro (36km)

Xtep 160X (48km)

Do note that only the Alphafly is brand new, and the other shoes have relatively low mileage as stated above. We do concede that mileage will affect the stack numbers a little bit, but that cannot be helped at this stage. 

Again, the aim is to check the accuracy of the measurement system, and to get an idea of how close to the actual published stacks these shoes are. Anything within 2mm of the actual stack would be considered pretty good, and acceptable considering manufacturing tolerances of both the shoe itself, and the insole/sockliner.


To get the measurements done, I enlisted the help of my good friend Choonwei, who runs a bicycle fitting service in Singapore (check them out at @fitskuul on Instagram). 

Choonwei is Retul University and FIST-certified from the USA, but more importantly, his fitting yu (called Zin) uses equipment that can measure distances between identified points in space even if there is material between the probe and the sensor. 

That makes for a formidable bicycle fitting device. I’ve been a subject of his bicycle fitting before during my triathlon and bicycle time trialling days, and he can basically use the probe and map out the full curvature of your silhouette onto a computer screen in seconds. More importantly, for our purposes, the probe can register a point in space from inside the shoe, which saves me the heartache of sawing the shoe in half.

The first step was to pinpoint the locations to measure, both on the surface of the sockliner, and on the surface of the “floor”. During the time of measurement, the shoe would be completely flush with the surface of the “floor” so there would be no problems with errors in measurements derived from potential space between the outsole and the surface. 

For the purposes of this measurement, we used a stool as the “floor” and ensured a level surface using a spirit level.

I then outlined the silhouettes of the shoe on a taped down piece of A4.

The next step was to identify where to make the measurements and for this we followed the World Athletics rule of 12% / 75% of the internal length of the shoe on the center line. That was straightforward enough and all the points were marked out with stickers on the insole.

Corresponding points were marked out on the A4 which would indicate where the outsole terminated.

The actual measurements were quite straightforward. Put the probe on the spot on the insole, press a button, remove the shoe, put the probe on the mark on the A4 paper, press a button, and the computer generates a number. Replacing the shoe in the exact same spot was easy because we just had to align the shoe back to the outline we have traced on the A4 paper.

The whole process is then repeated for the forefoot, ensuring that for each measurement, we are not depressing the sockliner, and that the outsole is completely flush with the “floor” surface. In the case of all these rockered shoes, it often meant the heel was a little elevated during the measurement of the forefoot stack.

The initial refinement of the process took a while, but once we settled on the methodology, the next couple of shoes were much quicker to measure. All in, the 5 shoes took about an hour to measure.


Nike Alphafly (RTR Initial Video Review)

Forefoot 38m, Heel 42mm

4mm drop

Nike Tempo Next% (Derek’s RTR Review)

Forefoot 38mm, Heel 42mm

4mm drop

Nike Vaporfly Next%  (RTR Review)

Forefoot 26mm, Heel 35mm 

9mm drop

Saucony Endorphin Pro  (RTR Review)

Forefoot 29mm, Heel 35mm

6mm drop

Xtep 160X (Derek’s RTR Review)

Forefoot 28mm, Heel 31mm

3mm drop

Since most of the shoes are lightly used, I expect the stacks to be slightly different from the manufacturer’s published stacks, but from the results I got, the numbers where not that far off, and so I am fairly confident in the accuracy of the methodology for measuring stacks.

Clearly, from these measurements, it shows definitively that measuring from outside the shoe yields very different results from the officially recognized method of measurement as described by the IAAF. I was very surprised to get the exact same stack for the Alphafly and the Tempo. I don’t think this necessarily negates the utility of measuring from the outside fully. The Alphafly feels a lot more cushioned in the heel, and this is better reflected is measuring the from the outside than from measuring in the centre. 

Bear in mind that our concern is not legality of shoes, but rather what the stack numbers could mean for runners in terms of predicting ride quality. The other big surprise is that at least in my current pairs, the VF Next% and the Endorphin Pro have the exact same heel stack. The Endorphin Pro is a very cushioned racer, but the Next% heel feels so much more forgiving than the Endorphin Pro. It just goes to show how important the quality and characteristics of a midsole are, and how 2 shoes of the same stack can feel quite different underfoot. 

The other shoe I measured is the Xtep 160X. Michael Patrick Ellenberger and I have done a review on this one pretty soon, and it is a PEBAX foam carbon plated racer out of China, which had very little information on its geometric data, and I was very happy to get some numbers on this shoe for the upcoming review. You will need to stay tuned for our review on this one.

Read reviewers' full run bios here
The product reviewed was a provided at no charge. The opinions herein are the authors'.
Comments and Questions Welcome Below!
Please let us know mileage, paces, race distances, and current preferred shoes

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Sven said...

It would be interesting to see the stack height IAAF style of the shoes in size 8/42 that are around the new limit for track racing, 25 mm. For example I have seen very different numbers for Adizero Pro and Metaracer. And Im not sure about my Reebok Floatride run fast. Hopefully world athletics will publish their list on approved shoes soon.

azer89 said...
This comment has been removed by the author.
freebird54 said...

Great work
Is there a table of stack heights please