37.5 Fabric for Cycling Apparel: A Technology Review

By Brandon Bilyeu

Cost:

Varies depending on brand using 37.5 Technology

Trek Circuit LTD Jersey: $90

Pactimo Summit Baser Layer: $74

How obtained: review samples from company

Website: 37.5 Technology

RBR Sponsor: no

Tested: 60+ hours

37.5 Technology is a company that is not an apparel brand itself, but it licenses its performance enhancing materials to be used in all types of clothing.  The 37.5 technology is incorporated in garments ranging from sleepwear to exercise and everything in between.  The list of cycling brands using 37.5 include Ashmei, Trek, dhb, Katusha, Mavic, Mission Workshop, Pactimo, and Rivelo.  I tested out 37.5 in Trek’s new Circuit LTD cycling jersey and Pactimo’s Summit Base Layer.  Before getting into my experience with 37.5 let’s dig into exactly what 37.5 technology is and the claimed benefits.

37.5 Technology – What is it and How Does it Work?

The ideal core body temperature is 37.5 Celsius and ideal skin level relative humidity is 37.5%.  In simple terms, 37.5 technology is integrated into clothing and helps with thermoregulation to help keep you at the correct temperature for best performance, whether riding a bike or sleeping.  This is achieved with tiny active particles of volcanic sand imbedded into the clothing material.

The special volcanic sand has two features that lead to its thermoregulation abilities.  First, the sand absorbs infrared (IR) light in the wavelength that the human body emits.  You are probably familiar with IR images similar to below and might be much too familiar with the IR thermometers used during the Covid pandemic.  Second, the sand has billions of micropores which massively increases the surface area compared to a solid piece of sand of the same size.  This is like the classic ‘bubbly’ lava rock we have all seen, but in a much smaller form. 

The volcanic sand absorbs the IR light emitted by the human body and is porous on a much smaller scale than the volcanic rock shown above.

The majority of bike kit is made from synthetic materials with moisture wicking properties to move liquid sweat away from our skin to promote evaporative cooling and keep the material dry.  But we don’t start sweating until we are already hot.  As the body heats up our skin microclimate goes from low humidity, to high humidity, and then finally liquid sweat forms.  Wicking materials only begin cooling in the final phase, but 37.5 aims to begin cooling in the humidity phase.

The IR absorption, maximized by the large surface area, is the ‘magic’ that makes the thermoregulation work.  When the body is relatively cool the particles absorb and retain the IR radiation to help keep you warm.  If the body is warm and generating sweat vapor the active particles use the IR energy to move the extra humidity to the ambient environment which provides a cooling effect.  This cooling may be enough to completely prevent liquid sweat from forming, but if exertion levels are high enough liquid sweat can still form and wet out the material.  In this case the humidity transfer no longer works.

With the presence of liquid sweat wicking and evaporative cooling now take place and the particles play a role here too.  The porosity of the particles spreads the liquid over the larger surface area and this leads to faster evaporation.  So, when 37.5 is integrated into a wicking material it leads to very effective evaporative cooling and the material staying drier.

Lab Testing Yields Some Pretty Impressive Numbers

For maximum performance the real goal is to keep the body core temperature down.  Skin temperature is more of a comfort issue.  The University of Colorado Boulder did an independent study with cyclists comparing three cases: long sleeve shirt with wicking technology, same long sleeve shirt but with 37.5 added, and a cooling vest with water circulating at 4 Celsius (39 F).  In short, the study found that the test subjects were able to perform at high intensity up to 26% longer with the 37.5 technology shirt versus the standard shirt.  And the 37.5 performed similar to the cooled vest.  For those interested in more details a summary of the study and results can be found here. 

While the 37.5 lab test results are impressive and prove that 37.5 is not just marketing smoke and mirrors, the exact 26% likely does not transfer directly to the road.  The one variable I don’t see mentioned is airflow.  Unlike walkers or runners, cyclists move quite fast and airflow plays a big part in evaporative cooling.  The wicking shirt will work much better with moving air and this could close some of the performance gap to 37.5.  There is a good reason we use six fans at once when riding the indoor trainer.  This study in a static lab does point to good 37.5 performance when climbing steep grades as speeds are much slower.

On the Bike

First, a few disclaimers.  My body is not perfectly calibrated to measure external skin or internal core temperatures.  Complicating matters is that perceived skin temperature is not a good indicator of core temperature.  This was not a blind test as I knew I was wearing kit with 37.5 technology.  So, my observations of 37.5 performance will be limited to perceived comfort on the bike in terms of temperature and dryness.

I started out with Trek’s new Circuit LTD jersey at the beginning of summer.  The jersey itself is very nice, but definitely not a lightweight summer jersey.  After a few hot rides I did not notice any significant difference between the Trek jersey and others of similar weight.  It is worth noting that I run hot and sweaty, so the transition from sweat vapor to liquid happens very quickly for me.  Discussions with 37.5 informed me that strict quality control ensures a minimum number of active particles in the fabrics and that more particles makes it more effective.  To increase my particle count they sent me a Pactimo Summit Base Layer with 37.5 to wear under the Trek jersey.

I love base layers in the winter to keep me warm and dry, but I have never had a good experience in hot weather with a base layer under a jersey.  The two layers in the heat always felt stifling.  To my surprise the two layers of 37.5 actual felt quite comfortable on even the hottest days.  The time it took for liquid perspiration to appear was delayed and even when sweating heavily the two layers of fabric stayed relatively dry compared to a standard jersey.

Continued testing through the summer saw me mix and match layers as well as use the base layer while hiking.  Comparison experiences with and without 37.5 over a summer of riding and hiking confirmed that 37.5 works better than a standard wicking jersey and that more particles enhance the effect.  While airflow effects may have been omitted from the lab study, my outdoor riding showed that 37.5 is still more effective even with airflow.  On descents where I typically keep my jersey unzipped to cool down from the previous climb, I found myself getting cold with the 37.5 and zipping up to retain some heat.

In cooler weather it is claimed that the particles absorb and retain the IR energy to keep you warm, but my experiences did not reflect this.  I sweat even on cold rides and I suspect the excellent evaporative cooling was overpowering any warming effect.  Perhaps if I was not a heavy sweater the heat retention would have been more effective.

Bottom Line

My experience with 37.5 is that the technology works and is a definite improvement over standard wicking materials on hot days.  The increased cooling performance, while not a night and day difference, is discernible out on the bike and on hot summer days I’ll take all the extra cooling I can get.  Considering the reasonable $90 price tag on the quality Trek jersey it appears 37.5 does not add a significant cost burden to the manufacturers.

---

Brandon Bilyeu is an avid recreational roadie who lives in Regensburg, Germany. He’s a year-round bike commuter and is a mechanical design engineer by trade. Click to read Brandon’s full bio.

Next Article: Multi-Use Path Safety: 9 Ways You Can Help Older People “Keep on Truckin”