Introducing Grip Testing: Understanding our static grip test
Contents
Since the launch of Bicycle Rolling Resistance in 2014, we have regularly got asked how useful our lab test data is without including a grip test. We've worked hard over the past 12 months on developing a grip test to answer these questions, and we've now come to the point where we can present our grip tester. This article will explain how it works, why we made certain choices, and how to use the data to judge tires.
We started working on the grip test in November 2020, and our initial plan was to launch it before the end of Q2 2021. We've run into several challenges and unexpected behavior during the tests and had to delay this test several times. We just had to understand every aspect of grip testing before we could share it with the world.
The short story is that grip testing turns out to be extremely hard to get right. The big challenge was not building the grip tester but finding a suitable test surface, tire preparation, and cleaning protocol. We've had to run the grip test more than 1000 times while learning a bit more after every run to get this right.
As of today, you can find wet grip data (average grip for non-Pro Members, + center and edge grip for Pro Members) of about 40 road bike tires. We started with the road bike section as grip data on a hard surface is probably the most useful there, and we haven't gotten around to the other categories yet. Over the next few months, we'll continue with testing off-road tires as we still need to learn how the knobby tread affects the grip test. If all goes well, we hope to add grip tests to the other categories, but it's going to take time.
The easiest way for you to understand how we test grip is to see it in action. We've recorded a video of our grip test, which takes about 4 minutes to complete. You'll see the wet grip test that starts by testing the center of the tire followed by the edge of the tire (35-degree angle - cornering grip).
Static Grip Test
If you've watched the video of our grip test, you probably know how it works by now.
Technically, our test is a static friction test as the tire pulls the platform forward, and the maximum pulling force is measured. The pulling force is measured with a load cell connected to one end of the platform and records the maximum pulling force.
We made the test a little less static by pre-starting the wheel and platform. During the first 4 seconds of the test, both the wheel and platform move at the same speed to ensure the amount of water under the tire is the same for all tires. After the first 4 seconds, the platform drops to 33% of the wheel's speed, and the wheel starts pulling the platform forward.
During the measuring phase, the platform stays at the speed of 33% of the wheel speed as it then keeps on "feeding" new wet surface for the tire to grip. We found this gives the best results with the limited speeds we're working with, and it again makes the test a little less static.
Still, everything happens at a low speed, and we feel it's probably the most significant shortfall of our test. There could be some differences at higher speeds in a dynamic setting, but we think this test will give a good indication of the potential grip. If a tire scores low in this test, it's improbable it would suddenly be the best performing tire in a dynamic setting.
Test Surface
Finding a test surface that gives reliable and consistent results over a long time was probably one of the most complicated challenges we faced. Of course, the natural choice would be to use some kind of asphalt surface as that's perhaps the surface you ride on the most.
The problem with surfaces like asphalt or concrete is that they aren't stable surfaces. The pores in asphalt or concrete trap rubber from the tires (even during a wet test), and if you use the same surface twice, the following measurement will be down 10%. Subsequent measurements will then be even lower as more rubber will be trapped in the pores.
Using a new test tile for every run would be practically impossible. Cleaning also presents a problem as asphalt, or concrete surfaces aren't wear-resistant enough to allow for abrasive cleaning. What we needed was a stable surface that doesn't wear out when cleaning it after every run.
Our final test surface for all the tests is a ceramic tile with a rough texture. The ceramic tiles we use are slightly slippier than asphalt or concrete, but we feel that's an advantage. Where you need the grip in the wet is on the slippier parts of the road like road paint, tar snakes, or drain covers.
The significant advantages of ceramic tiles are their consistency as they're incredibly wear-resistant and have a synthetic texture that's nearly the same for every tile. The consistency is critical as there's only a ~ 30% difference between the best gripping and the worst gripping tires. Without a ceramic surface, this test would be impossible.
We run the test you've seen in the video three times with three different ceramic tiles of the same type. In total, that means there's six tests for the center grip and six tests for the edge grip. The result is the average score of those tests.
Wet Test Only
Our initial plan was to test both the dry and wet grip, but we had to drop the dry grip test results as it looks like dry grip is highly dependant on the temperature and surface. Simply put, our textured ceramic tile probably is too grippy at our standard test temperature to give a meaningful result for a dry grip test.
What we found is that some tires have a compound that is too soft to grip well in the dry at our standard test temperature of 22 C, some tires have a compound that is too hard, and some tires have a compound that is exactly in the sweet spot for our test surface and temperature.
When dropping the temperature by 10 C (probably when you need the grip), things change, and the tire that was too soft now comes into the sweet spot. The harder compound tires have less grip at the lower temperatures, and the total picture looks completely different.
The dry test results depend so much on the temperature and surface texture that we don't feel it would add useful information. Wet grip is probably the most critical metric as we have never experienced a lack of grip in the dry.
Analyzing Test Results
Let's look at a small selection of the test results and see if we can spot some surprises.
This small selection of test results include the best gripping tire (Veloflex Record) and the worst gripping tire (Schwalbe Pro One TT) out of the 40 tires that we've tested up to now. The difference between the center grip of the best and worst gripping tire is 50%, while the differences in the edge grip test are a bit smaller at 30%.
Let's run through the test results and analyze the results.
Continental Grand Prix 5000 We can't leave out the GP 5000 as it's probably the most widely used road bike tire. The GP 5000 is a good benchmark to compare other tires.
The GP 5000 is a single compound tire, and it shows average performance in both the center and edge tests. Its casing is quite flexible, and it uses an anti-puncture belt under the center of the tread. The flexible casing increases grip a bit, but the anti-puncture belt reduces the grip a bit as it makes the tread less flexible. The total grip levels look good, especially when considering the low rolling resistance and high durability of the GP 5000.
Continental Grand Prix 4-Season The Continental GP 4-Season is often used as a winter tire as it has a compound optimized for wet weather performance. We see the difference between the GP 5000 and GP 4-Season isn't all that big, and we attribute this to the much stiffer and more puncture resistant casing of the GP 4-Season.
The grip of a bicycle tire is a combination of the flexibility of the compound and casing. The result will still be average grip when you use the best gripping compound on an extremely stiff and inflexible casing.
Michelin Power All Season The Michelin Power All Season also comes with a compound optimized for wet weather performance. The rolling resistance of the Power All Season is extremely high. We feel it's a good indication its compound is even more optimized for winter performance than the compound found on the Continental GP 4-Season.
The Power All Season also comes with a casing that's a bit more flexible than the casing of the GP 4-Season. The result is a tire that is one of the best gripping tires that we've tested up to now.
Veloflex Record (silica compound) The Veloflex Record probably is the biggest surprise here as it's the best gripping tire that we've tested up to now. We attribute its extreme grip to the extremely thin tread of about 0.5 mm and extremely thin and flexible cotton casing of about 0.4 mm thickness.
We see that not the softness of the compound is the most important but the combined flexibility of the casing and compound. Flexibility should not be confused by softness, as a flexible tire will be able to deform around imperfections in the road surface better than a soft tire that isn't as flexible.
Veloflex Corsa Race (silica compound) The Veloflex Corsa Race is interesting as it uses the same compound as the Veloflex Record but comes with a thicker tread and anti-puncture belt, making it a much less flexible tire. The result is a tire with less grip and also a different distribution of the grip levels.
While the Record had less edge grip than center grip, the Corsa Race has more edge grip than center grip. We attribute this to the puncture belt that is a bit smaller than the tread. The edge of the tire is more flexible as there is no puncture belt along that part of the tire, and the result is more grip.
Wolfpack Road Race The previous tires were all single compound tires, but as you can see, our grip tests quickly show which tires come with a true multi-compound. The Center grip of the Wolfpack Road Race isn't all that good, but it's the best gripping tire when it comes to edge grip.
The combined grip of the Road Race is the same as the GP 5000, but there is a big difference in how you'll experience this grip. It's good to know the multi-compound works, and if you've experienced a lack of grip in a straight line with this tire, it makes sense.
Schwalbe Pro One TT TLE The Pro One TT is the worst gripping tire out of the 40 tires we've tested up to now. It comes with quite a thin tread, no anti-puncture belt, and a compound that is mainly optimized for low rolling resistance. The casing is a classic 127 TPI nylon casing, and it seems it isn't flexible enough.
The results of the Pro One TT suggest that a thin tread is only beneficial to the ultimate grip if the casing is more flexible than the compound. With a casing that isn't flexible enough, a thicker tread will be more flexible as thicker layers of rubber will better deform around the road imperfections.
Schwalbe Pro One TLE The Pro One TLE is also clearly a multi-compound tire, but its grip distribution isn't as extreme as the Wolfpack Road Race. We suspect the center compound is very close to the compound used on the Pro One TT, although it does grip a bit better even though it comes with a puncture belt under the tread.
The edge grip of the Pro One TLE is a bit lower than the Wolfpack Road Race, but it's among the best-performing tires here. A part of this can be attributed to the tubeless casing, as tubeless tires are often a bit stronger. Especially with nylon casing tires, this results in even less casing flexibility and slightly lower grip.
Conclusion
Even though our grip test is a static test, the test results clearly show differences among the tires we've tested. We understand there can be differences when using these tires on a bike under dynamic conditions at different temperatures, speeds, and road surfaces.
We aim to at least give insight into the grip levels of bicycle tires and how the construction of those tires affects their performance. You probably shouldn't make the grip level be the decisive factor when our test indicates just a couple of points between different tires. The real value lies in those test results where a tire clearly performs better or worse than other tires.
What we've learned from our test results is that the compound by itself isn't as crucial as we initially expected. The flexibility of the casing is just as important as the tread thickness of bicycle tires is very low compared to most other tires. The thinner the tread, the more critical the casing becomes, as it will make up a more significant portion of the tire's flexibility.
Bicycle tires with their thin and soft casings are probably unique in this aspect. We expect the compound to be the more dominant factor on car tires or other tires with much stiffer casings and thicker treads. We're also curious how off-road tires perform in our test, but that will take some time.
Please visit the road bike section where you can find grip test data of about 40 tires from this moment on. Every new road bike tire test will also include the grip test data.