Tire Tips – Every Rider Should Know
More than keeping the rubber side down
With Frank Stacy
Frank Stacy is the owner of Stacy Testing & Tire Specialist, Inc. He has over 30 years of experience designing, developing and testing tires for motorcycle, ATV, UTV and bicycle applications, and does consulting for major tire manufacturers and OEMs worldwide. To say he knows a thing or two about tires might be the understatement of the year. Frank’s expertise covers all aspects of tread design, casing structure, rubber compounds and manufacturing. He’s worked to develop some of the best tires for iconic rubber brands like Dunlop, Pirelli, Maxxis, Specialized, and most recently Trek and Bontrager. We sat down with Frank to ask him some of the burning questions we have about tire technology and what it means for the future. These are a few of our favorite responses from the man who knows more about rubber than anyone.
What basic tips can you give for selecting the right tires for your terrain?
As we all know, mountain bike terrain can vary significantly throughout the world. It’s impossible to design one tire to perform at a high level in all terrain. But, the good news is that the leading tire brands spend a ton of time, effort and money to test their tires in all types of conditions worldwide. This allows tire brands to determine which tire tread works best on which type of terrain. The leading brands typically provide application charts on their websites, and these charts are super helpful for the inexperienced rider.
When I’m speaking with customers who are unsure of which tire they need, I try to educate them on some of the key points I use for designing treads. The following are some of those key points:
Dry dirt over a hard base: This application would be hard terrain, which basically means if you step in the dirt and you don’t leave much of a shoe impression, it’s considered hardpack. Hard- terrain tires ride more on top of the dirt (like your shoe impression), relying on tread footprint for traction. Hard-terrain tires have shallower tread depths, which reduce rolling resistance, reduce weight and provide a uniform footprint at all lean angles. They typically feel fast and light on the pedals. The downside is the tread spacing is typically close together, so if the dirt is too loose, the tread may not clean out very quickly and can clog easily if the dirt becomes wet.
Mixed—loose, loam, rocks and roots: This application would be intermediate terrain—soft-enough dirt to leave a shoe impression, but not so deep that you sink into it. Intermediate terrain is the most popular category for MTB tires. It’s basically the do-it-all tread. It may not be perfect on any one type of terrain, but taking into consideration the variation in intermediate terrain, the tread is designed to perform very well. Intermediate treads are designed to provide a balance between low-rolling resistance, traction and tread cleaning, and typically have somewhat tall tread depths. They are designed to dig into dirt, find traction, provide predictable traction on rocks and roots, and self-clean. The downside is that intermediate tires tend to have softer rubber compounds (higher traction), which typically wear out sooner.
Soft dirt, deep loam and mud: This application would be wet, muddy terrain. It’s the type of dirt that if you stand there long enough, your shoes begin to sink in. And when you pull your shoe out, dirt is clinging to the shoe. Mud tires have tall knobs with wide gaps between the knobs. They’re designed to dig into any type of loose, loamy, wet or muddy terrain. They also self-clean efficiently with each tire rotation. It’s always good to have a set of mud tires in your garage or race vehicle for wet weather. If it’s muddy, these tires are like cheating.
How do you feel about the plus-sized and fat-tire movements? Which riders would benefit most from using those bigger rubbers?
In my opinion, the fat-tire movement has slowed. Yes, there’s still a place in the market for fat-tire bikes, but I believe they will head back to what they do best—and that’s snow and deep sand. It’s no secret they appeal to a special type of enthusiast and, in the proper application, are fun to ride. But, they have their limitations.
In my opinion, the plus-sized tire market has a bright future for the weekend enthusiast, weekend warrior and maybe even the weekend pro. These bikes are fun to ride and provide a really good balance of performance, weight and handling with mega traction.
Will plus-sized or fat tires ever be race-worthy outside of the Fat Bike Nationals?
I don’t see the fat-tire market going crazy on the racing side, but plus-sized tires I do. Look at the bikes the plus tires are coming on. They are totally performance-driven and are way more fun to ride than fat-tire bikes.
What tips and tricks do you have other than lining the label up with the valve?
Not sure if this is a tip or a trick, but one thing that always seems to come up when I’m speaking with riders, customers, dealers, etc., is a tire’s life cycle. It’s obvious a rear tire wears down quicker than a front tire, and it’s easy to see this wear evidenced by a disappearing tread depth. Most people install a new rear tire and they’re happy; however, what people don’t realize is that the front tire is wearing too. The tread depth still looks good, but the casing structure is worn out. Over time the casing gets soft, which reduces puncture resistance and flattens the tread profile (arc), which affects handling and cornering traction. Also, the shoulder knobs take a beating. If you look closely at the inside edge (biting edge) of a worn front tire’s shoulder knob, you can usually see excessive wear and likely chunking (cracking) starting at the base of the shoulder knob. Most people don’t even notice this because they are only focusing on the remaining tread depth. This type of wear has a huge effect on front tire traction and steering.
Obviously, MTB tires affect ride quality. How would you recommend riders experiment after they burn down their stock, original-equipment tires? How should they evaluate their current tires and upgrade them?
I always say, “Right tires for your terrain.” In many cases, the OE tires may not be the proper tread for the terrain where the rider lives, so the rider will need to determine what terrain he or she rides and then select the tread that best suits that terrain.
The other is the casing structure/rubber compound. An OE tire may have the tread pattern that works in the rider’s area, but the casing structure and rubber compound may not be the best option for their riding. The casing structure (i.e., ply material, cut/puncture resistant layers and rubber compound) completes the “recipe” when making a tire. The proper casing structure is very important to achieve a high level of durability.
My suggestion is to invest in a stronger casing structure to get the most out of your tire’s performance and improve durability. Upgrading the casing structure may add some weight, but in the end, the rider will be happier. Stronger casing structures allow for lower tire pressure and thus more traction, stiffer sidewalls to minimize tire roll-over when cornering, improved tire sealing when running tubeless and extra rim protection, all adding up to a win- win.
Which is fastest—high volume, low weight, low tread pattern, better traction or all of them?
We have ongoing tests to prove these types of things, especially with the movement to wider rims and plus tires. Here are a few things we’ve determined so far:
—Higher-volume tires and/or heavier tires, once they are in motion, have almost equal rolling resistance (within a few watts) compared to a lightweight tire when all other things are the same—same tread, same size, same casing structure, same compound, same rim, same inflation pressure, etc.
—Low-tread-depth tires almost always roll the fastest.
—Better traction (using the same tread pattern and size) typically means the tire has a softer rubber compound. If you have two identical tires—one with a harder rubber and one with a softer rubber—the harder rubber typically will roll faster.
I’d like to mention that most tire brands test rolling resistance one of two ways (both provide usable data):
—Rolling resistance can be tested in a laboratory using a large rotating metal drum and attaching a wheel/tire assembly to roll against the drum. This measures the contact area friction in watts.
—Rolling resistance can also be tested by riding the bike on pavement using an SRM crank. This also measures contact-area friction in watts.
Why do the most expensive tires wear down so quickly?
Typically, these tires use high-quality, high-traction rubber compounds. If you want them to grip at a high level, they’re going to wear out quickly. It’s no different for tires in motorcycle, F1 and NASCAR racing.
Rubber compounding is an art. There are four basic properties of rubber compounds.
Polymer: Rubber itself. Can be natural, synthetic or a mixture of both.
Carbon black: Powder mixed in. Affects durability, hardness and traction. Also makes the rubber black.
Oils: Several different types mixed in. Affects traction, wear rate and hardness.
Fillers: Mixed in to prevent cracking and to protect against ozone and those types of things.
I’m not a chemist, but I have been designing basic formulas for rubber compounds for many years and have a good understanding of how changes to the main properties affect the end result. I determine the basic formula by analyzing a variety of compounds through field testing and laboratory research.
Rubber compounding is all about finding the right combination of durometer (hard or soft), rubber strength, durability, traction, weight and TLR (tubeless-ready) sealing.
How do tire designers decide between a round and square profile?
For me, it’s all about understanding the tire’s intended application. For example, rounder, inflated profiles are typically found on XC tires to achieve a fast-rolling, lightweight feel at the pedals.
Flatter, inflated profiles are typically found on trail and enduro tires designed to perform on a wide range of terrain and provide a good balance of characteristics for climbing, braking and cornering.
Downhill tires have the flattest inflated profile. The focus is on hard braking and a quick transition from straight-up to extreme lean angles when cornering. You want the shoulder knobs to bite hard.
Do you ever sipe or clip tires for your racers? Do you do anything else special for the fastest guys?
We’re still clipping knobs on the World Cup circuit. Not so much in XC, but definitely for enduro and DH. We’ll clip hard-terrain tires to make semi-slicks, only using these on the rear. We’ll clip mud spikes for certain wet and muddy applications, mostly when it’s wet in the wooded areas where there are tree roots or rocks. The clipped spike tread provides more traction in those conditions.
We’re also playing with a variety of air pressures, depending on rim width and tire choice. We’re searching for a balance of traction, handling and minimal burping. Weekends can get very busy!
Is tubeless the best solution for everyone? Why would anyone use tubes anymore?
For XC and enduro applications, tubeless is pretty finely tuned; however, tubeless for DH applications still has some downfalls. But, it’s getting better. The biggest issue is the high speed, high loads and low tire pressure the riders want to run. Rims just can’t take the punishment over and over again before something has to give. Once the rim loses its seal, the tire is flat.
We’ve been working on new technology for DH TLR applications, and we’re currently testing on the DH World Cup circuit. It’s working very well. Hopefully, we’ll have something to show the world before the start of the 2017 DH World Cup season. Stay tuned!
How much does wheel size affect tire footprint?
I can’t provide percentages or dimension details because they’re confidential, but obviously, with a 26-inch to 27.5-inch to 29-inch tire, the tread footprint lengthens, which means more tread on the ground and more traction. The wide rim movement is adding another level of traction. It’s all good.
Would you prefer a fat bike or a 26-inch bike for better traction?
I’ve never compared the two bikes back to back, but in my opinion, the fat bike with fat tires is going to have a higher level of traction on almost any type of dirt compared to the 26-inch tire; however, when the terrain gets tight, bumpy or there are high cornering loads, the 26-inch bike should outperform the fat bike. The fat tires tend to bounce around a lot at speed, and the sidewall rolls over easily when cornering hard, mostly because the fat tires require super-low tire pressure to perform properly.
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