bontrager tyre engineer frank stacy_-2
bontrager tyre engineer frank stacy_-2

Bontrager’s tyre engineer Frank Stacy hunts for the ultimate trail bike mud tyre...

From Dirt Issue 147 - May 2014

Words by Steve Jones. Photos by Steve Jones.

“It’s the wrong time of year to do a mud tyre feature", a friend from a neighbouring mag told me. Although wet weather grip is primarily a must from October to April I reminded him of the weather at last years first Enduro World Series in Italy, the summer time monsoons of Les Gets and Gee Atherton winning Val di Sole in 40º heat on his Continental Mud Kings.

Judging from numbers we researched from riders using Fly Up downhill service in the Forest Of Dean, on any given wet day the percentage of riders using wet weather rubber was around the 30% mark. It seems that many people don’t understand the need for mud tyres.

Whereas downhill is pretty well covered in terms of rubber for full–on monsoon weather, at the other side of the spectrum general duty trail bike mud tyres are available mostly as a 50/50 options. The best trail tyres to date are the Specialized Storm, 60a compound, the Panaracer Trailraker, 55a, and the Bontrager XR Mud. All around the 2.0 width, the Specialized and Bontrager have roughly the same pattern whilst the Trailraker was unique in its very angular mud shedding pattern – close to a tractor tyre.

The Bontrager remains one of the best all–rounders but as a trail bike mud tyre it is slightly compromised if you’re taking on longer rides. It’s arguable that there is demand for a lightweight mud tyre for a trail bike for full–on mud conditions for gravity style riding. But even though we’re not talking uplift tyres here, in heavy conditions and with higher pressures these tyres do a very good job on harder hitting 160mm bikes given their wide remit.

Last month I got a chance to ride some proto mud tyres with Trek and Tracy Moseley as they set out to produce a new improved version of their XR Mud, giving feedback in the process. The man behind all Bontrager’s tyre development is an American called Frank Stacy.>>

Click through to keep reading...

[part title="The Search for the Ultimate Trail Bike Mud Tyre - Page 2..."]

bontrager tyre engineer frank stacy_
bontrager tyre engineer frank stacy_

Frank: The current XR Mud has been around for several years and is one of Bontrager’s best selling mud tyres in the world market. Over the years the tread pattern remained unchanged, simply because it worked very well. However, there were updates to the casing structure (added Inner Strength sidewall) and dual rubber compound which improved overall performance and reduced weight to 660g for 29x2.00 size. The current XR Mud rubber compound is 62a/60a, 29x2.00 and weight is 660 gram. The weight was increased since we added our Inner Strength sidewall which improved cut resistance and sealing for TLR.

As such it’s a 50/50 tyre so is possibly compromised – not perfect for total mud riding and possibly not strong enough for downhill rock/mud terrain? I guess you cannot expect to cover all bases in a mud tyre?

The intended application for the new XR MUD is wet–muddy terrain. However, all tyres need somewhat of a cross over on varying terrain. For example when it’s muddy you still have slippery rocks and tree roots to contend with and maybe a fire road in the forest that connects the trails together where rolling resistance plays a key role. If the tyre and/or tread is so specific to it’s application, the performance will be sacrificed on these key areas of the ride.

bontrager tyre engineer frank stacy_-7
bontrager tyre engineer frank stacy_-7

Every tyre has a lifespan and we have to continue to evolve. There’s new technology, there’s things we’ve learned from our field testing and also new competitor’s products. The current XR Mud is a very good tyre but we felt we could improve its rolling resistance, cornering traction and reduce the overall tyre weight without sacrificing durability and performance.

[part title="The Search for the Ultimate Trail Bike Mud Tyre - Page 3..."]


bontrager tyre engineer frank stacy_-6
bontrager tyre engineer frank stacy_-6

In wet–muddy conditions you typically do not want a wide tyre, the wide tyre tends to slide around on top of the mud, instead of biting into the mud to find traction. Muddy terrain can get very rutted and the wide tyre doesn’t fit easily into the ruts, which can cause the bike to cross rut making it difficult to keep it in the line you are choosing. Plus the wider the tyre, typically the more mud the wheels carry, making the bike feel very heavy and increase rolling resistance.

Casing. Stiffer than previous, but similar weight?

The new casing stiffness is very similar to the current version, but we have a new material for our Inner Strength sidewall which has higher cut resistance and is lighter. It’s very effective when using low tyre pressure and seals tubeless amazingly well.

Compounds vary massively between brands it seems, where are we with these protos and why?

The previous XR Mud was 62a/60a compound… 62a centre and 60a shoulder. We are continuously developing rubber compounds throughout the Bontrager tyre line. The new XR Mud is a prefect example of our hard work. We’ve developed a faster rolling centre rubber with higher traction and a softer rubber in the transition knobs/shoulder knobs for higher cornering traction. Plus it’s lighter weight.

The durometer is 62a/50a on the new tyre but please remember the rubber compound performance is not based on durometer alone. There are several factors and properties to consider when making a rubber compound. There’s the polymer, oils, carbon black/silica and others… all of which have an effect on the rubber compound performance and durometer. It’s much like making a cake to eat and the outcome depends on the ingredients and the percentages of those ingredients.

It's not just about the compound though?

Correct, the rubber is a key part but only one part of the puzzle. Tread pattern, casing structure and weight all play a key roll to the finished product. It’s about finding the proper balance between all the parts.

Knob spacing compared to old tyre?

Very similar to the current XR Mud. However, we’ve sharpened up the knob edges and angled the shoulder knob both to increase traction and to help shed mud. The shoulder knob angled edge is designed to move the mud away from the contact patch easier. The small high sidewall knobs (between the shoulder knobs) help find traction in rutted areas.

bontrager tyre engineer frank stacy_-8
bontrager tyre engineer frank stacy_-8

We definitely reduced rolling resistance on the new tyre. It’s a combination of tread shape, tread spacing, rubber compound, casing structure and lighter weight. We’ve been able to find a good balance between all these key points.

[part title="The Search for the Ultimate Trail Bike Mud Tyre - Page 4..."]

bontrager tyre engineer frank stacy_-4
bontrager tyre engineer frank stacy_-4

The majority of Bontrager Team Issue tyres are Tubeless Ready (TLR). They all have our new sidewall insert that works amazingly to seal the tyre casing. Plus with the Bontrager TLR rim band the tyre holds secure to the rim even at low tyre pressure. We use this set up on all our factory race bikes. Bontrager MTB tyres can be used with inner tubes but once a rider rides with the TLR set up, they rarely go back to inner tubes. The performance difference is huge, especially in muddy conditions when using low tyre pressure.

Specialized produce a Hillbilly, 2.3 or 2.5 and around 1200g, Michelin do an Enduro 2.0 at 1050g for 29", Specialized Storm at 620g for 29". When are we going to see a mid–weight enduro mud?

Bontrager has several new enduro tyres in a variety of sizes coming soon to complete its SE tyre range… I hope to have a few of these for you to test later this summer.

But this new tyre could still be used as an enduro tyre?

Yes, the new XR Mud can be used for Enduro. However, with only the 2.00 width currently available it will likely limit its application to light duty enduro riding.


Two protos, how many people have been involved in testing?

I have approximately 50 test riders of all levels and all disciplines worldwide. They all play a key roll in our tyre testing programs. Specific to the new XR Mud I have 12 test riders riding the tyres.

Do you have a system for test days?

My system for tyre testing is something I’ve carried over from my motorcycle test riding experience. It’s all about paying attention to detail and focus on the little things. It’s not about being the fastest rider or the bravest rider, it’s about consistency, hitting your marks and doing those things the same each and every lap. Plus minimize the test variables, keep the bike the same throughout the testing and only change the tyres. I tell all my riders, I don’t need you to show me how fast you are or how high/far you can jump… that’s not what testing is about.

Riders might well ride in different styles/speeds which might lead to different outcomes?

I typically do not mix riders with big differences in ability at the same test. Typically what happens is the less experienced rider just follows what the pro–experienced rider is saying or is trying too hard to keep up with the pro rider’s pace, which means he or she is not focusing on the tyres. Or the less experienced rider is too intimidated with his or her comment.

Is this tyre designed for a certain style of bike?

The New XR Mud is designed for XC trail to light duty enduro riding. Once the larger sizes become available, it will open up to more aggressive enduro application.

How will you arrive at a conclusion?

bontrager tyre engineer frank stacy_-3
bontrager tyre engineer frank stacy_-3

During the day it became apparent very quickly that Frank and his team have every reason to hit a new mould pretty soon. Having started with the existing XR Mud and then trying the two new tyres it was noticeable how much better the rolling resistance was. Crucially however was the new profile, which allows a smoother transition when leaning the bike over and when you get it leaned how improved the traction was. Mud tyres need to be narrow to chisel in, they need to be self–cleaning, roll well and be easy to understand. Don’t kid yourself that it’s going to be dry all summer and keep an eye out for some great new tyres.