Based on what, ideal coefficient of friction between rubber & rock or something else?
If that logic was all that mattered, why have side knobs (or any for that matter) on MTB tires?
This isn’t about hardness or pure friction between two simple materials. Once you get beyond super hard packed dirt/pebble roads on par with concrete or tarmac, things change in a big way.
Simple surface roughness increase can take your perfect smooth tire and negate traction to a degree if/when it doesn’t drop down in the the negative spaces below the higher surface contact. This is one place where an actual knob has the opportunity to ‘dig’ into those low spots for contact to the bottom and/or sides of those depressions.
This all escalates once you then include the fact that many of these gravel roads include loose material on the surface. These bits of dust, sand, pebbles and such drastically alter the friction case. Ever roll through some dirt debris during a road ride on smooth tires while trying to make a corner? I have and I’ve nearly been put on the ground when the that awesome flat tire failed to grip through and around that loose material. I’ve done those same types of corners at similar speeds on my gravel bikes with side-knobbed tires and the experience is very different (much better for control and confidence).
I agree this would be super interesting, but rather difficult. We have enough issues with road tire testing on drums so I expect this would be another level of tough. Add in the fact that it would require definition and use of multiple surface conditions to be of any real value since there is far more than one use case.
You’re welcome to it, just report back if you ever try to rip a corner at speed on something less than “hero gravel”.
The problem with this discussion is that there are few actual “slick” gravel tires - Rene Herse doesn’t make any, for example.
Most slicks are actually file tread or diamond tread and there is compelling argument and evidence that both patterns are more effective on pavement than a slick tire.
Which of course leads to the question of, if 1-2mm deep file/diamond pattern helps cornering and traction on pavement, do 3-4mm deep blocks of various shapes help on rougher surfaces? I think yes, for the same reasons.
conforms more easily to micro-irregularities of the road surface
Side knobs & knobs in general, help the tire contact more of the unpaved surface by reaching past the macro-irregularities of that surface. A tire doesn’t conform exactly to surface features and depending on the irregularities (among many other factors) a slick can often have less contact between the rubber and surface than a tire with knobs or tread. If you’re riding loose over hard and the loose is 2-3-4mm stones then a slick is not going to work very well, it can’t move the stones effectively out of the way, the stones increase the void ratio and reduce traction. With a tire that has knobs 2-3-4mm deep the concentrated ground pressure at each knob can push stones out of the way of themselves and allow the rubber to contact the hardpack and provide traction - even if that amount of traction would be less than if the slick was fully contacting the same surface - it is more than the slick is able to contact due to the loose material that remains between the slick and surface. As well the stones then contact the formerly void areas of the knob tire so there’s often an interlocking/packing effect as well.
I’ll never forget my first race on Snoqualmie Pass (700cx44) filetreads after a couple years on Steilacooms (700cx38) knobbies - grabbing a handful of front and rear brake after a familiar long straight descent into a stop sign and the rear just sliding on the top of the gravel and dirt, whereas before the knob tire would have enough friction to slow without sliding. The same issue exists at cornering extremes, with knob tires I can feather the brake effectively but with slicks (file treads) the window before washout is much smaller and often nonexistent.
This is always a fun argument because there’s not much hard data, but it’s possible to spend $200 and perform any experiment you can think of to figure it out for yourself.
Yeah, there are some variables in my testing that might account for some of my differences, but I try to test in a way the reflect real-world use. The big variables I can think of is that I was testing with well worn ramblers (surprising if they get faster with wear), I run different tire pressures than BRR, and I tested the tires on different rims (but the race kings were on lighter, wider, and arguably more aero carbon rims). My ramblers were also 45’s (not 40’s), but my race kings are the protection version. My test speed was a little higher and the aero difference between tires would favor the ramblers, but not enough to explain the difference vs. the BRR testing.
As much as I appreciate and respect what BRR does, the results are just a data point. Looking at their testing protocols, I’d think they would be a reasonable proxy for smooth roads, but not representative for any irregular surface. Most all of the tires test significantly better as pressure increases, but that just doesn’t translate to real world when you move off the smooth testing drum onto chunky terrain (or even heavy chip seal). The other tire I’ve compared to the 2.2 Race King is the 2.25 Aspen. BRR would tell you the race king is ~10 watts faster per tire than an Aspen on a smooth surface, but my testing has them only 5 watts faster for the pair. Also, just a nit pic, but everyone seems to take the BRR wattage number and doubles it to get the wattage for a pair of tires. It doesn’t work that way since the front and rear tires are weighted and loaded differently. I also speculate that the steady application of power on the BRR drum is missing part of the story since power is applied to a rear tire in a very “surge-y” fashion while actually riding which I assume affects efficiency depending on the interface to the road/dirt.
I can’t explain all the differences I see on the road compared to BRR. My testing clearly isn’t as controlled or repeatable as BRR (and maybe I’m doing something really dumb), but I am convinced they are not getting the entire story and real-world testing should be used to validate things.
Yeah, I struggle to understand that mentality, but you are in the overwhelming majority of cyclists with that perspective. I’m a weirdo and I tend to embrace anything that goes against the cycling look/fashion norms, especially if it’s a performance boost. I love being the dork wearing a hydration pack on road rides and races. FS MTB on a group ride with skinny tires and top tube bag dropping the guy on a new s-works tarmac - makes for a fun dynamic. I know lots of strong and weak cyclists that put a huge emphasis on the fashion/image side of cycling. I give them all kinds of crap, while they give me crap for my stupid setups. It’s all in good fun, whatever makes people excited to get on the bike.
Looking at the way track bikes are going it seems that seatsays and fork legs that hug the tyres are less aero and running 40mm tyres in a frame designed for 2.2" tyres might be more aero? But then track bikes are going at 50-80kph.
Big vote for Rene Herse. They have a little grip pattern if the road gets wet, but they don’t have nob that will actually decrease your traction on pavement. In the winter I put them on my road and gravel bikes when not racing. The big advantage IMO is that they have thickened rubber on the contact patch which makes a massive difference in puncture resistance. All the Kevlar layers in the world don’t stand a chance against a nail or a car tire tread wire, but the thickened layer of rubber does actually give you some resistance.
Plus the Rene Herse ride like a dream. Super super smooth and quiet. They also have different casings if you want a different feel or have a different idea in mind.
I just rode my MTB with knobby tires on my gravel course yesterday for a few hours at endurance pace. similar to you, my most ridden course can go from super smooth to grass to non-maintained rooty / rocky / holes.
there were at least 3 times I was SUUUUPER glad I had those tires. I can’t say for sure how slicks would have handled
where I’m coming from is my handling skills are likely below average for the group on this forum (1 year under my belt and mostly on road)…and i’m likely heavier than most (86kg)
conditions on my ride were slightly muddy…rained 3 days previous
one time my rear wheel hit mud and I could feel it slide like an inch and the tire just grabbed in and caught the ground and the slide ended. I would not have fallen if the tire didn’t catch me, but if I had to brake or steer then for whatever reason - dunno, I’d rather not. very unusual feeling after being so used to riding on the road
another time there was a small gravel patch (say a foot wide) dug out of grass. I had to transition from the patch to the grass to avoid people…the gravel was like 4 inches below and it was sorta dug in a wide U shape. so to get out of the patch you go up that U. I dunno. I feel like my knobs HAD to have helped.
the third time I needed to take a detour off the course and wound up in a construction zone with a somewhat steep short pointy rocky assent. no idea how the knobs helped me but the tire being flat rather than a bulb I am sure helped. I really had no fear that the MTB tires would explode
So I guess until my skills come around, I am thinking knobs are best for me. even though the minority of my riding needs them, I’m more likely to hurt myself there. I will just keep in mind my cornering ability on dry surfaces is degraded
sounds like another wheels set is gonna make it on the to-buy list
The knobs are unnecessary idea that is spread by JH from René Herse is flat out wrong in my experience.
I’ve ridden most of his tyres over the past 10 years. My personal gravel riding experience goes back over a decade with some years of 10,000km a year entirely on gravel and dirt roads. People that live in most towns or cities would not clock up that distance on gravel in 10 years yet alone one year.
So knobs are your friend off road. Slicks are fine in a straight line but if you go around corners at speed there is no question that knobs help on dirt.
100%….for the type of gravel I ride, sone tread is mandatory.
Our trails are hard pack sand / dirt with “kitty litter” on top. You need some tread to give the loose stuff somewhere to go. Slicks are dangerous AF on the turns of our trails. You are essentially riding on a batch of micro-marbles.
Ride Report:
Bike: C’Dale Scalpel 2.
Wheel Set: Custom DT Swiss alloy 30mm IW, straight pull spokes, brass nipples (they heavy training set)
Tires: Panaracer Gravel King SK 2.1 30 psi F 33 psi R
I had a 4h 30m High Z2/Low Tempo ride on the schedule, but am still recovering from bruised ribs I picked up at The Rock Cobbler on Feb 8th, so the heavy wheels on the full sus for some time in the SF Baylands (FLAT, but rough hence the full suspension XC bike)
As expected, the SK rolls great on pavement, and true to their nickname can fling bits of gravel and sand at you depending on the makeup of the surface. Average speed considering the surface tilted towards unpaved (rough levees, loose over hard, hard packed fine sand/gravel) and not the smoothest pavement, the 14.5mph average speed over 67.6 miles and sticking to the assigned Z2/Z3 power. The SK is a fast rolling MTB tire for sure. Now if only they wouldn’t pelt me with bits of sand…
I’ll keep them on my heavy training wheels for days like this.
