WEIGHT REALLY MATTERS?

It's simple physics that it matters, it's brutally complex to measure how much. For most riders the marginal cost of chasing weight is high, and the marginal gains of a little more weight in terms of durability/traction/suspension performance, etc, are much more appreciable and intrinsically valuable. YMMV, get over it... this is a question probably as old as cycling. We talked about it it as BMX kids in 3rd grade in the 90s.

Light. Cheap. Strong.......Pick two.

The speed difference between a WW (Weight Weenie) XC bike and a good pedaling AM bike, is 70%+ in the tires. Maybe 20% in the rotating bits (mostly wheels) and then just a bit in the travel and extra chassis weight.

As an experiment some years back, I removed the drivetrain (wheels/ tires/ rotors/ cassette, basically just swapped the wheels over) from my WW Spur to my AM SJEvo and did an all out timed loop on a 35-40 minute section that is pretty XC style. It for sure had rocks and a handful of small drops/ ledges (1-2.5') but was certainly not a trail anyone needed an AM bike for.

The SJEvo, once it had the Spur drivetrain, was essentially the same speed as the WW Spur. This was a LOT faster than the SJEvo on a full AM wheel set up. several mph average faster in fact.

I even considered putting the SJEvo wheels/ tires on the Spur and then adding 6#s of weight to the Spur so it weighed the same as the SJ Evo and then testing if the Spur was now just as slow as the SJEvo was with the AM wheels.

But 3 timed loops, 1 on the stock Spur, 1 on the stock SJEvo, and 1 on the SJ Evo with the Spur drivetrain was enough for me.

That's a cool test with a really interesting takeaway.

Do the experiment again! But this time for science!

I'm going to need you to do 19 more runs with each setup, if that's cool. Just send me the numbers and I can compile the data.

Thanks!

Weight doesn't matter to me personally.  It's all about tires, rotating mass weight and suspension kinematics.  I pedal a 39lb nomad with DH tires on 4-5k climbing days, I do both big/small days on my 19lb gravel bike with 48c tires.  The difference in average speed, climbing pace and how I feel has more to do with me then it does the equipment.  I'm not racing, I'm not a pro athlete but I do like to suffer.  

I'm honestly not sure why people get so wrapped up with weight of their bike and quite frankly struggle to understand how/why they are shooting for such light weight hardware.  I think this just outlines the subjective nature of the ride feel, terrain, aggression level and riding style and how much that impacts perception and gear selection.  

I'm 6'2", 175lbs and ride on an advanced level and still like to do jackass things that I shouldn't for my age.  I cannot fathom having a trail bike that is under 31lbs.  Even my 135r/150f trail bike that I use for XC stage events with rekon/forekaster combo weights 31lbs with carbon frame, wheels, bars, cranks.  With krypto enduro casing tires (still carbon wheels) that same bike weighs 33.5 lbs.  It feels plenty pointy and agile at either weight and the delta in handling is minimal.  If I were to go any lighter with the bike or the tires... the frame would be holding me back and the tires would be destroyed in minutes.  

TLDR: Bike weights don't matter much, fast rolling tires and light wheels are the most important thing if you're concerned about weight and want a specific ride feel.  Mountain bike is pain and suffering and unless you're racing for a living... who cares about a difference between a few % of efficiency.  The suffering is the journey and the reward.  If being "fast" is your goal in the sport, that's like has a limited time frame of existence/enjoyment before that is unsustainable... besides, one thing is always true about this sport... there is always someone faster, more badass and capable of more suffering then you are just around the corner.  So, act accordingly, go fast, take chances, hesitation kills and mistakes break stuff.  

I agree with everyone here, it's all about the tires. My daily driver was a 2019 Transition Patrol (27.5) with 160/170. When I swapped over a fast rolling rear like an Aggressor or Nobby Nic, big days with up to 10k vert, all trail felt great!

I now have a shorter travel bike to go along with it, a 125/140 29er Norco Optic. Sure, it's no xc whip, but going back and forth between the bikes, they're very similar efficiency even at a 2 lb weight diff. Yeah, the Optic feels a bit sportier sprinting, but on big pedal days they feel about the same in terms of how cooked I get.

My bias is big days instead of 1-2 hours all-out, so YMMV.

This. 
To re-iterate much of what Suns_PSD said, MTB speed is primarily related to tires (rolling efficiency & weight) and wheel weight (rotational mass). 
After that, pedaling efficiency and weight make up a smaller percentage. 

The role overall bike weight plays is related to how much climbing and how steep the climbing is. Very long, steep climbs, then weight is obviously going to play a bigger role. However, say your average male rider is 175lbs + 8lbs of gear, water + 34lb bike. That's a system weight of 217lbs. Dropping down to a modern 'downcountry/Xc bike might mean a system weight of 212lbs. That's a 2% decrease in total system weight going from enduro bike to a Spur with light weight build. Unless you are racing XC at a very high level and/or you have a ton of steep climbing, weight will not matter nearly as much as fast rolling tires. 

Put a 1350g wheelset with 720g XC race tires on an "enduro bike" and the thing will feel like an absolute rocket. If you firm up suspension a bit, you can even race it in an XC race. I've done this many times. 

Take your 2,000g wheelset w/1300g MaxxGrip Assegais & Cushcore pro, and mount them to your XC bike. Will feel sluggish and slow as hell. 

To say another way, if a world cup XC racer had the choice between racing a Transition Sentinel w/1350g race wheels and 720g XC race tires. Or a Transition Spur with w/1300g MaxxGrip Assegais & Cushcore pro, they would take the Sentinel every day of the week. And if they firmed up the suspension a bit to get sag around 17-20% and it would pedal even better. 

It would be fun to get more data for sure, but I'm lucky to get to ride 1-2x per week and don't love burning it on an experiment day. This is a trail system I typically take my wife/ daughter/ or newbie friends to so it's not my favorite to say the least.

When I hit a more typical trail around CTX, I'll get about 125' of elevation per mile, which isn't a lot compared to better locations, but it's also insanely chunky the entire time. Not to mention 111' yesterday!

However, this easy test loop is only 191' in 5.3 miles, so pretty tame.

Ebruner above mentioned that weight doesn't matter to him. I suspect that on a flat trail like this easy test loop, maybe weight doesn't matter very much because there just isn't a lot of gravity working against you, but once the trail gets steep it could be an entirely different thing. It would be fun to take my XC bike and time it on this loop at stock weight, then with maybe 8#s added to the frame. I bet the difference would be small. But then run the same experiment on a long climb, with and without weight. I'd bet the delta would be much larger on the climb. So the steeper your trails, the more the weight matters (if you care about climbing time/ energy).

The outright speed difference is damn large between the AM rig (with appropriate tires) and the WW XC/ DC rig on the easy trail. On this trail I'll average about 11.4 mph on the fast bike and about 8.5 mph on the AM. It's a short easy route so I'm not limited by the slippery fast tires, limited suspension travel, or even my fitness. Payson McElveen has the KOM at this trail and is quite a bit faster than my all-out sprint on my Smuggler (it has fast trail tires and Berd wheels) I think he was in the high 13 mph range as I recall. The extra fitness he has doesn't surprise me at all, but I'm no slouch through the turns and yet he is averaging considerably faster there too!

Trails like this (easy) are immensely more fun when I'm traveling 35% faster and slipping and sliding the entire time. I'd take my WW fast DC bike against my coil suspended 1400 gram tire having Relay e-bike on this trail. I think the bike would be faster, it would certainly be more fun in these conditions.

"[S]ame watts..."

Here's one problem that the mtb science scene has struggled with (and that some media people who should have known better have spread misinformation around). How do we measure effort? Given some riding challenge, which of two different setups lets the rider complete the challenge with less effort? Or controlling for effort, which of two different setups gets the challenge done faster? Even when we don't care about the actual number from quantifying effort, we need to know it in order to make it the same across the test, and power meters don't really give this information. Heart rate has been used as a really coarse way of estimating effort, but the idea that we could use it to distinguish between bike setups is likely the realm of fantasy (not that I couldn't see a wearable company like Whoop partnering with a misinformation outlet like Outside for just this kind of thing).

I love seeing all the calls to measure stuff. Just be prepared for what may feel like pushback from the people with technical training in measuring stuff. It's not gatekeeping, there is just a really high bar to reach a reasonable level of credibility and confidence. The central flaw in bro science isn't a lack of training or resources; it's the belief that if high quality information has a high price then so-so quality information must only cost you a so-so amount of effort and that's good enough for Youtubing. The reality is the effort-reward curve isn't linear, and so-so quality experimental design gets you basically nothing that could ever stand up to scrutiny.

We get it, rolling resistance and geometry make a bigger difference than weight, but the OP was asking about weight, ceteris paribus. All else being equal. 

This calculator is for a road bike, but could prove to be useful: 

https://www.broleur.com/hill-climb-calculator/

So a rough estimate of a 2kg, or about 4.5 pound weight savings in your bike requires less than 2% increase in watts if you weigh 86kgs (190 pounds) kitted up and your bike goes from 34 pounds to 29.5 pounds. Thats a 6 watt difference. Seems pretty insignificant, until you realize thats roughly the claimed efficiency decrease of adding a well lubed idler pulley, or going up a casing toughness. 

Another way to frame it is that with this   500 meters / 1650 feet climb over 6km/3.75mile, you'll be almost exactly 1 min faster, 44 min instead of 45 min on the climb. To me, that actually matters. This climb is pretty typical for me, if anything on the short side. Finishing that climb a whole minute after your fit buddy is an ego buster.

We need someone to get one of these and do the tests.

https://vo2master.com/

Properly measure the amount of energy burned at set watts to compare different set ups. Then we know what is actually more efficient.

So I agree with everything you've said, but things the physics calculators completely miss about the reality of MTBing, is that we have to shift/ lift/ push & decelerate/ accelerate these bikes around every second of nearly every ride.

It's so much more than just fighting gravity while climbing on a smooth surface. 

I should point out that my SjEvo was pretty trick and in reality, only weighed 4-5#s more than my WW Spur. A lot of people are working with much larger weight deltas.

Unless you're riding Whistler BP level single blacks, something like the new Top Fuel with soft Nobby Nics or Forekasters will thrash the average 32lb trail bike IMO. 

what are you talking about, I only ride IMBA approved trails so its a direct, 1:1 comparison with road bike climbs on tarmac. 

Good thread but cmon guys, a big blinking light that says something to the effect of "NERD ALERT" might be warranted here.  

I do believe we are desperately in need of more scientific(ish) testing in all outdoor sports. Heck, as flawed as it may be, I'm still okay with "bro science", too. If enough of that type of testing gets done, I still think there is signal buried in there that starts to rise to the surface over time. Even if the test is critically flawed, if you are even trying to be objective it often forces a little more intellectual honesty in lieu of baked in psychological biases. 

Back to the OP's post - I feel one thing that makes your question a bit difficult is how broad it is. Don't forget, we're optimizing enduro bikes for the descent, not for the ascent. Hence, I'd be a lot more interested to see how different tire/rubber/weight combos work in a DH type of situation (with enduro bikes), but I'm not too interested in how much it matters over the course of an entire ride. Conversely, with respect to XC or "XC+" bikes, I'd be more interested in the exact type of test you are suggesting. What I'm really looking for is someone to objectively breakdown how much rolling resistance vs wheel weight vs crank weight vs bike weight all really matter when it comes to overall efficiency of translating rider wattage into kinetic movement (speed at a particular wattage). A simple equation could be built, so long as you could ascribe some kind of number to a particular tire's rolling resistance, which can be measured. I know, mountain biking is more dynamic than road riding, but XC has more in common with road than it does something like enduro (rider position is pretty static, most climbing isn't all that technical etc). 

One more resource for you all to play with https://www.bicyclerollingresistance.com - this is another way to objectively analyze some of what is being discussed here. 



 

In my experience, people who are faster than me kick my ass on any bike. The same principle tends to work in reverse for people slower than me. And when I'm riding by myself, the determining factor is typically my ability to focus on speed instead of dwelling on how my bike is or isn't performing*.

*assuming all bikes in question are, at a base level, functional. Wheels roll, tires holding air, suspension goes up and down, brakes work, etc.

You're overthinking this. 

Weight really matters, in fact its critical, if my bike is lighter than yours. If your bike is lighter than mine then weight doesn't really matter, ride quality matters more (of which my bike is superior, of course)

Let me give my best anti-science pitch and anyone can decide the merits for themselves. You raise excellent questions and I like the example of the serious article based on rigorous experimentation, so I'll use your idea. If we want to tell a story that is true to the best of our knowledge about bike weight (how much, where on the bike-rider system, rotating or not, etc., with respect to performance), and it's gotta be probably true and useful to most of our audience, then what we learn from the test has to be generalizeable, and that's why we need the $$$ budget and the fancy rigor. What we learned can't suddenly not be true with a different test rider, or on a different day, a different trail, etc, or else what's the point to the many different readers / different riders / different trails / etc.

So the first question I would ask is, do you really care if the info you get from testing is going to apply to the next guy? If not, then you don't need to do science and can save the $$$ budget for tires and spare yourself the extra time. Factory race coaches and technicians aren't doing stuff like randomized controlled trials but it seems like they are achieving more than putting on a pageant to increase their rider's confidence. So my feeling is it's probably smarter to emulate them than to take a crash course in ANOVA, especially if your plan is to feed your calculator a bunch of sus data from some corny quantified-self subscription wearable (sorry, sorry, please don't shoot the messenger!). Upgrade your membership to 'Pro' and we'll to the p-hacking for you. Tap the Share button to show all your followers the tightest error bars they've ever seen<3 <3 <3 

Whenever i think about putting lighter tyres on any of my bikes, i remember the brendog video where he tried to push a Scott spark as hard as he could to show what the bike could do... The stock tyres popped off on the first corner.

https://youtu.be/-ihU_iRMnOY?si=qQ2jN5YJlZP3IVuK