Stiffness versus Compliance

Frame stiff but maybe the chainstay compliant, wheels compliant radially yet stiff laterally. Stem stiff, bars compliant but then only in one plane if you line up very faded little marks. Seat post stiff, but compliant seat. Fork stiff fore-aft, yet compliant in some other direction for better tracking. Hub should have instant engagement, except while rolling at which time they should have decidedly poor engagement. That should do it, the perfect bike if you will (wheel?).

Oh yah, add compliant radial tires.

Personally I feel that most frames are overbuilt and too rigid for the average rider.  After watching in person the pros at MSA, it’s clear that the forces they are putting into the bike are absolutely unreal.  So a frame to a top 30 rider I can understand why they get to that stiffness level to give them the support they need.  

I’m a believer in that we should have a consumer spec and race spec. Right now, the Commencal Supreme V5, Frameworks and Atherton are riding the fine line of what feels good to a consumer yet can deliver  support to their top pro athletes(if they are in fact on a production frame).

Not sure how we got to this point where everything is so stiff. But I’m happy things are moving towards a more compliant and less of a teeth rattling ride.

Carbon wheels have come a long way and wayyy over built when they first came on the market. Check out this from Enve new rims and Crank Bros, things are definitely going in a better direction for consumers.

Another area that can help the ride feel that I think is really over looked is spoke diameter and tension.

 But the biggest area for improvement in my opinion that is needed is frame compliance, everything is a bandaid fix for stiff chassis. Personally I’ve never rode a bike with a front end that is too soft, rear end yes.

Such a tricky thing, right?

I've noticed that in my constantly loose terrain with poor traction, that the more noodle like a bike is, the better it seems to work.

Someday, we'll be 3D printing our custom frames with custom geo, leverage ratios & even frame layup.

One thing I think we ought to note here is we have so many methods of tuning stiffness and compliance. Some are cheap and easy, others are expensive and difficult. 

What I'd wager we start to see, for those who are actually good enough/ride enough to care about this, is most end up tuning compliance through the handlebar, wheel and even fork (38 vs 36 etc), but frame stiffness is usually left alone. That will suffice for 98% of riders.

That said, manufacturers have already recognized compliance in the rear end of the bike makes for a better riding bike if done correctly, so we'll see a little acknowledgement of this, too. I'm just a much bigger believer you can get more flex out of a wheel than almost any other part, if you so desire.

It depends so much on the rider, their weight, the trails they're riding, etc.

I've ridden wheels in the desert that felt great, only to ride them in the pacific northwest and finding them to be noticeably flexy.  You can put a lot more lateral load on a wheel when you're hooking up hard in grippy loam.  I think vertical compliance matters more in rocky, sandy places, and lateral stiffness matters more in loamy, grippy places.

I've ridden frames that felt solid and precise on flowy jump lines, only to find that they're overly stiff and get chattered off the line in chunky, rooty terrain.  But I'm not a huge guy, so I could see that frame being perfect for someone who weighs 50 lbs more than me.  And the frame that I'm riding that has goldilocks levels of stiffness and feels just right for me maybe feels like a wet noodle for a big guy.  

I've ridden bars that I think feel great - they're precise, and my hands feel fine.  But other people ride those same bars and say they're brutally stiff.  I think different people are sensitive to different frequencies.

Which is all to say, I don't think there's a universally correct answer.  Different people in different places will want different things, and they're all correct.

The engineers know far more than I do, so I'll take their word that the compliance is in the right ballpark. I'm not much of a test rider anyway; the only flex I can ever feel is when I try to push a Harley-Davidson into a parking spot and the steel handlebars flex about an inch and a half. 😅

I'm sure there's a rider weight factor in this, too. Lighter guys like me don't always flex things as much, perhaps. 
From a philosophical standpoint, I'd argue that zero flex anywhere is ideal, but of course that's not true. 

"How is compliance quantified?"

Whichever best fits your marketing requirements.

I’ve tried 28h front and rear aluminum rims with thin spokes to negate a stiff frame and I didn't like the feel, it was too wobbly in corners. I think there needs to a compromise of both.  

Ya know the first step is understanding there is a problem. Hopefully this thead will make riders start to think about their bike setup choices.

For example, I was riding a bike park last weekend and a friend hands were going numb by the bottom of the run. I look at his bike and notice 20mm rise bars, straight gauge stiff spokes, too much air in tires, full carbon frame.  I definitely won’t lie and sometimes it’s riders fitness is the cause but giving yourself the last mechanical advantage will only help.

There is a podcast on Bike And Big Ideas with the guy from Enve. They are just comparing to it’s previous model. I think it’s in a rig with rim only.

https://podcasts.apple.com/us/podcast/bikes-big-ideas/id1461144682?i=10…

It's really all relative to how much you weigh and how hard you ride. Both of these factors put more force into a frame and require more stiffness to compensate. I have a light friend who can get away with 24 spoke wheels and lighter casing tires, at 220lbs on an XL frame, I put very different stresses on the bike than he does and have very different requirements from my equipment.

I think we could figure out some type of compliance scale based on these factors below.

Rider weight: 

125lb more compliance needed.

250lb less compliance needed.

Rider ability:

Beginner rider could use a softer setup.

Pro rider could push a stiffer setup harder.

Riding style: 

Playful park rider could use a stiffer setup(but not always).

Downhill/enduro smashing into rocks and off chamber could use a softer setup.

Dirt type (grip): 

Dry dirt/hardpack could use more compliance for extra grip.

Tacky dirt, clay, loam can run stiffer setup.

Everyone is going to have a different take on all of this, but I've always believed fitness + skill > bike or bike setup (within reason). To add, like I posted in the fork thread, testing all this stuff becomes incredibly challenging with how many variables are in play (and out of one's control) day to day to day. 

I'm not trying to say people shouldn't seek more compliance out of their bike if they want it, but I also feel its important to know what its really worth and to know the adjustments at our disposal aren't as straight-lined or easy to implement as they may seem. For example, going to a lower spoke count and thinner gauge spokes may actually create more of a "spring" sensation than actual compliance (I'd argue compliance would come from lower spoke tension). This actually goes to show a the big point - I've been around bikes for longer than I care to admit and I can't tell you with confidence the "most compliant wheel setup" for a rider. I can make some assumptions (alloy rim, lower spoke tension etc) but I've seen no data to actually inform the gauge/quantity part of the equation. Embarrassing. 

Regardless, I'd argue outside of handlebar (which is highly personal and very important with respect to fit), most of this stuff is more for the racer at the very tip of the traction/speed spear, not as much something most people are really going to need to worry too much about.

I totally get what you mean on the wheels. Those experiments for me are endless, so many combinations to try.

But as far as the ride feel only being for a “pro”, I don’t agree. Maybe a beginner or intermediate rider won’t say, “wow this bike is super compliant and has grip” but they could notice more comfort over their previous bike, maybe they think the tires are better or the suspension is better.   

Agreed with the "only for pros" part not being true. Getting the right type of component is VERY important for beginners too as it defines the confidence levels.

Trying out very soft skis for example showed me how skis should actually perform doing carving turns, finally getting me to improve my technique by miles compared to before.

A good biker (skier) will also be able to work around the gear, a beginner has so much work to do with her/himself that the correct gear is essential for the initial model building. 

I think what drives me nuts is, you would think the suspension would be able to be good enough to allow the bike to be stiff.

One thing from Avalanche, he suggest Boxxers (35mm) for light riders and Fox 40s for heavy riders (or actual racers), for the stiffness difference.

I think that the frame compliance needs to work together with the suspension, I don’t think suspension will ever soak up every bump and frequency. I think that’s why the mass dampers are coming onto the scene. It can help eliminate some frequency that the suspension doesn’t need to, so then you can tune the suspension to soak up other forces.

Totally agree about forks!  I try and suggest to friends to try something other than the Fox40. I find it way too rigid and I’m 180lb.  The Ohlins has a much better feel. The new Boxxer 38 is the second most rigid chassis IMO. I’ve done a lot of testing in that fork to reduce the rigidity but could never get it better than Ohlins. I like the damper and Airspring better on the Boxxer but chassis was stiff!

Rigid to Compliant Forks (That I’ve tried)

Fox 40

Boxxer 38

Ohlins DH38

Dorado carbon Pro 

Boxxer 35

There is also a durability factor that is being designed for. An over built, durable frame is less likely to deforme or break in a significant way, and the trade off is weight and stiffness. Compound that with the fact that very few of riders actually maintain their bikes the way they should, and it's easy to see the reasons most of us have stiffer bikes than we actually need for the strain we put them through. If you are selling a bike, do you think the average consumer will complain that their bike is a bit stiff or harsher through rough terrain and they get pushed off line, or that they are breaking components? Considering the average state of peoples bikes that come in for me to work on, I say it's the latter. 

As far as frames, they could make estimates based on frame size (XS gets very light layup, XL gets much heavier layup, etc.) and then just offer a removable frame and/ or chain stay brace for those of us that actually think we know what we are doing.

That would likely cover a lot of scenarios, including maybe different terrain.

This is also why carbon is such a good material for bikes, since through layup and the resins used you can tune flex and durability with less compromise, and more specifically across the frame in ways that you can't with aluminum. 

I think there is a lot of room for bikes to improve in the stiffness and compliance space from what I see from the racing world, but I personally don't have a good sense of it. It would be really interesting to try more and learn how much of a difference it makes. Generally, it will be very hard for people to really understand the impact of stiffness and compliance without having a bike (like the Commencal) where you can adjust seat stay bridges and things, as you will always be comparing two different frames. I also think the 'symptoms' of too flexy are also a lot easier to detect than too stiff. For example, on my Forbidden Dreadnought, I had quite a few issues with the rear wheel buzzing the frame/chainguide when sprinting or cornering, and getting a lot of "load up and release" sensations from the rear end. On other bikes, which may be 'too stiff', the symptoms would be more subtle (chattering, getting fatigued after fewer runs, etc.). The too stiff symptoms are a lot easier to put down to other things (the trail is rough, my arms aren't strong enough) than the too flexy symptoms. 

I also think a lot of it will depend a lot on the rider weight/style/speed and the frame size they ride. I ride medium sized frames, am a racer, and I am pushing 200lbs in gear. I suspect I am quite a ways outside of the 'median weight' that a manufacturer designs their medium frames for (assuming they do actually change the carbon layup between sizes). I think this is where those adjustable bridges could come in really handy - allowing a 5 foot 6", 130lbs rider to not get rattled to death, while also allowing someone like me to have a bike that doesn't spring back unpredictably. In terms of adjustability, it would be really interesting to see how flex adjustments would compare to say, 0.5 degrees of head angle adjustment (does it have a bigger or smaller impact on the ride characteristic?). 

Lastly, I think this stuff also comes down to the terrain you ride. Like suspension, I imagine a stiffer frame will work well on short, rough trails, with big G-forces, whereas a more compliant setup will perform better on longer trails with medium sized hits, off cambers, etc. I notice this a lot with my own suspension setup. My local trails are mostly around 2mins with either small bumps, or massive rock slab G-outs, with very little medium sized chatter. As a result, I tend to run a stiffer setup that holds up for the big hits and allows me to pump the bike well. However, when I go places with 5+ min descents, my bike gets pretty hard to hang onto. I imagine the same goes for frame stiffness. 

All in all, I think its very hard for most of us non-pros to actually know how big of an impact it has, as we don't really have the means to test it. And like everything else in MTB, terrain and rider will matter a lot in determining what is "good" and "bad". 

Everyone bitching about stiff wheels putting 20psi or 120 psi into the tires?

I do believe that industry overall made amazing progress on making fork stiffer and passing down 35/36 size to trail bikes.

The wortst thing is typically have noodle front end riding downhill and not able properly control it

If we're going to dismiss things because fitness and skill always matter more, then we might as well abandon the whole damn website.  Same for the doesn't-matter-for-98%-of-riders argument, which could be (and often is) trotted out every forum discussion.  

On compliance vs stiffness specifically, I think there is HUGE application for non-racers.  The way the bike industry markets products based on their race pedigree exposes the hubris of many consumers and is just plain silly if you really think about it.  WTF am I supposed to do with a frame optimized for a world cup downhill or enduro pro?

It often seems like the arguments to dismiss the importance of technology like compliance, DAQ, mass dampers, etc. are predicated on the idea that "better" means "faster."  But I think for the proverbial 98% that's not the case at all, especially for compliance.  If the stiffness/compliance of my bike as a whole were tuned for me and that meant my 40-something body could hold up for an additional day of park on a riding trip or that I was less fatigued towards the bottom of runs, that would absolutely make a massive difference to my riding experience - not because it made me faster, but because it allowed me to have more fun.

I'm not dismissing it. I literally wrote - "I'm not trying to say people shouldn't seek more compliance" and went on to suggest how murky of an understanding we all seem to have with respect to to the topic at hand. I could write a lot about this, but ultimately am really just trying to ascribe a "weight" or "coefficient of value" to what we're talking about in this thread. Unless you have some outlier uber stiff 2x4 setup or are super light/super heavy, this is probably one of the last things I'd be thinking about when chasing a better setup. Handlebar being the exception, but I put that in the "fit" category, not in the "chassis compliance" category. 

YMMV.

I can’t remember who said it (robot maybe?) but I definitely agree. I’m always riding fastest right when my wheels lose their spoke tension. 

How do I get that compliance to last more than one turn?