Based on a few comments from folks such as @Daniel_Y and @Equinox I figured it would be worthwhile to have a topic on some of the myths that all of us have come across over the years and put some numbers to them in order to provide some rigor to the answers that we have for customers.
To piggyback off the list that @Daniel_Y has already put together, what are some of the myths that need “busting” aside from the ones listed below?
Rear Triangle stiffness
Front Triangle stiffness (NEW!)
Effect of Headtube diameter on Front Triangle stiffness (NEW!)
Seatstay brace or no Seatstay brace
Frame fatigue life
Fatigue of different joining methods
Validity of ISO standards for custom framebuilders
Effect of Headtube/Downtube gusset on Headtube stiffness (NEW!)
As with any good study, we would also need to validate any FEA work, but we can start digitally and work to physical validation later. Would there be any interest in any of these topics?
Can you tell me more about what the myths are? I mean, what is it that people believe about rear triangle stiffness, or fatigue life, or whatever, that isn’t true?
I guess honestly, for me, the biggest myth is that lateral/torsional stiffness is actually desirable beyond a certain point. Early aluminum motorcycle subframes in the 1970s were super, super stiff, and the riders all thought they were great (they felt fast), but they were a lot slower, because it turns out you want quite a bit of lateral flex out of a frame for something with wheels that don’t track each other in a corner.
Totally agreed. @Neuhaus_Metalworks and I try to tune the torsional stiffness of our frames. Its easy to build prototypes and do back to back AB testing. The hard part is translating that across an entire size range. As frames get smaller, they deflect less (stiffer), and the riders typically way less too. That’s where I try to use some modeling to guide our decisions.
I’m not an FEA expert at all, so take this analysis with a grain of salt. I tested three loading modes:
With this janky study, I found the weakest bending mode is lateral torsion. So I concluded that front triangle torsion is what dominates the ride feel. A pure lateral load at the headtube never really happens, and fore-aft torsion is probably more affected by fork stiffness (rigid) or suspension travel.
Interestingly enough, the numbers are very similar to one of the few published frame stiffnesses i could find:
I think the ideal stiffness is subjective. For me, the question is: what parameters have the biggest impact on the outcome? Hypothetical question: let’s say we wanted more rear-end compliance. Which parameter has more of an impact?
drop the seatstays 50mm?
20mm longer chainstays?
downsize the seat stay from 16mm to 13mm
downsize the chainstay from 22mm to 19mm
Or maybe none of these change the stiffness. That would be great to know!
I pretty much only ride mountain bikes, so for me, the flex/give inherent in the various other components overwhelms any conceivable rear end flex (it’s a truss…) to such an extent that there’s no point in really worrying about it. Hell, even on a road bike 5psi of tire pressure is probably more noticeable than anything you can do to the frame. You’ve got tires/rims/seatpost/bars/grips/saddle/etc all moving quite a bit under typical loads.
Velonews did a seatpost flex test maybe 10-15 years ago that was pretty neat. It wasn’t super surprising, but it did make it clear that seatpost diameter (duh) and extension (duh) were the main factors. Material mattered less. On a long 27.2 carbon post you can see the saddle deflect noticeably when riders hit bumps, so really I’ve always felt the most meaningful thing you could probably do for in the saddle comfort from a framebuilder’s perspective is make things as compact as possible and put a mile long post on the bike. Of course some people don’t like the aesthetics or want a big frame bag or whatever.
Even riders on the same size of bike may have different ideal springyness. For example in the BQ double blind tests I couldn’t reliably tell the riding difference between 9/6/9 and 7/4/7 standard diameter tube sets, but Jan and Mark could. However I’m quite sure that I can tell the difference between 9/6/9 OS (which feels slow as molasses to me) and 9/6/9 standard. I’m 30lbs heavier than Jan and Mark (155 vs 185) so it makes sense that we’d have different ideal bikes.
If you do the test frames please have a test administrator who can make sure the test is double blind. We did this by having only the stem cap differentiate the bikes. Hahn Rossman swapped the stem caps between most rides and wrote down which cap went with which bike (he only knew them as 1,2,3,4 until the testing was done when he called Jeff Lyon to find out which tubes were used). We wrote notes based on the stem cap color and which ride it was. It’s a lot of work, but protects from biases.
Unfortunately I don’t know of a good way to build a universal model that works for all riders.
One thing I’ve wondered about is chainstay length and rear tire traction. If you look at those climbing motorcycles or sand bikes they have very long rear centers for the purpose of traction but mtb designers always claim shorter means more climbing traction.
I know it‘s not as simple as a single parameter of chainstay length and involves front center and your overall center of mass but like compliance arguments I wonder if there’s a generalization to be made with rear center and traction?
I’d also like to know if there’s another variable worth considering like the horizontal distance between the rear axle and the center of the saddle, instead of rear center. So basically the Jones LWB with a 71 sta and 483 CS vs a frame with 75 sta and 420 CS.
I know what I think from the bikes I’ve ridden but I’m curious if there’s a way to dial in the numbers and ride characteristics.
FYI (I have only attempted ride one once, up a very small hill) the long swingarm on hillclimb motos is for loop-out prevention, not traction. Paddle tires take care of the traction.
Traction, assuming the same tires, is just a function of how much weight you’ve got on the tire that’s doing the pushing. Just like your truck in high school with the sandbags in back (I may be dating myself…)
The difference is pretty subtle in the range of chainstay lengths we’re talking about, though, IMO.
That’s kinda my question about the Jones versus a steeper sta and shorter CS bike, if your ass is the same horizontal distance away from the rear axle, will traction be the same? Or is there something else involved than short stays that adds to the better traction? I don’t know how close those two style bikes are in that distance in reality but I’m imagining they’re not that far off. I’ll have to measure but I don’t have a Jones to test ride. He likes to show how easy they are to wheelie, but that’s more than just CS length.
With the new long front centers shorter chainstays have fallen out of favor because of too much weight on the rear and not enough on the front. So builders are steepening STA’s a lot and adding 1cm or so of CS length. Anecdotally people say these bikes climb well or better than previous models. Is this a real effect or marketing?
I’m not sure I would agree that there are a lot of myths in framebuilding. There is a lot of marketing puffery involved in the sale of frames and bicycles though.
I have a large Jones LWB, and have about an 810-20 cm saddle height. I also love doing wheelies. It wheelies wheely well, but takes a bit more effort to get it up! I think the upright riding position that you’re meant to have on that bike means that traction is quite good when climbing steep loose stuff even when out of the saddle. However, I don’t know how the LWB feels for smaller riders, I imagine traction could suffer in the smaller sizes.
I did look at the horizontal distance between saddle and rear axle as a key comparison between the my Jones and frames I’m building. I found I prefer a steeper STA, so I shortened the chainstays to maintain a similar horizontal saddle to axle distance. They climb similarly to the Jones FWIW.
I recently got to chat with a friend I made a bike for. He told me to make the chainstays “as short as I can”. His rear axle is sitting at 410mm front center is at 785, with a really high BB 343mm. He rides in the bellingham, wa area and says it climbs like a beast. Not super confidence inspiring on the downhills, but he still has fun.
Some people like playful bikes others like to feel planted. When I run a longer CS its harder to wheelie and feels sluggish, to me. However, I end up configuring my bike set up at the time changes the ride for a while, but ultimately, I adapt to how the bike handles. I think there is a lot of marketing hype with regards to tube sets and frame material. I understand that the materials themselves have different characteristics, but as far as ride influence, I think that’s mostly determined by the component choices and tire pressure.
I’m not saying you’re doing this, but one person’s playful is another person’s sketchy. I’m referring to the bicycle journalist industrial complex and the way they describe how a bike rides, assuming that their impressions translate perfectly to all the other frame sizes (and even to frame sizes that don’t exist for that particular bike). I don’t think chainstay length really captures the whole picture obviously, but I still think describing them as being long or short should be relative to proportions, not an absolute. I like my bike to be really playful, I love doing wheelies and jumping and slapping turns etc and it’s annoying when regular heighted people are like oh ‘those are long chainstays, must be really stable’. Ok rant over.
Back on the topic of this thread, it is totally in my intention to bust some of the myths laid out on this tread, I’m just too stacked with other projects right now!
I would love this. As someone on the opposite end of the size spectrum, the options in terms of fit and geometry are equally terrible and I’d love to bounce ideas off of others.
I’m digging this up because I wanna hear some more opinions on this. I’ve been wrestling with the, shall we say, lack of stiffness in my current bike. Wrestling isn’t really the right word, noticing? Observing?
Just sitting there static, minding it’s won business with no weight on it, it approaches wiggly. Ok, it flies way past wiggly. Between the wheelbarrow bearings in the main pivot and the spherical bearings on the linkage and connecting rods, it has a noticeable amount of looseness. As soon as I weight the suspension, it feels like it goes away. At least, it’s not noticeable while riding. I don’t notice it climbing or descending and despite my best attempts to get it to feel weird, it just doesn’t. I will say that when it’s leaned over and pushing into a corner, it feels a little vague (?) for a second, nothing scary or weird and I have previously chalked it up to the really high pivot stretching the wheelbase out as I push into it. The side effect of the static wiggle (Truffle shuffle?) is that it’s super supple, I can get the 500lb coil to break free with just the pressure of pushing the seat by hand when the dropper remote is pushed. It’s a sofa. Add to that, the anti squat is about 35% in the big gears and it’s fast. Scary fast.
Not quite sure what the point here is…wiggly bikes FTW?
He commented how much smoother the HB was, even though Honzo had more travel and bigger tires. I think that came down to lateral flex.
For the Honzo, the primary requirements were to make a lowcost frame that does not break. The more flexible the frame, the shorter the fatigue life and the closer you are to the ultimate yield strength. If I were designing the Honzo, I would be conservative on the tube spec, just to reduce the percentage of RMA’s.
Full suspensions are a totally different story. Lateral flex is good, that is why the world cup pro’s de-tension their wheels. But too much flex can contribute to bearing and shock wear. It must be a tough balancing act.
