Road bike geometry: handling with large offset fork + slack headtube?

[Mark McM]

Originally Posted by palincss

Typical geometry of the early 1900s. The Raleigh DL-1 rod brake roadster is a fine example. 66 degree angles, fork rake way out to there.

I wonder what the handlebar width of those bikes was? The tendency to oversteer due to flop increases with the amount of weight over the front wheel (the bike in the picture appears to have a very rearward and upright riding position), and also wider handlebars can help a rider to resist flop.

[Spoker]

I agree there is something to a longer stem and short front end to make things rideable.
The holy grail (for me) is neutral out the seat climbing, and stable "no hands" riding.

[Heisenberg]

Originally Posted by andrewsuzuki

Thanks! I got the front center down to 620mm though not really sure what the ideal is (I'm 6'2). The CAAD10 58 has 601mm but I'm not sure how much of a difference to expect from +19mm? Also decreased the trail to 53mm for a more moderate HT angle and to match the trail of the CAAD10. 700x32 tires. Updated BikeCAD: https://i.imgur.com/gKN6gfH.png

Edit: Funny, Path Less Pedaled just released a video on the Rodriguez Phinney Ridge today. https://www.youtube.com/watch?v=L0g7uIijBPg (describes the high-offset fork around 1:30)

be looking at mechanical trail.

why is the bb so high? gonna go crit racing with your tractor?

[marciero]

Originally Posted by Mark McM

Clearly, all the components in the system flex to some degree - tire, wheel, fork blades, crown, and steerer. Jan Heine's test does show that there is some flex in the fork blades. but it also shows that this flex is small, and that there is far more flex in the rest of the system (particularly the tires). ...

While Jan Heine's test shows that there is some flex in the fork blades, it doesn't answer how it compares to flex in the crown/steerer. To do that, he could have mounted some type of indicator arm to the steerer tube to compare to the arm attached the fork tips, to compare the magnitudes of the total flex in the fork to the magnitude of the flex in the blades themselves. Without that, he has only shown that there is some flex in the blades, but hasn't shown how significant it is.

Sure, he could have done a lot of things to compare with flex in other places. But he wasn’t out to do that. He demonstrated pretty convincingly what he set out to show: fork blades flex, and not an insignificant amount if they are of a certain design. Note that the low rider hoop rises about 3mm as it gets closer. That is pure vertical deflection, and not "amplified" through a large radius. And to my eyes, the video does not show there is "far more flex in the rest of the system". But that is beside the point. Even if flex at the crown is significant or greater, the fact is that fork blades do flex, thinner blades will flex more, and curved thin blades will flex more still. Most of us can feel very small amounts of frame flex in the ride. One bike has "all day comfort" while another one beats us up. Those sensations are about very small amounts of frame flex. And here we have a not insignificant amount of flex right where it has the most influence on the ride.

[Mark McM]

Originally Posted by marciero

Sure, he could have done a lot of things to compare with flex in other places. But he wasn’t out to do that. He demonstrated pretty convincingly what he set out to show: fork blades flex, and not an insignificant amount if they are of a certain design. Note that the low rider hoop rises about 3mm as it gets closer. That is pure vertical deflection, and not "amplified" through a large radius. And to my eyes, the video does not show there is "far more flex in the rest of the system".

The tire is part of the system, and clearly flexes more than 3mm. Plus, this test only simulates landing on a flat surfaces - when hitting small radius bumps (such as cobbles or the edges of potholes) the tire flex will be even greater, and the relative contribution of fork flex will be less.

Originally Posted by marciero

Most of us can feel very small amounts of frame flex in the ride. One bike has "all day comfort" while another one beats us up. Those sensations are about very small amounts of frame flex. And here we have a not insignificant amount of flex right where it has the most influence on the ride.

Actually, blind testing has shown that most riders can not feel small amounts of frame flex. Here's some comments from Josh Poertner regarding blind test of frames and wheels while he was an engineer at Zipp:

I've participated in numerous blind product studies over the years where we controlled bikes or the wheels (I've done this twice with a bike manufacturer during development work around a pro team, and many times with wheels) with fabric shield tensioned between seat post and stem, flat black rattle can paint on everything, etc. In each of these studies, the entire subject group including pro riders, engineers, and other industry people with LOTS of experience, struggled to find any real differences between any of the bikes, until after the study was de-blinded and everybody (including me) instantly began to try and rationalize it all… This is just human nature, we all do it, and from experience, it is nearly impossible NOT to do it.

One of the major discoveries was that after controlling for seat post (round post shimmed into aero frame so as to not give it away) not a single rider found the aero road bike to be less comfortable, less compliant, etc, than the identically setup 'endurance' or 'roubaix' bike (clearly this leaves room for the aero seat post to be why people feel aero bikes are less compliant..seatposts generally have more effect on bike compliance in the lab than frames do, but that's another story). We ran blind wheel tests a couple of times a year at Zipp to benchmark competitive wheels and our own prototypes, and we also found that blinded riders were generally unable to tell the difference between stiffness and inertia, had no reliable feedback on weight, lateral stiffness, or comfort in general, and in the end were generally only able to pick out the aero wheels because they were riding laps around a closed park environment using power, so the more observant ones would notice speed differences. In the end, we sort of determined that when riders didn't know what they 'should' feel, they really struggled to find differences in stiffness, compliance and weight between frames or wheels. The strongest correlation we ever saw was to tire pressure, but not in the way you would expect. Almost everybody assumed the setups with lower tire pressure to be the endurance bike and would then score it exactly as you would expect a magazine review of a comfort bike to look…so we determined that we all naturally would latch onto something we were confident in, in this case comfort, and then would proceed to perceive everything you expected from that bike: less aero, less stiff, better damping, etc. Imagine the shock for the group when it turned out that the it might have been a super stiff race bike, or an aero road bike! Let the rationalizing begin!

Others have also done blind testing of frame compliance, and had similar results.

[unterhausen]

Originally Posted by palincss

Typical geometry of the early 1900s. The Raleigh DL-1 rod brake roadster is a fine example. 66 degree angles, fork rake way out to there.

actually doesn't look to have that much fork rake in comparison to a lot of the current low trail bikes. It's just exaggerated by the slack head angle. I'm curious what the trail is on the pictured bike. That sort of explains why the wheels would flop over annoyingly when parked. The head angle in combination with the high flop from mid to high trail meant there is no way the wheel was going to stay straight.

[andrewsuzuki]

Originally Posted by Mark McM

Actually, blind testing has shown that most riders can not feel small amounts of frame flex.

Mark, I'm 100% with you on frame compliance (at least for metal bikes). But forks are clearly different because they're cantilevered. Just like how Josh mentions they had to control for seatpost geometry...because seatposts are cantilevered as well (though he didn't mention controlling exposed seatpost length, but I assume they were all compact geometries). Though most rigid forks, including steel ones, are so stiff that they can basically be factored out of the overall comfort equation, and I'm guessing that was the case for all of the frames they tested.

[colker]

Colnagos (55cm) have close to 71 degr. head angles. Just slap a fork w/ whatever rake you think of on a Colnago and see how it feels.

[andrewsuzuki]

Originally Posted by Heisenberg

be looking at mechanical trail.

Originally Posted by Butch

I agree with what has been said, way more to handling than trail and a very shallow head angle creates wheel flop.

Using the v2 bikecad geometry above, I've nearly matched the trail, mechanical trail, and flop to the caad10.



Now it's time to CNC an elliptical bending die. Expect another post in about a year

[Mark McM]

Originally Posted by andrewsuzuki

Mark, I'm 100% with you on frame compliance (at least for metal bikes). But forks are clearly different because they're cantilevered. Just like how Josh mentions they had to control for seatpost geometry...because seatposts are cantilevered as well (though he didn't mention controlling exposed seatpost length, but I assume they were all compact geometries). Though most rigid forks, including steel ones, are so stiff that they can basically be factored out of the overall comfort equation, and I'm guessing that was the case for all of the frames they tested.

Sure, a cantilevered fork can be made with some meaningful compliance. Well, meaningful if the rest of the components are very stiff. But in the video, the fork is mated with a wide and very compliant tire, and it can be seen that there is far more compliance in the tire than in the fork, making any fork flex less meaningful. Like in Josh Poertner's test, it is likely that adjustments in air pressure can influence compliance more than the fork.

For years, bike manufacturers clung to the myth that rigid frames/forks can be built with built-in "ride compliance" (remember all those "stiff yet compliant" claims?). But more recently they've given up on this myth, and there are now frames that incorporate moving/flexing joints (such as Trek's IsoSpeed), which can actually create meaningful compliance.

[andrewsuzuki]

Originally Posted by Mark McM

But in the video, the fork is mated with a wide and very compliant tire, and it can be seen that there is far more compliance in the tire than in the fork, making any fork flex less meaningful.

I certainly don't disagree. Not sure if you missed it but I currently use those same tires on my full time road bike. This new bike however will be constrained by 32mm GP 5000 tires because I believe those are the fastest practical road tires available (if there were a 40mm tire available that was as fast on pavement, I would use it). So that's why I'm looking to the fork for more compliance -- even with 32mm tires, wide for paved road standards, road buzz gets though on bad roads, chipseal, etc. I've considered the Lauf Grit fork but it's probably overkill, too expensive, and less aero.

Edit: and even with those extralight 40mm tires at the lowest psi I'm comfortable with, road buzz still gets through, so I'm interested in fork compliance regardless.

[Butch]

One more thing to consider in the design is weight distribution. If you increase the front center, strongly consider increasing the chainstay length so when you sit up to pull your jacket off while riding not all the weight is on the back wheel.

[marciero]

Originally Posted by Mark McM

The tire is part of the system, and clearly flexes more than 3mm. Plus, this test only simulates landing on a flat surfaces - when hitting small radius bumps (such as cobbles or the edges of potholes) the tire flex will be even greater, and the relative contribution of fork flex will be less.

Was not disputing that tire flex is greater, just that the fork contribution (in this case anyway) was enough to be significant, and noticeable. And you can always decrease pressure enough to mask flex elsewhere, but not without affecting handling.


Point regarding Josh Poertner's remarks well taken. Still, without drilling down into one study or another it is hard to apply these general remarks to the present situation. If riders "struggled to find any real differences" does that mean they could not tell them apart? If not, in what way were the bikes in fact different? In fact, it sounds like they all were able to distinguish between the aero and the endurance bike, they just didnt correctly label them.
I do recall blind tests tests of near-identical bikes-steel with the same diameter tubes with similar butting-that is, near identical flex characteristics-and riders could generally not tell them apart.

[Hellgate]

You need a 333fab Air Land Sea frameset.

I'm struggling with needing 40mm tires on chip seal. Seems a bit excessive.

Have you tried a Brooks saddle? It does an amazing job of killing road buzz.

[Heisenberg]

why don't you just jet to CPH or AMS? the bike of your dreams is probably right outside the front door of your terminal, and a ripping €80. (also the food, THE FOOD)

no really. wheelbase is a thing. weight balance is a thing. fc is a thing. the bike you want to build doesn't really exist because it (probably) sucks. buy the fox gravduro suspension fork and be done with it if you won't deal with rough roads+road bike.

otherwise you're not building a road bike.

you're building a city bike. which is fine, i guess. if that's your thing. geometry is a very overall/holistic endeavor, not one pivoting on a pretty simplistic & VERY nebulous one-dimensional approach around making your tookus feel less discomforted.

ps kaisei is stiff as **** if you think it'll improve ur smoove. it won't.