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

[andrewsuzuki]

I built my first frame (a mtb) earlier this year and now I'm designing the geometry for a road bike.

Ideally it will handle like your average road bike with ~59mm of trail. Something like my old CAAD10.

I'm entranced by the the flex offered by low-trail steel forks with thinwall fork blades (see this video by Jan Heine). I believe typical low-trail forks offer more flex because of the larger offset and a relatively tight bend at the bottom that allows the lower section of the fork to be near-parallel to the ground. However, I don't plan on riding with a front load so I don't think a low-trail fork is ideal for me.

Here's my idea: use the same fork offset of a typical low-trail bike (around 61mm), but then slacken the headtube (to around 70 degrees) to achieve high trail (around 59mm).

My question is if this theoretical bike will handle identically to your typical high-trail road bike, or if there are any gotchas? Has this been done before?

Bikecad mockup:

[kingpin75s]

Looks solid.

I am actually helping guide a build for a family member that is very similar to this. The Builder has offered a design with a similar fork rake and slacked HTA to achieve the same effect your are looking for, albeit we are going for a little higher trail number than you.

Part of the reason we are going this way is to avoid toe overlap with fat tires and fenders for 700c and a small build. Bike will be disc so will not be getting the supple fork effect.

Should work just fine regardless of your driver.

[v531xc]

I would suggest considering the front-center that will result with a slacker head tube angle and a higher rake fork. That might effect the overall handling characteristic more than just the trail alone will.

[mt2u77]

Originally Posted by v531xc

I would suggest considering the front-center that will result with a slacker head tube angle and a higher rake fork. That might effect the overall handling characteristic more than just the trail alone will.

This! Search the threads across the hall. There’s a good one on the topic of front-center, wheelbase, and trail with input from some of the heavy hitters in frame building.

Handling is more than just trail. It’s also wheelbase, and weight balance front-back. If you use your std top tube length, the front-center is going to put the front wheel way out in front of your mass and it will be wash out prone. If you want to try this, maybe plan on a shorter top tube and using a longer stem to shorten the front center.

[marciero]

The slacker head tube affects handling beyond its influence on trail, wheelbase, and front-center. That is, two bikes that are the same in those dimensions but with different HTA will handle differently because the front wheel sweeps out an arc that is less parallel to the ground with shallower HTA, resulting in the bike being lowered more when you turn the wheel. In other words, decreasing HTA increases wheel flop factor if trail is kept constant (at least for the range of angles that are practical)

Another idea is to sweep the fork back - sort of a reverse rake- then rake it forward. I think this could look elegant if done by a skilled builder, and actually think it's worth experimenting with.

[Peter P.]

There's a reason you don't see your design implemented by any builders, custom or production.

I wouldn't do it. The bike will steer like a lazy dog.

Any fork flex it going to come from the crown area, where the leverage is greatest. Tuning the fork leg diameter to permit fork flex near the dropouts is wildly hit or miss.

If you want to stick with slack, mountain bike-like head angles, choose the fork rakes typically offered on production bikes.

Otherwise, stay with tried and true numbers.

[Mark McM]

Originally Posted by Peter P.

There's a reason you don't see your design implemented by any builders, custom or production.

I wouldn't do it. The bike will steer like a lazy dog.

Any fork flex it going to come from the crown area, where the leverage is greatest. Tuning the fork leg diameter to permit fork flex near the dropouts is wildly hit or miss.

If you want to stick with slack, mountain bike-like head angles, choose the fork rakes typically offered on production bikes.

Otherwise, stay with tried and true numbers.

Yes on all of the above. It is a common misperception that a pronounced curve in the fork blades near the dropouts increased (vertical) flex. Most of the flex in a fork is near the crown (and in the steerer). As noted, increasing fork offset while keeping trail constant requires making the head angle shallower head angle, increasing the flop (tendency for the fork to want to over-turn, especially at low speeds).

Also, the video referenced does show some flex in the fork (primarily at the crown), but it also shows that there is more vertical compression from the tire. Tires are also better at soaking up small radius bumps than forks. If you want to improve compliance, look at the tires first.

[spoonrobot]

You don't want to do this, it'll ride poorly. The compliance is from the blade tubing/steerer diameter - not the bends. Although in my experience there is compliance in the fork from the dropout to the crown - it's not all at the crown/steerer.

Aside, 59mm is mid-trail still. High trail is generally 70mm+ as that's when the differences start to be obvious.

I put the Soma low-trail disc conversion fork (65mm rake) on my DB Haanjo (70° headtube angle) and it rode poorly. It had much less steering consistency during cornering and felt much more floppy than the arithmetic flop number would indicate. There was very little compliance gain as that specific bike has a massive ht/dt and whatever fork flex was gained was not enough to matter in ride feel. It also required adjustment to reach and setback over my prior position but never felt quite right since the weight distribution was off as well.

Here are two gifs to help confuse the issue. First is me putting mild pressure on the Soma low-trail fork mentioned above, with feelers mounted at the dropout and midfork eyelet. Figure 1/3 of my body weight (~70kg) moved the feelers ~5mm closer together (camera shrinks the distance, I had set prior to filming). This was done with the tire at max psi, very little flex there.


Note the lack of fork blade movement here, it's all in the crown for big hits.


You want fork flex? Use a 1" steerer, those Kaisei blades and don't worry about an additional 15mm of rake, it won't matter.

[Butch]

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


I think one of the best things that has happened to comfort riding road bikes is tire volume and rim width. The wider rims allow the use of a wider tire with less pressure and minimal tire roll while cornering. This also allows the tire to stick to the ground, roll well and absorb more of the small bumps while maintaining excellent handling characteristics. The tough thing is you need a stiffer/stronger fork if you are going to run disc brakes. The ISO test for disc forks is very rigorous so the carbon forks that pass for strength weigh 100 grams more than a rim fork. The company I used to test forks told me they had tested several steel disc forks and none passed the ISO standard, they bent. Always a balancing act.

Rim brakes definitely allow for a more compliant fork but obviously can limit rim and tire size.

[andrewsuzuki]

My use case

Speed and comfort are both incredibly important as I'm trying to win a long ultraendurance race. We're talking a thousand+ miles of chipseal. I've been riding on 40mm Babyshoe Pass tires for the past year. They're comfortable and certainly fast enough for most people, but I'm still not sure they're as fast as tires like the Continental GP 5000 (especially the new 32mm variant) for typical paved road riding. Also, narrower tires are more aero. So yeah, I'm a believer in comfort=speed and suspension losses, but not necessarily in the ultimate performance of Rene Herse tires.

And yes, I'm planning on rim brakes so disc brake + supple fork isn't an issue.

Fork crown vs blade flex?

Originally Posted by Peter P.

Any fork flex it going to come from the crown area, where the leverage is greatest.

Originally Posted by Mark McM

It is a common misperception that a pronounced curve in the fork blades near the dropouts increased (vertical) flex. Most of the flex in a fork is near the crown (and in the steerer).

The above video came from Jan's article stating this is a myth, at least for typical low-trail forks. Is Jan wrong here? In case you missed it, in the video there's the hoop of a low-rider rack attached to the bottom and a rando rack attached near the crown, and clearly the two move closer together (though to be fair, it's amplified by the length of those racks). But if you play it in slow motion you can also see the dropout moving relative to the mid fork area before the bend.

The second gif spoonrobot posted does indeed seem to show flex in the crown/steerer/headtube area. But those fork blades could simply be stiff enough that the flex is biased towards the crown/steerer. Additionally, that fork has a relatively straight blade. Regarding the Soma low-trail fork flex (first gif), is the wall thickness of those blades known? Because I'm guessing they aren't quite as thin as the Toei special blades.

Location and radius of the curve

With the assumption that the fork blades do flex and can contribute significantly to overall compliance (assuming thin walls and small diameter)...

Fork blades have a taper -- in the case of the Kaisei Toei Special blades in the video, the radius is at its thinnest in the lower third of the blade (from the start of the curve to the dropout).

Obviously steel tubing is incredibly strong in compression but less so laterally.

It seems to follow that the geometry of the curve does matter. On a straight-blade fork the tubing will be roughly parallel to the bump forces. But if you bias the geometry so that the smaller-diameter part of the fork blade (the lower 1/3) is closer to perpendicular to the bump forces (and is therefore deflecting laterally instead of being compressed), the blade will deflect more overall. Or am I missing something?

Back to the original question...slack headtube + high offset?

A few things:

• I realized that the CAAD10 actually has relatively low trail for a road bike at 53mm. So now my headtube is at a steeper 71.3deg and offset still at 60mm.
• There are now some gravel bikes popping up with slack headtubes. The Whyte Gisburn for example has a HT angle of 70 degrees!
• The kicker I found an article from Rodriguez Bikes suggesting similar numbers, here: https://www.rodbikes.com/articles/ph...vel-bikes.html. See example 2b for the closest numbers -- the Phinney Ridge fork has a 55mm offset and they seem to be using it for both low and mid-high trail bikes -- example 2b suggest a HT angle of 70.5 degrees. This is enough validation for me for a $250 experiment and more framebuilding experience but I'm interested to hear what you all think.

[Ernesto]

I'll add one data point: I have a Velo Orange Pass Hunter in size 53cm. It has a HTA of 71 degrees and fork offset of 60mm (see link).
https://velo-orange.com/products/disc-hunter-frameset

I really enjoy the handling of this bike - neutral and for whatever reason incredibly easy to ride no-handed. Anyway, I realize handling is more than just trail... just my experience with this particular frame/geometry.

[Mark McM]

Originally Posted by andrewsuzuki

My use caseThe above video came from Jan's article stating this is a myth, at least for typical low-trail forks. Is Jan wrong here? In case you missed it, in the video there's the hoop of a low-rider rack attached to the bottom and a rando rack attached near the crown, and clearly the two move closer together (though to be fair, it's amplified by the length of those racks). But if you play it in slow motion you can also see the dropout moving relative to the mid fork area before the bend.

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). And it also shows that the statement in the referenced web page, "As the camera zooms in, you can see how much the fork blades actually flex. "That is what takes the edge off bumps that are too large for the tires to absorb," is at best an exaggeration. Since all the components are flexing, all components are absorbing bumps, to a greater or less degree. There is no single component that is "taking the edge off of bumps", and only when the tire bottoms out are there bumps "too large for the tires to absorb".

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.

[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.

[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.