Reflections on fat-tire road bike geometry

[beeatnik]

Originally Posted by joosttx

650B needs to enter the discussion as they are the future of the gravel bike.... or potential future.

The future of the gravel bike is the skateboard.

[ergott]

I guess I'm an outlier, but I prefer my bars just as low and long as my other road bikes. I have no problem shifting back on the saddle for gnarly descents. Never slowed me down and on my cross bike (same position as well) I ride all the technical mtb trails exclusively in the drops. I'm also old school on my mountain bike and have narrow bars and they are near the same position as I would be if I were on the hoods.

For me, I see mixed surface rides with lots of challenging climbs and my power output has to be pretty high just to keep going. My road position is dialed for this and being more upright wouldn't get my weight over the cranks as easily.

With regards to tire size, I prefer 650b as well. It seems to make more sense to me especially on smaller frame sizes as it's easier to keep road bike proportions. The chainstays are only a little longer at 418mm. 650 gives me room for 42s and fenders. I've even had 50mm tires in there without.

BB drop and front end dimensions are much too personal to standardize. Everyone will have what works for them. My bike is 70mm and my FC is 581mm for a relatively compact 989mm wheelbase.

[dogrange]

Originally Posted by Kontact

My instinct is that a gravel bike would benefit from low trail to deal with slow, technical stuff. The problem being that trail increases with tire size, so when you put the fat tires on you can end up with high trail compared to the skinny tires, and if you set the trail for fat tires, it gets REALLY low when you have the skinny tires on. This would be a good reason to use a 650b for the fat tire and 700c for skinny - to preserve the same diameter and therefore trail.

Totally agree with this. Riding a high trail cross bike is less than optimal on chunky gravel at medium speeds (doesn’t want to turn) and gets truly hair raising at high speeds on pavement (it really doesn’t want to turn and then flops over when you do get it turned.) it is totally baffling to me why virtually all the off the shelf gravel bikes that have been coming out (from the big makers, not elephant or other small shops) are going with slack head angles and high trail. I’m waiting on something like an OPen UP with a 65 take fork - hasn’t happened yet.




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[marciero]

Originally Posted by Bonesbrigade

Ive always wondered about this. Are all trails created equal? Will a bike’s front-end with a shallower HTA and higher fork rake handle the same as a steeper HTA and less rake when both have the same trail?

Ex: 73 HTA and 43mm rake = 56mm mechanical trail
71.5 HTA and 52mm rake = 56mm mechanical trail

Has anyone ever done a comparison. Kind of hard to do as the other geo values would be difficult to keep constant.

Absolutely not. With the shallower HTA the axle sweeps through an arc that is less parallel to the ground when turning. Keeping trail constant, the wheel flop is a function of HTA only, and will change. You can feel this, even stationary, turning the handlebars side-to-side.

[merckx]

Originally Posted by Bonesbrigade

Ive always wondered about this. Are all trails created equal? Will a bike’s front-end with a shallower HTA and higher fork rake handle the same as a steeper HTA and less rake when both have the same trail?

Ex: 73 HTA and 43mm rake = 56mm mechanical trail
71.5 HTA and 52mm rake = 56mm mechanical trail

Has anyone ever done a comparison. Kind of hard to do as the other geo values would be difficult to keep constant.

When wheel flop is considered, all mechanical trails are not the same. Very shallow HTAs with long rakes often have more WF than steeper HTAs with less rake despite the same MT.

[AngryScientist]

good discussion. Thanks hank, insightful and well thought out initial post.

i would really like to ride one of the newest crop of 650b gravel specific bikes, just to see the difference between them and the stuff i'm currently riding. a lot of the conceptual talk is hard to make sense of until you're out on the [g]road.

i definitely agree that 650b is a better wheel size for someone of my small size and keeps the bike's proportions a little more normal, while allowing for more rubber. i'm in the process of putting together a 650b road conversion now, and appreciate all the new tire choices that are out there.

anyone riding cannondale's new road/gravel specific lefty shock?

[Mikej]

The span of bicycle tire width and o.d.'s in my garage is so crazy, 23mm 700c -4.8"x27.5, you would lose your mind thinking you could make them all work how you'd like (well, think you'd like?), but have fun chatting!!!

[marciero]

Originally Posted by velotel

...
Given all this performance criteria, a custom build can be super attractive with the ability to meet any and all needs. Like forks. Apparently off-the-shelf forks for fat-tire road bikes, from Enve for example, are available with only one fork rake, so a front end has to be built around that fork’s rake. A builder who does his own forks could build a frame with a custom fork to provide exactly the handling characteristics desired. I don’t think too many builders do this. Building forks is apparently a pain and expensive. Forks used to be as much the domain of the builder as the frame itself but carbon forks pretty much killed that. Not that people are apparently complaining. My Stoner Bike’s got an Enve CX carbon fork and I’m definitely not complaining...

Custom steel fork is not more expensive than the Enve. Building a custom bike for a dedicated purpose around a front-end geo based on what's available makes no sense to me. If your bike is perfect with the stock 45-ish rake Enve-that's great. But front end geo has the greatest effect on handling compared to things like bb height. Innovative design approaches would necessarily incorporate that. (And I'm not suggesting Erikson and others dont do this and arrive at the 45 rake number.) As far as I know, only Seven does the variable rake with carbon, with offset dropouts, a less than ideal approach IMO.

[Wayne77]

Great thread. Can an expert weigh in on how the effects of pneumatic trail are addressed in designing a bike with 650B wheels? Or is that only an issue when putting 650B wheels on a bike that was built for 700c?

I ask because there are a lot of newer gravel bikes on the market that are positioned as accommodating both sizes equally well...typically stating something like “will take up to a 45mm tire with a 700c wheel or 2.1” tire with 650B wheel”. Maybe they are striking a balance in the middle somewhere?

In any case I’m excited for an upcoming gravel-dream build. Saving my pennies. Currently looking at the Why Cycles R+

-as above, tire clearance for 650B 2.1” or 700c 45mm
-thru axle
-hydro disc
-1x drive
-even a dropper post ;-)
-extra bottle cage mounts

[Kontact]

Originally Posted by marciero

Absolutely not. With the shallower HTA the axle sweeps through an arc that is less parallel to the ground when turning. Keeping trail constant, the wheel flop is a function of HTA only, and will change. You can feel this, even stationary, turning the handlebars side-to-side.

In theory, but what's the reality when the difference between shallow and steep is 2° of HTA and the steerer is never turned more than a few degrees right or left?

Specifically, if you have a 72° and a 74° HTA on two bikes and you set both fork rakes to produce 56mm trail, the difference in "flop" is 12%. Both bikes will have flop (which is good, it helps us turn the bars), but the actual amount of force difference between the two is small.

And it is especially small when you consider that the influence of flop comes from gravity. The amount of weight pushing down on the front end is what produces the flop feeling, and smaller bikes with more flop usually have lighter riders. That also means that flop changes when you are going up and down hill when the front end weight or vector of gravity changes.

So flop is a factor, like stem length is a factor. But who among us is so sensitive to these forces that we notice that the bike seems to handle different when we change the trail with larger larger tires or change to a longer stem?

[marciero]

Originally Posted by Kontact

In theory, but what's the reality when the difference between shallow and steep is 2° of HTA and the steerer is never turned more than a few degrees right or left?

Specifically, if you have a 72° and a 74° HTA on two bikes and you set both fork rakes to produce 56mm trail, the difference in "flop" is 12%. Both bikes will have flop (which is good, it helps us turn the bars), but the actual amount of force difference between the two is small.

And it is especially small when you consider that the influence of flop comes from gravity. The amount of weight pushing down on the front end is what produces the flop feeling, and smaller bikes with more flop usually have lighter riders. That also means that flop changes when you are going up and down hill when the front end weight or vector of gravity changes.

So flop is a factor, like stem length is a factor. But who among us is so sensitive to these forces that we notice that the bike seems to handle different when we change the trail with larger larger tires or change to a longer stem?

It's not so much the amount of weight that you are feeling. The action of turning is different-with shallower HTA there is more of a heeling over of the wheel. I think this is what amplifies small differences and makes it much easier to feel. Also, the front end is lowered when you turn; more so with shallower HTA. Both of these effects are independent of weight.

Tires and trail also produce very discernible differences. These are in contrast to some of the other design details discussed in this thread which are a) the same as for road geo, b) not discernible, or c) a matter of preference d) any combo of the above

[Kontact]

Originally Posted by marciero

It's not so much the amount of weight that you are feeling. The action of turning is different-with shallower HTA there is more of a heeling over of the wheel. I think this is what amplifies small differences and makes it much easier to feel. Also, the front end is lowered when you turn; more so with shallower HTA. Both of these effects are independent of weight.

Tires and trail also produce very discernible differences. These are in contrast to some of the other design details discussed in this thread which are a) the same as for road geo, b) not discernible, or c) a matter of preference d) any combo of the above

The things you are referring to are the wheel flop, and wheel flop is "powered" by front wheel loading. If you unloaded the front wheel of the higher flop bike, you wouldn't be able to detect a greater amount of "heeling over" because the motive force for the heeling wouldn't be there.

In other words, we only experience wheel flop as the tendency of the handlebars to want to turn and keep turning, but that tendency is powered completely by the vertical load on the front wheel. We are absolutely not detecting a geometrical change but a force change.

Wheel flop is a leverage problem. You can change the lever arm to an effect, but the amount of force on the arm is just as important. Higher flop at lower weight is not going to feel different than low flop at high weight.

[marciero]

Originally Posted by Kontact

The things you are referring to are the wheel flop, and wheel flop is "powered" by front wheel loading. If you unloaded the front wheel of the higher flop bike, you wouldn't be able to detect a greater amount of "heeling over" because the motive force for the heeling wouldn't be there.

In other words, we only experience wheel flop as the tendency of the handlebars to want to turn and keep turning, but that tendency is powered completely by the vertical load on the front wheel. We are absolutely not detecting a geometrical change but a force change.

Wheel flop is a leverage problem. You can change the lever arm to an effect, but the amount of force on the arm is just as important. Higher flop at lower weight is not going to feel different than low flop at high weight.

Yes, you feel the effect of wheel flop more dramatically when you load the front. But wheel flop itself is independent of weight.
flop=trail X cos HTA X sin HTA so is independent of weight. Rather than comparing high/low weight vs flop. I am comparing HTA with same weight and wheel flop. If you take bikes with 72 degree HTA and 74 HTA, both with fork rake adjusted to make the wheel flop the same, these bikes will handle differently.

[Kontact]

Originally Posted by marciero

Yes, you feel the effect of wheel flop more dramatically when you load the front. But wheel flop itself is independent of weight.
flop=trail X cos HTA X sin HTA so is independent of weight. Rather than comparing high/low weight vs flop. I am comparing HTA with same weight and wheel flop. If you take bikes with 72 degree HTA and 74 HTA, both with fork rake adjusted to make the wheel flop the same, these bikes will handle differently.

The flop itself has nothing to do with weight, but people experience flop as an acceleration in the way the bars turn. That acceleration is powered by the weight pushing down on the wheel.

If you adjusted rake to make flop identical the trail will be completely different.

[marciero]

Originally Posted by Kontact

The flop itself has nothing to do with weight, but people experience flop as an acceleration in the way the bars turn. That acceleration is powered by the weight pushing down on the wheel.

If you adjusted rake to make flop identical the trail will be completely different.

Yes, trail would be different. The affect I was getting at is separate thing from wheel flop. If you take flop out of the equation there is still a difference in the steering. In the extreme case of HTA =90, the wheel flop is zero. A point on the top of the wheel does not move when you turn the bars. Nor would the frame move if the bike were stationary. If HTA is less than 90 and you adjust the rake to give zero trail the wheel flop is still zero. But now when you turn, a point on the top of the wheel moves in arc. If the bike were stationary, the frame would move, pivoting about that point. I think that this gives rise to subtle but noticeable affect in handling.