Understanding Descending Mannerism of Bikes

[RyanH]

I've been thinking a lot lately about how different bikes behave going downhill. I've never been able to really put my finger on why a certain bike feels better than another. Bikes with seemingly the same geometry can behave differently going downhill.

Looking back at the bikes I own, the 2013 Evo (sz 50 with 63mm trail) still stands at the top of the list. It was a bike where I'd hit a familiar descent and get to the end thinking, man, I could've hit every turn faster and then going home to see that I set a PR despite that. It just urges you to push the bike harder and gives you the confidence to do so. Jump forward to present and I'm scratching my head why this Crux is so damn good (sz 52).

On paper, the geometry of the Crux and Evo are quite different. Evo had short chainstays, steeper HTA and lower rake fork while the Crux is longer and slacker. With that being said, the Crux with 30mm tires (actual) has a trail of 64mm. Geo difference here:



Lastly, my Litespeed T3 has 58mm trail and feels comparatively nervous going down the same roads. I have a longer fork and taller headset to hopefully slacken it to bring the trail number to 63mm so it'll be an experiment in altering one element and seeing how much of a difference 5mm of trail makes.

With that being said, what I'm trying to understand is what makes the bikes feel so planted going downhill that is absent in other bikes? What other bikes are notable in that regard? If someone asked me today to design a custom bike I'd be more confused than ever on what I'd want and less confident that I'd be able to replicate these characteristics unless it really is all down to the trail number.

[CarlosFerreiro]

I'm interested in what geo "feels like" too, especially after your comments in the other thread about not liking the SuperX.
Ironically I went from a 51cm US CAAD8 on 25/28mm tyres to a 54cm SuperX on 32/35mm tyres and really felt nothing negative at all, a very familiar feeling behaviour. A quick squint through Geo Geeks would actually put your SuperSix if anything closer to the SuperX than the CAAD8 :S
Although I'm not doing your style of descending and I do fear I may be pretty insensative to bike changes, and I am also not doing back to back changes to maybe highlight the differences more.

[Kayrehn]

I found this old article from cyclingtips.com very useful on this topic when I wondered about the same thing in the past https://cyclingtips.com/2018/11/the-geo ... -steering/

Sent from my SM-A125F using Tapatalk

[pdlpsher1]

A bike can be either agile or stable, or somewhere in between. But a bike can't be the most agile and stable bike at the same time. High speed stability and low speed agility are two competing objectives in bike geometries. So a bike that inspires high-speed descending confidence won't be well suited for a descent with lots of tight turns and switchbacks. On paper the Evo is better suited for slow speed descents, and the Crux is better at higher speed descents. So it begs the question for Ryan- does both the Evo and the Crux do well in all types of descents? The answer should be a 'no'- since a bike cannot be the most agile and stable bike at the same time. But I have a working theory to solve this mystery. In general the bike with the longer trail will be less twitchy and gives you more confidence. It's the feeling that the faster you go the more stable you become....very confidence inspiring. It's very possible that Ryan is finding the Crux to be a great descender because it inspires confidence as the speed picks up and encourages him to push the envelope. In contrast the T3 is jittery and unerving, causing him to hold back instead of pushing the envelope. Now why the Evo is also a good descender is a mystery. On paper it should feel similar to the T3. I still think without a back-to-back comparison it's not possible to judge and compare the Evo against the Crux. It's possible that Ryan only remembers the fond memories in the Evo. In a nutshell I think the Crux is in fact a much better descender than the Evo. If Ryan is setting new PRs on the Crux then that's an objective proof.

ps my main road bike is a custom bike, one that has a long chainstay (425mm), long wheelbase, slack headtube angle, and a low BB (80mm drop). I find the bike to be the bees knees on descents, as well as in any other riding conditions. That's why I find the Crux to be very intriguing as an option for my next road bike. And not so much on the Aethos at least on paper.

[Mr.Gib]

But a bike can't be the most agile and stable bike at the same time. High speed stability and low speed agility are two competing objectives in bike geometries.

I think whether this statement is true depends on how you define low speed agility. A bike with less trail will be easy to maneuver when it is upright at a slower speed and still may track in a straight line very well at high speed. What it won't do is respond as well to lean as a bike with more trail. If your definition of agility is response to lean angle then things flip somewhat.

I am fanatical about this issue and have learned my preference lies with bikes that are more responsive to lean. They feel like they can go around corners faster with less risk of sliding out. My Colnago Extreme Power was the king of this feeling - very responsive to lean, very good in high speed corners, and also rock solid at super high speed in a straight line. But it was also awkward at slow speed, out of the saddle climbing etc. due to steering flop. My Parlee Altum is the opposite - upright head angle, less trail and relatively unresponsive to lean angle - no steering flop. This geometry makes the Altum very steady out of the saddle, climbing, etc. - and still highly maneuverable, tossable at these slow speeds without any comprise in straight line stability at high speed - the thing is an arrow. However some gymnastics are required to get it around tight curves at high speed. So depending on your definitions of agility/stability you can have both. My new Factor O2 is good middle ground by both geometry numbers and performance on the road.

All of my findings are based on relatively solid bikes. Flimsy crap will be horrible regardless of geometry.

[RyanH]

@pdlpsher1 while I agree that a bike can't be the most agile and the most stable, I think if you plott ed agility and stability on a graph they are not directly proportional. At least in how we perceive them. When I think of needing an agile bike, I think of a crit and as counterintuitive as it may sound, if you told me I had to go race a crit on the Crux, I wouldn't have any reservations whatsoever. The one descent I've been meaning to hit up on the Crux to test how it does in a super technical descent is Tuna canyon in Santa Monica.

When I had the Evo, I was riding it back to back with the T3. Throughout the past 5 years, the T3 always served as the constant and reference point. There was no type of descent where I felt the Evo gave up ground and there's no bike that I've ridden since (except maybe the Crux) that felt that way going downhill. I've had bikes that felt better on big sweepers and then sucked in tight descents or vice versa but the Evo excelled at both.

Also, I'm seriously considering calling Carl (or Seven for a 622 XX) next week and getting a build with similar geo to what you described.

One last thought, I think bike handling isn't just geo, it's also how the stiffness of the frame in various parts meshes together and I think that's what makes Peter Denk such a great and revered designer.


@Mr.Gib responsiveness to lean is an interesting concept that would probably best describe my favorite descending bikes. The C59 felt that way too where you felt that no matter what you could just lean a tad more and get yourself out of trouble. The Evo was like that and I think the Wilier was too. Do you think that's just a function of more trail?

[Lucendi]

The agility vs stability tradeoff has mostly to do with the caster angle (all other things equal).



More positive castor angle => heavy steering (stability)

Less positive castor angle => easy steering (agility)

At high speeds more of a castor angle is preferable as road imperfections won't take as much rider input to compensate for.

At low speeds less of a castor angle is preferable as steering will be easy and thus quicker.

[tjvirden]

I'm of the view that while geometry is important, other factors can dominate it; I'm thinking mainly about out-of-plane stiffness (of the whole bike) and tire size and pressure - rider weight matters significantly in comparison to those and the "fixed" stiffness of off-the-peg framesets is an issue if one is far from the optimum that it is designed for.

Like pdlpsher1 I also prefer longer (than is fashionable) chainstays, slacker headtube and a lower BB, but I do prefer a low trail figure with that too (circa 53-55mm ground trail); however I'm not convinced that it's a big difference - and looking at the geometry of the Evo in combination with RyanH's happiness going downhill on it makes me think that, within reason, geo is secondary.

Edit: Hmmm. It perhaps reads that I think the Evo shouldn't descend well due to it's geometry, but that's not what I meant! That geo is uncontentious; other factors make it go downhill well.

[Lewn777]

The agility vs stability tradeoff has mostly to do with the caster angle (all other things equal).



More positive castor angle => heavy steering (stability)

Less positive castor angle => easy steering (agility)

At high speeds more of a castor angle is preferable as road imperfections won't take as much rider input to compensate for.

At low speeds less of a castor angle is preferable as steering will be easy and thus quicker.

I agree with this if you mean fork angle or head angle, slacker makes things slower but far more predictable at high speed.
However you can have a bike that makes up for it's lack of high speed ability by putting more of the rider's weight over the front wheel. Also when you look at mountain bikes you can see how slack they are which should mean that they should be hopeless at lower speeds - however they are pretty good at lower speeds because conversely the rider's weight is far more over the rear wheel. This means that active rider weighting and other geometry is just as important as castor angle or fork slackness.

IMHO all road bikes should be ridden in the drops on descents, otherwise the bike is not rider weighted in an ideal way for it's intended geo.

I've owned and rented about 50 bicycles, some I loved for different reasons including a Cervelo I hated and a 4 cross mountain bike I loved because it had rigid chainstays the perfect length for wheelies, damn wish I hadn't sold that.

Bike geometry is hugely complex, there is no perfect bike because we all have different postures, body shapes, local terrain and intended use for the bike.

All I know is that Cannondale CAAD12 geo is about perfect for me, others get really close like the Tarmac, TCR and Emonda. Loads of others equal to those maybe some better but haven't ridden them.

[Hexsense]

^
Cervelo had significant changes in their front end design for small sizes through the time. Not sure which one you tried.

Very very old Cervelo like RCA, etc. use really short front center with steep HTA and short fork offset. This genetion has rider's weight all over front wheel of short wheel base, low trail value bike. Toe overlap fest. And the bike feel unstable.

Then the previous S3, transit to slacker HTA. But then the fork still remains low offset. This increase front center length a bit but trail value raise up as well. What worse is the reach (top tube length) shrink significantly. It's like a a side way move rather than an upgrade.

The current gen though, small size got 51mm fork offset. Here is where it clicks. Trail value is back to 58mm like old generation, so low self center force of front wheel. But then both slacker HTA and longer fork offset increase wheel base length thus stability. It's a blend of agility from low trail value and stability from long front center.

I think slacker HTA+ longer fork offset are both good ingredients to cook a good descending drop bar bike. Coincidentally, almost all gravel bike use 50-55mm fork offset which is more than 43mm usually found on road bike.
The current Cannondale SuperSix Evo up to size 54 also use this concept to nail its handling. 71.2 Degree HTA+55mm fork offset.

[Mr.Gib]

@Mr.Gib responsiveness to lean is an interesting concept that would probably best describe my favorite descending bikes. The C59 felt that way too where you felt that no matter what you could just lean a tad more and get yourself out of trouble. The Evo was like that and I think the Wilier was too. Do you think that's just a function of more trail?

I suspect trail is the key factor. Colnago geometry is a bit slacker with more trail than most. It's telling that you found the handling of your C59 as I found my Extreme Power. You can't beat the feeling that no matter how hard you corner, you can always hold your intended line. As for other factors such as frame flex, weight distribution, etc., those things won't vary much in the majority of cases.

[PeytonM]

One bike that interests me geometry wise is the current super six in 54cm. 71.2 head angle with 55 off set fork giving 58mm trail but with a long front center.

I assume this would feel like you sit in the bike rather then on the bike, kinda like a modern mtb?

[dcj10]

I've soaked up the comments on this thread, as the question asked has occupied me for years. What are people's thoughts on weight distribution playing a significant role in this topic? When comparing the subjective handling of different bikes, did all the components affecting rider weight distribution occupy the same position relative to the other machine? Second thought: assuming a persons bikefit has taken into account their physical limitations, what necessary sacrifices to perfect weight distribution were consciously made to get there (for example: perfect saddle position achieved, but the rider lacks flexibility and so needs a shorter reach and more upright hand position)? Mostly I wonder if not having enough rider weight low over the front wheel might lead to vague handling, but that is just my theory. I've no knowledge to back it up. Interested if there are any thoughts on these aspects.

[GothicCastle]

Just for more data points, what bar / stem do you have on the three frames and what are the sizes?

[CarlosFerreiro]

I think slacker HTA+ longer fork offset are both good ingredients to cook a good descending drop bar bike. Coincidentally, almost all gravel bike use 50-55mm fork offset which is more than 43mm usually found on road bike.

The thing that made me more interested in this thread was that Ryan didn't like his SuperX, which is the same sort of base geometry that would fit that kind of idea. So what is it about the SuperX geometry, or his bike setup, that makes it not work as well for him as you might expect?
The SuperX BB isn't "low", and the trail number is based on bigger tyre sizes, but otherwise nothing much stands out to my newbie eyes.
The Cycling Tips article does end up admitting that all individual ingredients of a bike geometry are a lot clearer than the overall results from the combinations....