Understanding Descending Mannerism of Bikes

[Hexsense]

If you want to go faster get some 30/32 grippy tires and run them at 60-70psi.

Not at my 63kg weight.
I run 25c front tire at 58psi, the same pressure as rear 28c. 30/32 at 60-70psi would be brutally hard. Maybe 40-50psi would work.

Never thought of understeer on a bike, perhaps what people mean by understeer is that their bike is running wide from the intended radius. What I have experienced is particular bikes that require what feels like too much lean in certain radii at certain speeds. It creates the sensation of choosing between sliding out and running wide. But same corner/curve, same speed, same rider, same fit, same technique, same wheels, same tires, different bike, and I have no problem making the turn/curve with room to spare. So what's the difference? By the numbers it's the trail figure.

I had that problem when I'm new and didn't know how to corner. I kinda knew a thing or two about counter steering. But in panic, I corrected a wide turn by turning the bar further inward hoping the bike would turn sharper following my handlebar. It's a big mistake that make the bike lean less and turn even wider.
Once I recognize that and corrected my instinct to lean the back end of the bike more inward and handlebar more outward (more counter steer) then problem vanished.

[Alexbn921]

If you want to go faster get some 30/32 grippy tires and run them at 60-70psi.

Not at my 63kg weight.
I run 25c front tire at 58psi, the same pressure as rear 28c. 30/32 at 60-70psi would be brutally hard. Maybe 40-50psi would work.

I'm 87kg and run 70psi
Most people run their tires way way way too hard. Lower those pressures down until the tire starts to squirm, then add a couple psi.

[Mr.Gib]

Never thought of understeer on a bike, perhaps what people mean by understeer is that their bike is running wide from the intended radius. What I have experienced is particular bikes that require what feels like too much lean in certain radii at certain speeds. It creates the sensation of choosing between sliding out and running wide. But same corner/curve, same speed, same rider, same fit, same technique, same wheels, same tires, different bike, and I have no problem making the turn/curve with room to spare. So what's the difference? By the numbers it's the trail figure.

I had that problem when I'm new and didn't know how to corner. I kinda knew a thing or two about counter steering. But in panic, I corrected a wide turn by turning the bar further inward hoping the bike would turn sharper following my handlebar. It's a big mistake that make the bike lean less and turn even wider.
Once I recognize that and corrected my instinct to lean the back end of the bike more inward and handlebar more outward (more counter steer) then problem vanished.

Never had the problem, even when I was "new". Only noticed a bike running wide when, after 30 years of serious road cycling, I bought a bike with different head angle/fork offset. Never thought about turning the bars when cornering - that's a weird thought. To me it was always just lean and pressure on contact points. The bike would do it well, or wouldn't.

[Discodan]

I'm going to jump in here because I have a different view on a few points. I don't want to turn it into physics-fest but based more on well established motorcycle dynamics and personal experience

background on my frame of reference; I would consider myself a good/very good descender with a fair few downhill KOM's and have spent 15 years racing motorcycles, much on small capacity GP bikes where cornering speed is king

First up is the statements that have been made, which is that geometry doesn't make much/any difference on descending. I agree that the speed of turning into a corner doesn't really matter, it's easy on a bike to just turn faster using brute force as gyroscopic forces are relatively low. Where geo does really make a difference is in the stability mid-corner; a more stable bike (longer wheelbase, more rake) allows the rider to hold a more consistent line and take it right to their limit of comfort/traction. A longer wheelbase also allows slightly less lean angle for the same corner speed.

Given none of us really ride on the limit of traction let's say that the stable bike then lets the rider use 90% of available traction throughout the sweeping corner. On the other hand a nervous and quick steering bike will be reacting to inputs and bumps and will need lots of micro-corrections and may be varying between 85 and 95% of available traction through the corner, which is a lot less confidence inspiring and requires you to slow down to keep your chosen safety margin

This plays out in practice for me: I have a 2000 steel bianchi with really stable geo on 23mm GP4000s at 90 psi, and also modern bikes such as a 2020 SuperSix Evo on 30mm sticky tubeless tyres at 70psi. The SuperSix should be much faster on descent but every KOM has been set on the Bianchi. Even on old tyres you whistle through corners solid as a rock, thinking that you've got another 20% up your sleeve, and realise when you get home what your times were like. On the other hand the SuperSix or TCR feel fast (as in scary) but are bouncing around a lot more and do not give you the confidence to push to the limit despite having better mechanical traction. I do wonder what could be done after fitting modern tyres to the Bianchi.

I guess what I'm saying is that yes, theorically fatter stickier tyres are really important, but in reality confidence and stability override that by allowing the rider to push harder. I don't believe any/many of us are pushing that last few percent of traction out of our tyres (otherwise we'd be crashing a lot) so that 5% grippier tyre is not mechanically going to make us faster, it's the confidence to push it from 80% to 90% that actually makes us faster. That confidence can come from geo, or maybe for some people it's knowing they have better tyres.

[RyanH]

@Discodan I wholeheartedly agree with what you wrote. My favorite descending bikes give you the confidence to get closer to the safety margin than other bikes.

If people want to use cars as an analogy, a 911 is faster around a track than a Cayman but most will find they are able to put out better times in a Cayman. One instills confidence while the other takes true skill to extract out every last fraction of a second.

Back to bikes, I have the Litespeed at 72.5* HTA and 43 rake for a new trail of 63mm and it didn't resolve the straight line mannerism. I need a warm dry day to push the local descent to see if it feels better there but the straight line handling is my current issue and a 58mm trail to 63mm didn't make a noticeable difference. So, I don't think trail is the end all be all, some bikes feel better than others and I think that may move beyond the geo sheet even when all other variables are controlled.

[pdlpsher1]

That's interesting. I wish you were local to me and could test my bike. I have no doubt that I hit the jackpot with my bike's geometry (and perhaps tubing choices) but I can't prove it unless you try it out. Over the holidays I'm flying my kids from Loveland to Burbank via Avelo for $59 each way. So for a cost of $118 you could come here and test ride my bike. That's a bargain for an experiment don't you think?

[Lewn777]

@Discodan I wholeheartedly agree with what you wrote. My favorite descending bikes give you the confidence to get closer to the safety margin than other bikes.

If people want to use cars as an analogy, a 911 is faster around a track than a Cayman but most will find they are able to put out better times in a Cayman. One instills confidence while the other takes true skill to extract out every last fraction of a second.

Back to bikes, I have the Litespeed at 72.5* HTA and 43 rake for a new trail of 63mm and it didn't resolve the straight line mannerism. I need a warm dry day to push the local descent to see if it feels better there but the straight line handling is my current issue and a 58mm trail to 63mm didn't make a noticeable difference. So, I don't think trail is the end all be all, some bikes feel better than others and I think that may move beyond the geo sheet even when all other variables are controlled.

Very good point. I have ridden bikes that drop in to corners easily but then need to be stood up on the straights, and bikes that have to be muscled-in to corners but stand-up more easily after the corner.
I prefer to muscle my bikes in as I have plenty of moto riding in my past. Yeah, there is a lot more going on than the basic geo numbers, frame material and construction and tires/tubes play a part.

[ricerocket]

Back to bikes, I have the Litespeed at 72.5* HTA and 43 rake for a new trail of 63mm and it didn't resolve the straight line mannerism. I need a warm dry day to push the local descent to see if it feels better there but the straight line handling is my current issue and a 58mm trail to 63mm didn't make a noticeable difference. So, I don't think trail is the end all be all, some bikes feel better than others and I think that may move beyond the geo sheet even when all other variables are controlled.

Back to the whole confidence thing, the Litespeed is a traditional 1-1/8" headset, correct? And the Supersix is 1.5" lower and 1-1/8" upper? I know, lots of confunding variables here, but for me (sample size of one), a stiffer fork and headtube make all the difference.

There's been a recent trend on social media to drop full suspension MTBs from head-height onto the ground in slow motion, and while not directly relevant to this, you can see how much forks and headtubes actually flex, even trail bikes with huge travel that are supposedly overbuilt for "hucking".

Geometry is important, yes, but if the bike isn't stiff enough to hold that geometry, then as the rider, you feel that in the bike as being unpredictable.

[RyanH]

@ricerocket the Litespeed had an Enve tapered 1.5" fork and is now using a Moots 1.25" fork.

I was talking to a friend and he said that his SL6 had a tendency to move around on straight, high speed descents too.

If I get in the drops, it's better. On the Crux though, I don't need to be in the drops to experience a high level of stability. That to me sounds like a weight distribution issue but I'd think with a 140mm stem and 10cm of drop I'd have a decent amount of weight on the front, probably much more than it was designed for so getting into the drops should make things worse not better as that is further weighting the front, isn't it?

[tonytourist]

Over the holidays I'm flying my kids from Loveland to Burbank via Avelo for $59 each way. So for a cost of $118 you could come here and test ride my bike. That's a bargain for an experiment don't you think?

Ryan, beware of any "free candy"

[1llum4]

@RyanH if the main issue between the crux and the litespeed is the straight line stability/maneuvrability then I think it might be a case of BB Height difference between the two and it should be easy to measure at home to verify my theory.

My thinking is that we mostly check BB drop but in reality your point of rotation is not the wheel axle but the tire contact patch. Since you dropped the crux from its intended 38-42mm tire to 28-32mm tire you might have a lower BB height from the ground with the crux than with the T1sl.

[RyanH]

On that idea, does crank length affect the effective BB drop? Meaning, if you have two identical bikes (stack and reach stay the same so HT changes) except one is a bb drop of 80mm with 165 cranks and the other is a BB drop of 70 with 175mm cranks, are those the same?

I ride 165 cranks and I've been wondering if that places me higher on the bike which means a higher center of gravity and less perceived stability.

By the way, it's not out of the realm of possibility that something is off (out of spec) on the Litespeed and this may be influencing my perceptions. I don't have another similar bike to test at the moment and while I hit my local descent dozens of times back to back with the Medium T1sl and the T3 and the Crux to compare notes with how it takes turns, I didn't pay as much attention to how the bike moves at 40+ mph. The other hill near me is straight down in high double digit grade so it does a really good job highlighting that. It stopped raining so maybe I'll be able to test the Crux on that hill today.

[Hexsense]

If bb is statically set:
Shorter crank means higher seatpost. So, yes. longer crank has slightly lower CG. On MTB, it's negated by dropper post though.

If bb is moved with crank length as well, like 70mm drop+175mm crank vs 80mm drop +165 crank:
Shorter crank has lower CG in two levels depend on your position:
1) seated: Tiny bit lower. your butt is at the same y axis height but one leg is lower on shorter crank.
2) out of saddle: Significantly lower. cranks are usually rest at forward/backward before the corner, then pump outside leg downward into the corner. People usually align their position height based on the height when legs are forward/backward. Not when one leg up and one leg down. When cranks are horizontal, crank length doesn't matter, but bb drop does.

Another dimension that crank length effect is the stance stability.
Many people thought longer cranks put legs wider apart and it'd be more stable.
While actually, it's only true if you don't lose stability though tilted hip. The more one leg rest further vs another, the more tilt your hip is(, usually, unless you intentionally align it). And that reduce the body's ability to stabilize. Shorter crank align knee more closer to each other and it's better to stabilize.
Watch more about Triangle of Awesome. Skip to 0:45 if you just want to visualize it quickly. https://www.youtube.com/watch?v=gqyMM9H9EzM

[tjvirden]

@ricerocket the Litespeed had an Enve tapered 1.5" fork and is now using a Moots 1.25" fork.

I was talking to a friend and he said that his SL6 had a tendency to move around on straight, high speed descents too.

If I get in the drops, it's better. On the Crux though, I don't need to be in the drops to experience a high level of stability. That to me sounds like a weight distribution issue but I'd think with a 140mm stem and 10cm of drop I'd have a decent amount of weight on the front, probably much more than it was designed for so getting into the drops should make things worse not better as that is further weighting the front, isn't it?

My best guess is that what you're feeling (instability of a sort) is coming from a combination of the stem length + weight of bar & shifters. That 140 stem may just be exerting too much influence, in your particular setup.

I wonder how it feels if you descend with hands on the tops, next to the stem? That might give a clue to what is happening. Trying a shorter stem is worth thinking about; obviously your fit won't be right, but it might give a strong hint as to which bit of the bike is responsible for what you're feeling.

[DaveS]

The handlebars should not carry much weight. It's your saddle fore/aft position that has the greatest effect on weight balance. If you need a 140mm stem, you either have long arms and/or torso or the saddle is too far forward.

Any serious descending should be done in the drops. I do some very long descents, mainly on the brake hoods, but the slopes aren't that steep and the corners aren't sharp. I get into the drops when the speed gets higher or cornering needs to be more precise.