Discussion: Bike handling of modern small bikes.

Discuss light weight issues concerning road bikes & parts.
Hexsense
Posts: 478
Joined: Wed Dec 30, 2015 12:41 am

by Hexsense

While most medium size racing bikes are settle around 73 degree head tube and about 43mm fork rake. Which give Trail figure around 57-61mm with 27mm tire.
http://yojimg.net/bike/web_tools/trailc ... =622&tw=27

The geometry of road bike vary wildly on small sizes,
-Most medium to low end models from all brands including top model of some brands keep the same fork as their bigger size but slacken head tube angle (70.5-72 degree) to push front wheel away, reducing toe overlap. This bump Trail to 66-78mm.
-Some high end models on certain brands try to better retain the head tube angle, but use small-size specific fork which has more fork offset to push the wheel out instead. This option also push front wheel away but maintain trail figure around 60mm.
-Some bike don't bother to deal with any of these changes and let a lot of toe overlap happen. Older generation Cervelo is an example of this.

Discussion:
-small bikes normally pair with shorter stem, does it also means the bike needs more stability of extra long trail?
-chain stay don't normally scale with sizes. Tall people sit more over rear wheel than short person which sit noticeably in front of the rear wheel. Would front end length and trail figure needs to compensate for this?
-Small riders also run shorter crank, are small riders at disadvantage sitting too tall because the bike have mostly the same bb height as bigger bikes design to clear longer crank into the corner? Is the handling getting even worse with the wide tire which is also taller? If we lower BB height would front end has to compensate?
-in the same corner, which bike design approach lean at steeper angle: long or short trail ? Does it related to wheel flop?
-Ultimately which approach leads to bike that can carry most speed into the corner without losing grip or overshoot the corner?

-Does extra large bikes face the opposite problem? too steep head tube angle while keeping the same fork rake pair with chainstay that is not long enough to prevent rider to sit directly over the center of rear wheel?

Feel free to discuss anything about size specific handling and geometry.

shimmeD
Posts: 413
Joined: Thu Jun 28, 2012 10:52 pm
Location: eNZed

by shimmeD

A question, this obviously affects how a bike handles:
in terms of where the rider is positioned fore/aft, does this change with rider size?
Less is more.

by Weenie


C36
Posts: 270
Joined: Fri Mar 03, 2017 3:24 am

by C36

It is simple, small bikes should be in 650 to have proper geometry. 700 wheels are “so big” they do not let you build the rest of the frame properly.
Large frames do not have the same limit, you could virtually use all angles and weight distribution the frame builder would like. Now the limit on large frames is often on the stiffness... weight répartition should dictate longer wheelbase but nobody would like to accelerate with a 420mm rear end.


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2lo8
Posts: 441
Joined: Thu Mar 31, 2016 10:32 am

by 2lo8

Smaller bikes also have smaller handlebars, which like stem length affects handling and so on. It's not just trail, there's also the issue of wheel flop. More trail has more caster effect, but wheel flop goes up with it. Higher trail isn't just higher stability. It also means higher caster force, and your short stem and bars are a shorter lever, meaning force is increased by both factors, even if the amount of movement needed is less. The trail and flop and other geo things also affects how a bike responds in turns, the leverage of the bars and stem being relatively unimportant. Your fit also changes handling quite a bit. You know what else drastically changes leverage, changing hand positions. The other thing is unless you have very specific needs, or comparing bikes back to back, you eventually learn how to compensate for most handling characteristics.

Small wheels might be "right" but it's easier to just scale things down and tell you to go buy special wheels and tires and tubes, and set up gearing for 650c than it is to properly design a small 700c bike with actual parts support. It's a lazy way to make something work. I know what works for 700c M frames, scale it down and 650c should work for small frames, instead of figuring out how to make 700c S work. I mean you could scale a bike down to ant size, but that has nothing to do with the real world, it could turn on a dime and be very very slow. Scaling doesn't automatically make it right. It's more important for very small frames with very short top tubes though when you are pushing the limits of reach and stack.

Extreme toe overlap also is a lazy way of thinking. This geo works on 700c M bikes, it should work fine on 700c S bikes as long as I ignore toe overlap. I'll just tell people toe overlap doesn't matter and then when they're doign slow speed manuevers that can jam their toes into the front wheels. It also ignores the change in chainstay to front center ratio, or weight distribution between front and rear wheels.

And of course normal rake with a shallow head tube angle is just lazy and gives up any pretense about preserving good bike handling, although it's not unridable.

My opinion is a higher offset fork is the best compromise. Anything else is lazy design. It keeps trail (and flop) reasonable, it lets you use common and high end parts instead of expensive low performance specialty parts and it maintains FC for weight distribution and toe overlap.
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C36
Posts: 270
Joined: Fri Mar 03, 2017 3:24 am

by C36

2lo8 wrote:
Small wheels might be "right" but it's easier to just scale things down and tell you to go buy special wheels and tires and tubes, and set up gearing for 650c than it is to properly design a small 700c bike with actual parts support. It's a lazy way to make something work. I know what works for 700c M frames, scale it down and 650c should work for small frames, instead of figuring out how to make 700c S work. I mean you could scale a bike down to ant size, but that has nothing to do with the real world, it could turn on a dime and be very very slow. Scaling doesn't automatically make it right. It's more important for very small frames with very short top tubes though when you are pushing the limits of reach and stack
Sorry lazy design is trying to stick a very small person on a 700 size wheels to stick to stds cause it’s convenient. There is no good way to design a 700 frame for very short people. Spent enough time with frame builder to have this very clear. The reason you don’t have them more is, as you mentioned, to avoid special components.
650 is the only way to give freedom in angles, rake and head tube angle, have proper cockpit position and a decent seatpost extension (this last one is purely aesthetic).
I did face frame builder who didn’t go to the 650 size... cause they had no idea how to manage the geometry.


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NickJHP
Posts: 104
Joined: Wed Feb 14, 2018 2:22 am

by NickJHP

Even with slack head angles and steep seat angles, XS frames with 700c wheels usually have some toe overlap. For that reason, and also because of the excessive trail due to the slack head angle, I used 650b wheels on the last bike I built up for my wife, who's only 157cm tall. The Merckx Mourenx XS frame I built up is designed around 700c wheels, but using 650b wheels running Schwalbe G-One Speed 650b 30mm tyres reduces the trail from 70mm to 64mm and gives an additional ~17mm of toe clearance compared to running 700c 28mm tyres. As the frame uses disc brakes, smaller wheels present no problem from the point of view of getting good braking.

Although the BB is also 17mm lower with the smaller wheel size, as she uses 160mm cranks and the BB drop is only 70mm, the smaller wheels haven't led to a ground clearance problem. In fact, she has more ground clearance with that setup than I do with 175mm cranks on a frame with 76mm BB drop.

2lo8
Posts: 441
Joined: Thu Mar 31, 2016 10:32 am

by 2lo8

C36 wrote:
Wed Jun 27, 2018 2:16 am
2lo8 wrote:
Small wheels might be "right" but it's easier to just scale things down and tell you to go buy special wheels and tires and tubes, and set up gearing for 650c than it is to properly design a small 700c bike with actual parts support. It's a lazy way to make something work. I know what works for 700c M frames, scale it down and 650c should work for small frames, instead of figuring out how to make 700c S work. I mean you could scale a bike down to ant size, but that has nothing to do with the real world, it could turn on a dime and be very very slow. Scaling doesn't automatically make it right. It's more important for very small frames with very short top tubes though when you are pushing the limits of reach and stack
Sorry lazy design is trying to stick a very small person on a 700 size wheels to stick to stds cause it’s convenient. There is no good way to design a 700 frame for very short people. Spent enough time with frame builder to have this very clear. The reason you don’t have them more is, as you mentioned, to avoid special components.
650 is the only way to give freedom in angles, rake and head tube angle, have proper cockpit position and a decent seatpost extension (this last one is purely aesthetic).
I did face frame builder who didn’t go to the 650 size... cause they had no idea how to manage the geometry.


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Just because your frame builder doesn't know how to do it doesn't mean it can't be done. It takes more work to create a new front end geometry rather than just taking a calculator and scaling things down by 92%, which is lazy. A frame builder should be an expert in frame design. Multiplying by 0.92 and forcing all the issues onto you isn't expert design. It's just being lazy and forcing the isues on the rider. It's not important to preserve specific head tube angles or fork offsets. The rider shouldn't care about that. The rider wants good handling. It's the frame builder's job to design a bike with good handling by adjusting HTA and offset. 650c gives the freedom for the builder to use a 73 HTA, not the rider the freedom to have good handling or anything. The freedom to do that is pretty unimportant unless you only want the freedom to use the 73 HTA and 43/45mm offset template (or scaled down to 40/41mm offset, which is incdentally what stock carbon 650c forks are). If it was, then every bike above M would have exactly 73 HTA and 43-45mm offset. The thing about 73 HTA and 43-45mm offset is that is also gives you medium reach for the FC/wheelbase it gives you.

The cutoff for 700c is when you can't get stack/reach figures even when you modify the front end from the M size. Not just a crutch to maintain 73 HTA in a small.
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wingguy
Posts: 4042
Joined: Thu Mar 08, 2012 11:43 pm

by wingguy

NickJHP wrote:
Wed Jun 27, 2018 4:28 am
Even with slack head angles and steep seat angles, XS frames with 700c wheels usually have some toe overlap.
M frames with 700c wheels usually have some toe overlap.

Toe overlap on XS isn't a question of 'if' but of 'how much'.

Geoff
Posts: 5153
Joined: Sat Dec 13, 2003 2:25 am
Location: Canada

by Geoff

it is good to see the acknowledgement of the virtual requirement for 650c wheels. Maybe that will help the production of tires!

My boy is 10. While he is tall for 10 (all the better for pitching 4-seamers at 65), there is no way he should be on 700c. The biggest issue for that size is the wheel choices. Back 'in the day', tri-geeks made 650c wheels a "thing" and there was quite a variety of tubular wheelsets available. Today, that is not the case. Tires, too, are more difficult to get, without going to FMB or Dugast.

For full-sized adults, the smaller frames in 700c can also be a virtual requirement. For my frames, the prevalence of monocoque frames has caused me to choose frames which would traditionally deemed too small due to the massive headtubes. To get the bars low enough, I need to choose a 54 frameset and use a 140mm stem, whlist choosing a saddle that will allow the maximum setback. If I could, I would select a 56 toptube and run a 120mm stem on a 125mm headtube, but I cannot buy that frame unless it is tube-to-tube.

Even with a 54, there are 'issues'. As noted by others, with 172.5s, I just barely have toe-clearance. If my feet were bigger, that could be a real problem. I don't think there is an answer for that for anybody.

mattr
Posts: 3584
Joined: Fri May 25, 2007 6:43 pm
Location: The Grim North.

by mattr

I've had toe overlap on almost every bike i've ever owned.

It's never, ever been an issue. Even with mudguards, overshoes and ~4cm of overlap.

The handling is far far far more important.

wingguy
Posts: 4042
Joined: Thu Mar 08, 2012 11:43 pm

by wingguy

Geoff wrote:
Wed Jun 27, 2018 2:08 pm
Even with a 54, there are 'issues'. As noted by others, with 172.5s, I just barely have toe-clearance. If my feet were bigger, that could be a real problem.
What kind of problem? :noidea:
I don't think there is an answer for that for anybody.
Why does there need to be? It's a road bike, not a trials bike or a BMX. How it corners at 2mph has never concerned me in the slightest.

WheelNut
Posts: 9
Joined: Fri Jun 22, 2018 11:51 pm

by WheelNut

To understand if smaller wheels or so called "compromised geometries" are a problem I think we need to understand the problem in its full context and answer the question what is a "good handling" bike? It doesn't seem obvious to me that a bike which has a geometry that delivers stability in direction changes (low 'G' corner) should necessarily be faster through corners (high 'G' corner) in a on the limit descent. For all I know weight distribution might be the primary driver of at the limit stability. Maybe a good handling bike one that inspires rider confidence, but where does confidence come from? Geometry? Fork stiffness? Tire construction/width/compound/rim width/contact patch shape? Weight distribution? Frame alignment?

C36
Posts: 270
Joined: Fri Mar 03, 2017 3:24 am

by C36

WheelNut wrote:To understand if smaller wheels or so called "compromised geometries" are a problem I think we need to understand the problem
Compromised geometry comes forcing 700wheels on very small frame, not with 650. The 650 being smaller allow more freedom to choose angles impossible with 700.



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Hexsense
Posts: 478
Joined: Wed Dec 30, 2015 12:41 am

by Hexsense

Question i still don't know is, is moderate trail better than high trail number?
high trail also means high wheel flop.

More wheel flop means front wheel drop more height when tilted, but doesn't that also leads to more centering force and straighter (rather than more lean) of the rider and back end of the bike in the corner?
Would that be more stable (can carry more speed without loosing grip) and provide rider with more pedal-ground clearance?

by Weenie


2lo8
Posts: 441
Joined: Thu Mar 31, 2016 10:32 am

by 2lo8

Hexsense wrote:
Wed Jun 27, 2018 8:58 pm
Question i still don't know is, is moderate trail better than high trail number?
high trail also means high wheel flop.

More wheel flop means front wheel drop more height when tilted, but doesn't that also leads to more centering force and straighter (rather than more lean) of the rider and back end of the bike in the corner?
Would that be more stable (can carry more speed without loosing grip) and provide rider with more pedal-ground clearance?
No, usually wheel flop leads to less self centering, however caster effect increases with speed. It's like a windvane. Some people like a high trail geometry for rough roads and going mostly straight. It's also a matter of personal preference. Some people like the way an XXS Colnago Master handles. Wheel flop makes gravity want to turn the wheel. Trail also has to do with what angle the wheels want to be when the bike is leaned a certain amount. It's what lets you steer a bike while walking with one hand from the seat. You can always force the wheel to be in another position. Lean angle is a function of mass, location of mass, speed, and turn radius which affect centrifugal force. Think about how a car sways in a turn. You lean enough to cancel it out with gravity so you don't flip over since you don't have the extra wheels. I don't think very many people have been able to isolate wheel flop from trail to any meaningful degree, which is why you might see differing opinions on what people think it does. They don't diverge much within the range of 71-74 HTA given the same trail, so for the purposes of a bike, they can be considered to be correlated.

Wheel overlap isn't binary either. It gets increasingly more annoying as overlap increases because the front of the foot is round and the path of the wheel is an arc. It's not important on a race bike, a road bike used on roads, I think it's at least something I'd rather not have than have, same with 650c. It's much easier to borrow a tube or get a wheel swap in a race if you're running 700c. There's the issue of having to keep a stock of 650c tires instead of being able to buy one quickly if needed and so on. Standards exist for a reason, otherwise we'd all be running the perfect size wheels custom made for us like they had with penny farthings. Some people think the only way to acheive good handling is 73 HTA scaled down and it's worth suffering all these problems just to make a 92% carbon copy of what works for a M bike, as if there's something sublimely magical about 73 HTA.
Last edited by 2lo8 on Thu Jun 28, 2018 3:52 am, edited 1 time in total.
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