Questions on Geometry, Fit, and Ride

[exapkib]

Last night I had the chance to ride three different bikes around the neighborhood back-to-back. These are bikes I've been riding for years, so it was not simply a "test ride." Then this morning, I took the Yamaguchi out after not having ridden it for quite a while, and was struck by it's unique ride. This, in turn, led me to look at the three bikes next to each other, and contemplate the difference in geometry.

First, a Parlee Z2. Standard 56 square. 110mm stem. Super stable, but still quick to respond to steering input.

Second, Black Mountain Cycles Monstercross. Size 56. 120mm stem. Ponderous? Stable? Awesome. Does the dirty work--winter commuting, singletrack,

Finally, the Yamaguchi. 55 TT, 56 ST. 120mm stem. This bike move like a laser--so quick to respond to input. So fast changing its line. At the same time, once it reaches speed (say 23mph+), it stabilizes into a really confident ride.

In many ways, these three bikes line up very close--set up for my fit, they all look basically the same, with one exception: the wheelbase, and more particularly, the front-center measurement.

I understand how these bikes feel underneath me, but I don't really understand how the geometry translates into ride qualities.

Here are a few numbers:

Wheelbase
Parlee: 98cm
Yamaguchi: 94cm
Black Mountain: 103cm

Front-Center
P: 59cm
Y: 55
BMC: 60.5

Chainstay:
P: 41
Y: 41
BMC: 44

You can see that all of the 4cm difference in wheelbase between the Parlee and the Yamaguchi is in the front-center measurement. Why would a framebuilder (or client) make that decision?

My question to the collective wisdom of the board:

How does front-center fit into questions of geometry/fit/handling?

Is it simply a leftover measurement, tweaked to reduce toe overlap after everything else is already in place?

A local framebuilder has written a lengthy post on the topic here (http://waltworks.blogspot.com/2012/06/first-of-all-this-isnt-rant-really.html), though he is focused exclusively on mountain bike applications, and seems to indicate that it is a great measurement to focus on because it reflects the impact of so many other measurements--head tube angle, seat tube angle, etc.

I'm in the process of finding someone local with a frame jig who can help me to find the angles on the Yamaguchi, because I assume that those play into this conversation as well.

[Mark McM]

:

How does front-center fit into questions of geometry/fit/handling?

Is it simply a leftover measurement, tweaked to reduce toe overlap after everything else is already in place?


According to Damon Rinard, manager of road bikes at Cannondale, front center is not of primary importance (unless it is so ridiculously short that there is massive toe overlap), and that the most important dimensions are TT length (for fit) and trail (for stability and handling). After selecting the TT length, head tube angle and fork offset, the front center just falls where it may.

Although you listed the wheelbase, front center, and chainstay lengths of the 3 bikes, you left off what may be the more important dimensions - head tube angle and fork offset. These two dimensions are the primary variables that regulate trail (another variable is wheel diameter, but that is likely to be nearly the same for all 3 road bikes). You mention that the Yamaguchi has the quickest handling, and also the shortest front center. Because the Yamaguchi has a similar TT length and seat tube angle as the other 2 bikes, the short front center is most likely due to the Yamaguchi having a steeper head tube angle. Therefore, it is likely that the Yamaguchi has the shortest trail of all 3 bikes. This is probably the reason that the Yamaguchi has the quickest handling.

In regard to the blog post about front center on MTBs: The blog post mentions several factors that are affected by front center, including front wheel traction/washout (front center too long) and ease of going over the bars (front center too short). On hard, relatively smooth pavement, these are far less of a concern. I'd also add on the list that MTB front center can also affect turn radius and wheel track parallax, which can be a concern when picking your way through rock gardens and tight single track, but are far less of a concern on a road bike.

[cmg]

parlee z2 carbon, Yamaguchi steel? it has an effect in the bikes feel. carbon fork on the yama? of the 2 which has the more slack seat tube angle, probably the yama. are the head tubes the same length, is the amount of drop between the seat and bars of both bikes the same?

[exapkib]

parlee z2 carbon, Yamaguchi steel? it has an effect in the bikes feel. carbon fork on the yama? of the 2 which has the more slack seat tube angle, probably the yama. are the head tubes the same length, is the amount of drop between the seat and bars of both bikes the same?

Important information!

Yamaguchi--steel fork
Parlee--all carbon

The Yamaguchi has a little more drop to the handlebar, with a 14cm headtube. The headtube on the Parlee is 15cm. I think your guess on the Yamaguchi seat tube angle is accurate, as the slightly shorter top tube feels slightly longer than the numbers make it sound like it should.

Some photos for comparing:

Parlee:
https://farm1.staticflickr.com/829/26948953027_a950405fbe_b.jpg (https://flic.kr/p/H4oqRa)

Yamaguchi:
https://farm1.staticflickr.com/974/41774843702_3e707eb5ec_b.jpg (https://flic.kr/p/26DuZBU)

Black Mountain:
https://farm2.staticflickr.com/1808/42614706734_fb3d479c21_b.jpg (https://flic.kr/p/27VHvD1)

[tylercheung]

weight balance/distribution between front/back?

[David Tollefson]

weight balance/distribution between front/back?

Ding ding ding!

That, plus trail, FC and chainstay length, pneumatic trail, wheel flop (related to wheel mass, HTA, and fork offset)...

[Peter P.]

According to Damon Rinard, manager of road bikes at Cannondale, front center is not of primary importance (unless it is so ridiculously short that there is massive toe overlap), and that the most important dimensions are TT length (for fit) and trail (for stability and handling). After selecting the TT length, head tube angle and fork offset, the front center just falls where it may...

I agree with Damon Rinard; front center is a RESULT of all the other dimensions and if I were a framebuilder, it is something I wouldn't worry about.

Every sensation exapkib describes sounds to me like handling/steering characteristics, and to me that's all to do with head angle and fork rake.

When exapkib says, "You can see that all of the 4cm difference in wheelbase between the Parlee and the Yamaguchi is in the front-center measurement. Why would a framebuilder (or client) make that decision?", my answer is, every framebuilder has their own interpretation of how a frame should fit and ride, and they all don't necessarily have to agree because there is no one, perfect geometry for a given rider.

The good thing is, if exapkib puts all three frames in a jig and extracts the dimensions and angles, he can then make more sound decisions regarding any future custom frame purchases, or perhaps even a stock frame.

[exapkib]

weight balance/distribution between front/back?

Ding ding ding!

That, plus trail, FC and chainstay length, pneumatic trail, wheel flop (related to wheel mass, HTA, and fork offset)...

Yes!

But as asked and (kind of) answered above, I've been curious as to whether front-center is actually a dimension a builder thinks about, or simply the result of decisions made about head tube angle, fork length, etc.

[l0n3rider]

for a stock frame, the frame is designed to perform best at something. and normally this is designed around front center and trail. the trail is the result of headtube angle and fork rake. e.g a sprinter bike is designed for more weight distribution to the front center and stable at high speed.

so sometimes it depend on the speed that we ride. a stable at high speed normally twitchy at slow speed - very much depending on the trail and weight distribution.

[David Tollefson]

Yes!

But as asked and (kind of) answered above, I've been curious as to whether front-center is actually a dimension a builder thinks about, or simply the result of decisions made about head tube angle, fork length, etc.

When I'm designing a frame, I start with the contact points and location of the center of gravity, then go to the descriptions of ride feel to determine weight balance (which gives the ratio of front center to rear center). There is a continuum of "responsive" to "stable", and a series of factors that move the overall feel in each direction. Then you start balancing those factors.

For my own bikes, I like the front end to be light and very stable, so I maximize front center, go slack with the HTA, but stay with a "normal" fork rake to get trail up (my gravel bike is about 80mm of trail, and my road bike is near 75). But that's because I want the bike to respond as it does, and I wouldn't build one like that for some one else unless they really wanted it.

[bitpuddle]

Yamaguchi:
...


I don’t have anything clever to add, but that Yamaguchi is really nice.

[OtayBW]

For my own bikes, I like the front end to be light and very stable, so I maximize front center, go slack with the HTA, but stay with a "normal" fork rake to get trail up (my gravel bike is about 80mm of trail, and my road bike is near 75).Whoa! I like a lot of trail, but that is some serious trail. Doesn't it get kind of noodly at low speed?

[dddd]

Just thought that I'd add that the stem length is one more thing that will affect the weight distribution at the tires and the more complicated mass-distribution about the steering axis.
Depending on whether the rider is sitting or standing, much of the rider's mass is effectively pivoted at the saddle or pedals, with the other end of this mass being the rider's hands pivoted at some distance from the steering axis.
I wish that I could add this up into some kind of algorithm, but my sensibilities about all of these variables are perhaps limited to within ranges of frame and rider sizes close to my own.
I'll note that for off-road use, I like to keep the stem length shorter, while here the OP apparently has the longest stem length on the off-roader.

[David Tollefson]

Whoa! I like a lot of trail, but that is some serious trail. Doesn't it get kind of noodly at low speed?

Not sure what constitutes"low speed" to you, but I have no problem riding no-hands at 10mph.

[Mark McM]

Not sure what constitutes"low speed" to you, but I have no problem riding no-hands at 10mph.

I'm sure you can. In fact, with trail that large, you can probably ride no handed (in a straight line) at lower speeds than on most bikes. But how does it handle when riding up steep hills at single digit speeds, when you get out of the saddle and lean the bike back and forth?

For 700c wheels and with a "standard" fork offset of 43 - 45 mm, a trail dimension of 75 - 80 mm would need a head tube angle in the 69 - 70 range. Low speed flop becomes a serious concern with these dimensions.

[Heisenberg]

Also, can't forget BB drop.

Matters. A lot.

[David Tollefson]

I'm sure you can. In fact, with trail that large, you can probably ride no handed (in a straight line) at lower speeds than on most bikes. But how does it handle when riding up steep hills at single digit speeds, when you get out of the saddle and lean the bike back and forth?

For 700c wheels and with a "standard" fork offset of 43 - 45 mm, a trail dimension of 75 - 80 mm would need a head tube angle in the 69 - 70 range. Low speed flop becomes a serious concern with these dimensions.

I tend toward short stems as well - 70mm on the gravel and MTB, 100 Max on the road. Using Conti Speedride in 700x42, and that bike is indeed a 69 degree HTA,I haven't had issue with wheel flop on single digit speed uphills. And no problem on the road (70 degree HTA) either. It's not a bike I'd take into a tight crit, but my racing days are behind me.

It just works for me, YMMV.