So what does front center do?

[vqdriver]

I know what the measurement is but when people ask for that number in the classifieds I'm unclear what on what they're trying to determine.

It can't be as simple as toe overlap?

[Ray]

In addition to toe overlap, it interacts with the chainstay length and the steering geometry (HT angle, rake/trail) to determine the bike's weight distribution between the front and rear wheel. A given geometry will result in different weight distribution under different riders, given differences in rider weight and position.

I suspect most of us, except for a few handfuls of expert fitters and frame designers, don't know how all of those variables interact, but some riders probably know what combination of chainstay length (which tends to be pretty static on most racing frames) and front center has worked for them in the past.

-Ray

[mister]

i'd guess they are interested in toe overlap...too many variables that affect the number for it be of much use in comparing frame to frame.

[Tobias]

It can't be as simple as toe overlap?
Why not go with simple? ;)
Either that or it makes them sound more informed. :rolleyes:

[Peter P.]

Of course; I'm not a builder but I have opinions like one!

I say front center is a RESULT of all the other dimensions so build your top tube/head angle/fork rake to what you want. Then the front center "is what it is".

The number might help a builder who thinks in terms of how it might affect things such as; "There's likely to be toe overlap with this design. I better let the customer know before I start cutting tubes."

[Grant McLean]

I say front center is a RESULT of all the other dimensions so build your top tube/head angle/fork rake to what you want. Then the front center "is what it is".

I think you describe a pretty commonly held view, but I think it's backwards.

People have forgotten that angles and rake are the result numbers of the
frame's actual dimensions, not the other way around.

Wheelbase and trail determine how a bicycle handles.
Front center is part of the wheelbase, I start with that number
when designing a frame.

The rest of the numbers relate to how the bike FITS, not how it rides.

-g

[sevencyclist]

Okay. For those of us, like me, who are less educated. What is front center?

[Grant McLean]

Okay. For those of us, like me, who are less educated. What is front center?

the distance from the center of the cranks to the center of the front hub.

The amount of BB drop will effect what the "real" number is.
It should really be measured along the axis of both hubs
to remove this variable. The same thing goes for chainstay length,
or "rear center" if you like.

-g

[Peter B]

Feet, a$$ and hands yield position. Position relates to fit. Design renders position over the wheels to match a given handling/use goal. Think of it as weight distribution.

Wheelbase and trail are part of the handling equation. A given trail number can be obtained via a range of HTA/rake relationships, but each combination locates the front wheel a different distance from the BB. Front center gives the builder feedback regarding that distance and hence, rider weight distribution.

While FC may not be the sole end in itself, it is likewise not merely a resultant.

I'm with Grant on this one.

[happycampyer]

Okay. For those of us, like me, who are less educated. What is front center?Here is a concise explanation of wheelbase/front center as well as other measurements that affect the way a bike handles:

http://www.calfeedesign.com/frontendterms.htm

Grant is exactly right—the wheelbase (FC/RC) has a major effect on the way a bike rides, but many people overlook it (including some manufacturers). Two bikes with identical top tube and trail measurements, but different wheelbases, will fit a given rider the same but will handle very differently (as others have said, because the rider's weight will be distributed differently over the wheels).

[Grant McLean]

Feet, a$$ and hands yield position. Position relates to fit. Design renders position over the wheels to match a given handling/use goal. Think of it as weight distribution.

Wheelbase and trail are part of the handling equation. A given trail number can be obtained via a range of HTA/rake relationships, but each combination locates the front wheel a different distance from the BB. Front center gives the builder feedback regarding that distance and hence, rider weight distribution.


great explanation.
too bad it won't fit on a t-shirt.

-g

[sbornia]

Front center is part of the wheelbase, I start with that number when designing a frame.

Grant, could you explain this a bit more....do you have some ranges of front center measurements that work for you for different types of applications? (E.g., long distance bike, crit bike, Toronto snow bike, etc.) Or maybe there's another way to describe what starting with that number means in practical terms?

[soulspinner]

I have 2 bikes with the same trail but 10 mms diff rear center(chainstay length). The longer chainstay bike also has 6.8mm bb drop but the other has an 8 drop. The longer wheelbase bike turns in quicker by quite a bit. TT is the same as is bar height etc. It seens bb drop makes a bit of a diff when thrown into the equation.

[sevencyclist]

Thank you all for sharing the knowledge with me. It looks like front center, bottom bracket drop, chainstay length, and wheelbase are all inter-related, and dictates how a bike handles, just as head tube angle, stem length, and saddle fore/aft with seat tube angle all factor into part of the equation.

This makes me have more respect for the best builder who are able to create their "signature feel" for their bikes while having to customize a different bike for each different client, or those builders who are able to recreate in steel, carbon, or aluminum what the clients desire in their ride. :beer:

[ckamp]

Another daily visit to the forum.. another bit of knowledge obtained. :beer:

[dekindy]

The question on front center makes me wonder more than ever how a builder goes about designing a bike frame. Do they start with key measures like ST, TT, HT, front center, and bottom bracket height that are what they consider the primary determinants of fit and handling and then all the other dimensions are determined from this vital information? Do they go through a hierarchy of inputting dimensions and then see what compromises they have to make?

I wish Carl Strong or David Kirk would do a webisode on this process. Show the rider weight and key fit dimensions and desired handling characteristics and how they go through the process of setting the frame dimensions and angles, selecting the tube sizes, verifying the weight distribution, etc. Or is this considered proprietary information and something that would never be revealed by a builder?

[Grant McLean]

Grant, could you explain this a bit more....do you have some ranges of front center measurements that work for you for different types of applications? (E.g., long distance bike, crit bike, Toronto snow bike, etc.) Or maybe there's another way to describe what starting with that number means in practical terms?

The way I think about it, wheelbase and trail have a relationship.
It's difficult to put into the right words, but longer wheelbase bikes
have a certain "stability" and smoothness quality. By longer, i mean
41cm chainstays, and 58.5 to 59.5 front centers. The maneuverability
feeling comes from trail. I've found that the combination of the
slightly longer wheelbase, and a little less trail makes for an agile
feel, quick to turn and yet very stable in a straight line.

Many stock bikes have a different trail number across their size range,
mostly caused by using a different head angle with the same fork rake.
The smaller size frames have shorter wheelbases with more trail...
this works, as the higher trail number 'compensates' for the shorter wheelbase,
and the design works... but it's nowhere near as good a design as
the lower trail, longer wheelbase bike, in my opinion.

-g

[RPS]

Many stock bikes have a different trail number across their size range,
mostly caused by using a different head angle with the same fork rake.
The smaller size frames have shorter wheelbases with more trail...
this works, as the higher trail number 'compensates' for the shorter wheelbase,
and the design works... but it's nowhere near as good a design as
the lower trail, longer wheelbase bike, in my opinion.

-g
Grant, you must like riding a tandem. ;)

I'm the same. I prefer longer wheelbase and less trail. I know it's not for everyone, just what feels best to me.

[BdaGhisallo]

it's nowhere near as good a design as
the lower trail, longer wheelbase bike, in my opinion.

-g

I recall hearing that Merckx liked quite low trail numbers on his personal steeds when he was hammering all those guys years ago. And with his height he would have had a longer wheelbase so count him as a proponent of your belief Grant.

[Tobias]

Longer wheelbase also reduces weight transfer ...... rider shifting back and forth has less impact on weight distribution. And he's less likely to do an endo when braking hard. Ride is better too.