How long is too long for front center?

[mokmu]

Designing a new custom bike and am a bit concerned that between my 73 deg seat tube angle and 73 degree head tube angle, my front center is too long at 623mm. I'm already using a 13cm stem. I've got 58mm of trail right now, should I steepen up the head angle? This would reduce the trail to about 55mm while also shortening the front center a bit.

[TimmyB]

Are building the frame yourself? If not... why not just ask/trust your builder? They might, you know, be able to fit you / design the bike around you/your riding style/etc. Since you know, that is their job :bike:

[Kontact]

Aside from Timmy's excellent advice of not designing a builder's product, have you looked at other people's geometry to see how this is normally done?

In general, if you want to shorten front center you change the HTA and adjust the rake to keep the trail where you want it. Front center is the primary reason for different HTAs across frame sizes. Handling does not drive HTA. That's why 58s generally have 73.5 or 74 HTAs.

Why the long stem?

[Bob Loblaw]

What's the bike to be used for? 650B or 700c? Fenders or no fenders? Clearance for big tires? Do you suffer from toe overlap paranoia? Can you scratch your knees without leaning over?

I would definitely say talk to your builder.

BL

[Kirk Pacenti]

623mm is on the long side for a traditional road bike. But as others have pointed out, there are a lot of variables. We'd need a more complete picture of the design intent of the bike to give any meaningful suggestions.

Fwiw, you can also manipulate FC with a slacker seat tube angle; assuming you would still be able to get your saddle set back (relative to BB) positioned correctly.

With the given FC dimension of 623mm, and twin 73* angles, I will assume that you're on a large(er) bike, with a long TT, in which case [generally speaking] you'd likely benefit from a slacker STA anyway...

Cheers,
KP

[Chance]

Fwiw, you can also manipulate FC with a slacker seat tube angle; assuming you would still be able to get your saddle set back (relative to BB) positioned correctly.

Can’t follow this idea and recommendation.

Assuming he picked 73* STA and 73* HTA along with a 13 CM stem because that is the reach he needs to get the proper fit using his ideal contact points, simply making the STA more slack will only change the type of seat post and saddle rail position relative to seat tube to get it back where he had it, but how does that change anything beyond that? In this case doesn’t making STA slacker just make the top tube longer? Seems the head tube will end up right back where it started relative to BBKT and therefore with same front center.

[mister]

Can’t follow this idea and recommendation.

Assuming he picked 73* STA and 73* HTA along with a 13 CM stem because that is the reach he needs to get the proper fit using his ideal contact points, simply making the STA more slack will only change the type of seat post and saddle rail position relative to seat tube to get it back where he had it, but how does that change anything beyond that? In this case doesn’t making STA slacker just make the top tube longer? Seems the head tube will end up right back where it started relative to BBKT and therefore with same front center.

kirk is talking about make the sta slacker and tt the same length...
that will in fact make the fc shorter

[Kirk Pacenti]

kirk is talking about make the sta slacker and tt the same length...
that will in fact make the fc shorter

Yes, that's what I was talking about.

Can’t follow this idea and recommendation.

Assuming he picked 73* STA and 73* HTA along with a 13 CM stem because that is the reach he needs to get the proper fit using his ideal contact points, simply making the STA more slack will only change the type of seat post and saddle rail position relative to seat tube to get it back where he had it, but how does that change anything beyond that? In this case doesn’t making STA slacker just make the top tube longer? Seems the head tube will end up right back where it started relative to BBKT and therefore with same front center.

Like I said, we can't make meaningful recommendations without more info... For all we know, he could have a 35mm offset post with the saddle slammed all the way back.

[Chance]

kirk is talking about make the sta slacker and tt the same length...
that will in fact make the fc shorter
And if so we’ve just made his bike and reach shorter, making it necessary for him to go to an even longer stem. Unless we just “assume” mokmu doesn’t know what he needs and located his saddle arbitrarily. Maybe that’s the case but why assume the worse?

Mokmu asked a simple enough question. Why do we have to make it more complicated or question his competence. Isn’t it just as easy to assume he measured an existing bike and is using that for his design rather than assume he shouldn’t be doing it himself? That part is hard to follow.

[Chance]

Like I said, we can't make meaningful recommendations without more info... For all we know, he could have a 35mm offset post with the saddle slammed all the way back.
Assuming he got his saddle to bars dimension right by measuring an existing bike or having a proper fit then it doesn’t make any difference to front center. All your example means is that he should use a longer top tube so he can use a standard seat post with saddle rails at center.

[mister]

And if so we’ve just made his bike and reach shorter, making it necessary for him to go to an even longer stem. Unless we just “assume” mokmu doesn’t know what he needs and located his saddle arbitrarily. Maybe that’s the case but why assume the worse?

Mokmu asked a simple enough question. Why do we have to make it more complicated or question his competence. Isn’t it just as easy to assume he measured an existing bike and is using that for his design rather than assume he shouldn’t be doing it himself? That part is hard to follow.

i don't think anyone is questioning his competence.
he is asking for advice about front center and people are giving it him.
if he's not willing to adjust setback, then the answer is very simple, adjust the front center with the head angle and fork rake.

kirk already stated that mokmu needs to give more info, if he does that then he will probably get some very good advice.

[Kirk Pacenti]

Assuming he got his saddle to bars dimension right by measuring an existing bike or having a proper fit then it doesn’t make any difference to front center. All your example means is that he should use a longer top tube so he can use a standard seat post with saddle rails at center.



Let me say it again; unless we have more info we can't make meaningful recommendations. All assumptions, however logical, are most likely incorrect...mine included.

A FC dimension well outside the norm indicates [to me] that there is an issue elsewhere in the design.

The experience of having built and designed several thousand custom frames tells me that you can't assume anything. Its not difficult to replicate the dimensions from an existing bike(s) to a new one perfectly.
That tells us nothing about whether the design / fit was right to begin with.

[eddief]

the relationships can be tricky...especially if one is into the liberal arts. the one dimension missing from the diagram is the trail which is 58 as designed here and that is with the input of a 43 mm raked fork. how about some Slant 6 lugs on this thing?

[Kirk Pacenti]

mokmu,

Have you been able to reconcile your frame design with regard to the front center dimension?

Cheers,
KP

[Fivethumbs]

With the long front center I'm sure the OP is on a big bike. This means the entire wheelbase will be large too. Most bikes I've have seen increase the HTA as the frame gets larger. My 63cm Bottecchia has a 75.3* HTA. The trail is about 49. A stock 62CM Mondonico has an HTA of 75*. I don't know the trail but I'm sure it's high 40s low 50s. These larger bikes can handle well with the lower trail. As an example, I descend the fastest and most confidently on my Bottecchia. If the OP increases the HTA from 73 to at least 74.5 it will bring in the front center and still handle well (I believe).

[Peter B]

Provide your builder with the locations of your feet, a$$ and hands and your intended use, then let them do their job while you ponder colors. If you can't trust your builder with design how can you trust them with fab?

[Chance]

Provide your builder with the locations of your feet, a$$ and hands and your intended use, then let them do their job while you ponder colors. If you can't trust your builder with design how can you trust them with fab?
Because it can be like having your custom dream house designed by an architect. You can give him a good idea of what you want and need, budget, and what it will be used for and so on, but as the project takes shape you may want to make changes to his design.

Some people are OK with letting their “architect” make most decisions while others want to be more involved in what they get. Maybe it’s lack of trust as you mention but it could also be just knowing what you want and having a non-compromising personality.

[Chance]

Let me say it again; unless we have more info we can't make meaningful recommendations. All assumptions, however logical, are most likely incorrect...mine included.

A FC dimension well outside the norm indicates [to me] that there is an issue elsewhere in the design.

The experience of having built and designed several thousand custom frames tells me that you can't assume anything. Its not difficult to replicate the dimensions from an existing bike(s) to a new one perfectly.
That tells us nothing about whether the design / fit was right to begin with.
Didn't know you were in the business of designing and building. Will try to remember and not reply to your posts.

[Kirk Pacenti]

Didn't know you were in the business of designing and building. Will try to remember and not reply to your posts.


Huh? Why wouldn't you reply to my posts? Difference of opinion is fine; ask 10 builders to build you a frame and you'll almost certainly get 10 different designs.

A perfect example of that can be found right here on the Serotta forum. A couple of the most esteemed builders on this forum have over 30 years experience building road racing bikes. Both are both very successful and well respected for what they do. Yet, if you were to look at their geometries on paper they'd be on nearly opposite ends of the design bell curve; there isn't always a "right" answer...



Cheers,
KP

[mister]

to give the original poster a rough guideline, and this of course is only the start of things to think about...you'll end up with toe to tire interference around 580-590mm (variable of shoe size, cleat position, crank length and tire size) and if you're planning on using fenders. a well designed fender will probably take up another 15-20mm minimum.

personally i would probably want front center around 580-590mm on a srtaight road bike.
another bike i have is an all-arounder type design and the front center i think is 601.2mm, handles nice, the chainstays are longer to go with the longer FC.

[Chance]

Huh? Why wouldn't you reply to my posts? Difference of opinion is fine; ask 10 builders to build you a frame and you'll almost certainly get 10 different designs.

A perfect example of that can be found right here on the Serotta forum. A couple of the most esteemed builders on this forum have over 30 years experience building road racing bikes. Both are both very successful and well respected for what they do. Yet, if you were to look at their geometries on paper they'd be on nearly opposite ends of the design bell curve; there isn't always a "right" answer...



Cheers,
KP
Maybe in theory but not sure about in practice. Nothing personal against you.

It doesn’t seem like a good idea for any non-builder to disagree, question, debate, or correct a builder. Nothing to win and everything to lose.

If it is something important enough that needs to be addressed another builder can always step up and get involved.

Thanks for your open mindedness.

[rphetteplace]

My .02 is that your builder should be the one and only person worrying about this. If it isn't you've chosen the wrong builder, or not put enough faith in your builder.

[Peter B]

<snip>

Because it can be like having your custom dream house designed by an architect.


Find a good Architect.

[RedRider]

Give your framebuilder your fit dimensions and let him execute his design. I know a few builders that do not give out design specs and consider them proprietary. A friend of mine had a new fitting and then gave a few framebuilders the results. Over then next 3 years he had 5 new bikes made from 5 different builders. Each one was unique and executed differently but they all had the same riding position and all delights to ride.

[Kirk Pacenti]

I know a few builders that do not give out design specs and consider them proprietary.

I would tend to agree with this. Not because there is anything secret about the design, but rather, how the "numbers" might play into preconceived ideas the customer may have about geometry, fit, handling, etc... I would rather have the ride, not the spec sheet, inform the customers impression of the bike's performance.

A friend of mine had a new fitting and then gave a few framebuilders the results. Over then next 3 years he had 5 new bikes made from 5 different builders. Each one was unique and executed differently but they all had the same riding position and all delights to ride.

This is exactly what I was referring to when I said getting frames from 10 different builders would yield 10 different designs. The assumption being they could all replicate the position and deliver a bike that performed as intended, yet each have unique characteristics, or "personality".

Cheers,
KP

[RedRider]

This is exactly what I was referring to when I said getting frames from 10 different builders would yield 10 different designs. The assumption being they could all replicate the position and deliver a bike that performed as intended, yet each have unique characteristics, or "personality".

Cheers,
KP[/QUOTE]

+1 Nicely stated.
And this is what I consider the real value of a handmade frame builder... the personality of their product.

[Kontact]

I think Chance just meant that having any back and forth with an industry builder can get you banned on this forum. Even if the builder is happy with the conversation.