Why the long front center on road bikes?

[cyan]

Why some road bikes have fairly long front center (e.g. ~595mm or greater on a 54cm) with only 28-30mm tire clearance? For example, 2020 Cannondale SuperSix Evo, Colnago C64, etc. 580-585mm seem more common for that size.

So what does a long FC achieve on road bikes besides avoiding toe overlap (but I'd imagine once beyond 590mm on a 54cm the issue is minimal)? And how do they handle?

[dddd]

Originally Posted by cyan

Why some road bikes have fairly long front center (e.g. ~595mm or greater on a 54cm) with only 28-30mm tire clearance? For example, 2020 Cannondale SuperSix Evo, Colnago C64, etc. 580-585mm seem more common for that size.

So what does a long FC achieve on road bikes besides avoiding toe overlap (but I'd imagine once beyond 590mm on a 54cm the issue is minimal)? And how do they handle?

A longer front center increases rear wheel weighting, which in itself tends to lighten/quicken the steering (except when the longer front center is achieved by use of a shallower head tube angle).

The fork's rake also affects the trail so the steering response can be adjusted back to normal while using different HT angles, with one net result of a slacker HT angle and increased fork rake being more vertical flex in the front end structure (unless the tubing sections are adjusted).

A longer front center also allows harder braking before the rear tire lifts, which I've found is most useful in cyclocross.

[fignon's barber]

Originally Posted by dddd

A longer front center increases rear wheel weighting, which in itself tends to lighten/quicken the steering (except when the longer front center is achieved by use of a shallower head tube angle).

The fork's rake also affects the trail so the steering response can be adjusted back to normal while using different HT angles, with one net result of a slacker HT angle and increased fork rake being more vertical flex in the front end structure (unless the tubing sections are adjusted).

A longer front center also allows harder braking before the rear tire lifts, which I've found is most useful in cyclocross.

That's a really impressive display of knowledge. Well done

[LouDeeter]

I also like to look at the reach. When you subtract the reach from the top tube length, you get the setback, which is driven by the seat tube angle. Knowing your desired setback will allow you to compare frames when looking to purchase new bikes. If you rely solely on the top tube measurement, you might be off by several cm with what you really need. A long front center and a short setback might defeat the handling that you desire, particularly the rear weighting.

[Dave]

These days, stack and reach are what tells me if a bike will fit. The STA only tells me what amount of seatpost setback might be needed. Steering geometry is separate, IMO. The smaller frame sizes that I ride all have fairly slack HTA and trails in the 63-68mm range. Front center is what it is - the result of the reach and steering geometry.

My latest Cinelli has the most slack HTA of 71 degrees, 45mm fork offset and long 415mm chain stays. It handles great. It's rock steady at 50mph and makes low speed U-turns better than any bike I've ever owned.

I just looked at the supersix geometry. In my size they have a slack HTA, but a large 55mm fork offset to get 58mm of trail. That increases the front center. I like more trail.

[Mark McM]

I don't know about the Colnago, but I think I can see what is going on with the Cannondale SuperSix EVO. I attended a talk on frame geometry given by the current Cannondale road bike manager/designer frame designer, Damon Rinard. He felt that the main factor in road bike handling was the steering trail/flop, and that the front center dimension played only a minor part. The primary limitation on the front center was the CPSC regulations on toe overlap, which limits the minimum front center allowed.

If you take a look at the SuperSix EVO geometry table, you'll see that all sizes have the same trail dimension (58mm). In addition, you'll see that there are only two fork offsets (55mm for smaller frames, and 45mm for larger frames). This means that there are also only two head angles used (71.2 deg. for smaller frames, 73.0 deg. for larger frames). Using a a longer fork offset and/or shallower head angle will increase front center.

So, appears that the smaller frames use the long fork/steep head angle combination to limit toe overlap, while the larger frames use the short fork offset/steep head angle combination to keep the wheelbase (and front center) from getting too long. The transition occurs between the 54cm and 56cm sizes. So, the 54cm SuperSize EVO ends up with a longer front center than the same size frame from other brands, which may use a shorter fork offset/steeper head angle for this size.

(I tend to agree with Rinard about the front center not playing much factor for road bikes. I've ridden a number of road bikes with different front center dimensions, and I haven't noticed any meaningful differences in handling that can't be accounted for by differences in steering trail or flop. On MTBs, front center can play a big role in bike handling, because these bikes often see slopes that are steeper and turn radii that that tighter than typically seen on pavement.)

[benb]

Damon Rinard has always seemed top notch but any time there's discussion from a major manufacturer and they're talking about making a bunch of sizes share the same key geometry #s you can't get away from cost savings.

A whole bunch of sizes having the same HTA, STA, only two fork offsets, common chainstay length is all about manufacturing cost.

It is amazing this stuff hasn't all been completely nailed down and been out in public in say mechanical enginering theses for decades considering how similar today's bikes are to bikes from a hundred years ago.

I look for bikes based on frame Stack and Reach too... as long as it fits I have extremely little doubt that any production frame geometry is going to ride or handle badly given it actually fits.

[jkbrwn]

Originally Posted by benb

A whole bunch of sizes having the same HTA, STA, only two fork offsets, common chainstay length is all about manufacturing cost.

Totally. Look at this C40 geo chart for example from 20 years ago. Granted, there's no stack or reach and I can't tell you what fork rake is from this chart, but look at the differences in tube sizes and angles for each size frame.

[benb]

The C40 looks like it's WAY better than average though.

They make an absolute ton of sizes and they're way more optimized with differences than a lot of other big brand bikes.

ISTR stuff like early Giant TCRs trying to get down to 3-4 sizes at one point and every one having the same chainstay length and other stuff like that. And that was right in the same time period as the C40.

Didn't Cervelo also do lots of goofy geometry stuff that resulted in tons of sizes having identical specs for some of the frame angles/tube lengths? They had a bunch of scientific sounding marketing to make it sound desirable but that could have been a lot of cost saving too.

Theoretically if all the sizes take the same wheels they can make the whole back triangle identical on every size if they want to. The bike might look goofy of course but they can attempt it.

All this stuff seems like it has been getting better again in recent years though. At some point the Asian manufacturers get/got so good at making Carbon they could start to make more variations without the cost skyrocketing.

[jkbrwn]

Originally Posted by Mark McM

This means that there are also only two head angles used (71.2 deg. for smaller frames, 73.0 deg. for larger frames).

This is incorrect, though, no? There's a whole 3!

[jkbrwn]

Originally Posted by benb

The C40 looks like it's WAY better than average though.

Yeah, true, probably an unfair example to pick.

[benb]

C40 vs SuperSix is a perfect example..

9 Different Chainstay lengths on the C40.

1 Common Chainstay length on the SuperSix. The dropped seatstay design probably means they can use the same back triangle for every size. The factory can now pump out identical back triangles with no need to worry about which sizes actually sell better or worse. They can even make the expensive models use the same back triangle as the cheaper models and just design weight/stiffness changes into the front triangles of the more high end models. Now they don't even have to worry about which models in the lineup will sell the best as far as the cost of the back triangle.

It looks like some SuperSix models are not available in the biggest and smallest sizes and that will get those models down to only two HTA angles.

[Mark McM]

Originally Posted by benb

Damon Rinard has always seemed top notch but any time there's discussion from a major manufacturer and they're talking about making a bunch of sizes share the same key geometry #s you can't get away from cost savings.

A whole bunch of sizes having the same HTA, STA, only two fork offsets, common chainstay length is all about manufacturing cost.

The other bike mentioned, the Colnago C64, is worse in one way. The C64 use different HTA, STA and chainstay lengths across its range of sizes, but since each size frame needs different molds, Cannondale isn't saving much by using common dimensions. But at least Cannondale used 2 different forks - Colnago uses the same 43mm fork offset from its very largest to its very smallest frames. This results in a trail of 66mm for the smallest size, and a trail of 58mm for the largest size. This will give the smaller sizes rather slow and ponderous handling, which is the bane of many small bikes. At least Cannondale used a large offset fork (55mm) for their smaller sizes, so that they can get a short reach dimension without compromising the handling.

[joevers]

Originally Posted by benb

C40 vs SuperSix is a perfect example..

9 Different Chainstay lengths on the C40.

1 Common Chainstay length on the SuperSix. The dropped seatstay design probably means they can use the same back triangle for every size. The factory can now pump out identical back triangles with no need to worry about which sizes actually sell better or worse.

Colnago is pretty exceptional about the amount of sizes they offer but ridiculously short chainstays has been cannondale's thing for what, 20 years? They're not going to make them longer on larger sizes. They certainly don't all use the same rear triangle. The way that carbon is constructed I'm not sure there's a single advantage to that anyways though.

The new Supersix's don't ride like any road bike I've owned, the handling is a lot of fun. It's.. different but it's really enjoyable to me (work at a Cannondale road shop, ridden them at events but have not owned one).

[robt57]

Two take a ways / thoughts here for me.

I wish before I read that data point, [dddd] in my head was the thought of TT-Reach for that bit for setback for comparative data points. This just never occurred to me.

The other take a way quite possibly explains why I feel more in tune on my Classics Edition Domane. Being I ride 60 +/- CM bike, the Team Issue Classic a 62CM, the 425 CS for a Race frame. The constant CS lengths across sizes for guys on big frames presents a compromise.

I use a lot of saddle setback due to my riding position and long femurs, and have huge feet. So front center is always desirable for me, and long femurs on a lot of RR geometry frames has a bit too much weight rearward.