Tuesday, January 1, 2008

Low-Slung Fun

When presented the opportunity to build a bike start to finish, I elected to put my theory that cyclocross bikes are built with too little BB drop to the test. I had theorized for some time that a bike that handled more like a traditional stage racing bike would be easier to lean through the tight turns of a cyclocross course.

So the $64,000 question is: How does it handle? My experience in riding the bike I built under Stanton's guidance has been that the greater-than-usual BB drop does help the bike corner better than other cyclocross bikes I have ridden. Cornering clearance has not been a problem. On broad, sweeping crit-style corners where I have seen other riders pedal through the whole of the corner, I have been able to pedal through as well. On the super-tight corners that I have only ever seen in cyclocross courses, corners where the course sometimes literally doubles back on itself is where my rig really excels. I have been able to corner much more aggressively than many other riders. Likewise, I’ve noticed in watching other categories race over the same course that the corners I coasted through were corners other racers coasted through as well.

While I expected the bike to corner well, there were two other, subtle, handling assets I didn’t anticipate. I noticed that in riding through frozen ruts, or any ruts for that matter, that I tended to be thrown off my line less than other riders around me. And on the opposite end of the spectrum (though I believe the handling issue to be related), on sandy stretches I was much more comfortable allowing the bike to slide and drift than I have been on any other ‘cross bike. Indeed, one of my favorite memories of riding this bike was in my district championship and and feeling the bike drift slightly while hammering under full power over sandy hardpack.

But don’t take my word for it. When Tim Rutledge, the former product manager for Redline first got the green light to introduce a Redline cyclocross frame and fork, he elected to design a bike that would handle like a traditional road bike when equipped with a 23mm clincher. He modeled the geometry after two bikes he saw reviewed in Bicycle Guide: an Eddy Merckx and Mario Cipollini’s custom Cannondale. Both were built around 7.5cm of BB drop. Rutledge has moved on to other pastures and Redline’s geometry has evolved to reflect that the company now offers complete road bikes, but Rutledge won a master’s national championship (as did several others) aboard the bike he designed.

The tragedy here is that no one truly understands the interplay of all aspects of bicycle geometry. By that I mean, there isn't an engineer out there who can explain in objective terms how each dimension relates to the others. We know in broad strokes how they relate, but as the previous comments have shown, there is some disagreement about what one truly experiences as bottom bracket height changes. Our use of terminology confuses the issue: Is a bike with a low bottom bracket (7.5cm of drop or more) more stable or more maneuverable? It comes down to how you think about bicycle handling. And while the specific differences in physics between how two-wheel and four-wheel vehicles handle are substantial, I do believe the differences in handling between a Mini Cooper and an SUV do help to illustrate the difference in sensation, because what is at stake is a matter of perception--if the rider or driver perceives confident handling, greater speed seems possible and not unreasonable.

While it is true that I could have shortened the bike’s trail or wheelbase, both those approaches have liabilities related to tire clearance and toe overlap that must be worked around. I think most builders would say that less trail will make a bike more responsive, but that isn’t the same thing as cornering easily and what I was looking for was a bike that I could lean without fighting, a bike that increased my sense of confidence when in a corner. I found a design that I like, no more no less. Ultimately, the question is why the industry continues to follow a convention based on a piece of equipment no longer used, a convention which can be validly questioned.

19 comments:

KMY said...

Keith Code wrote some interesting stuff about motorcycle geometry, in his books, which crosses over somewhat. I like your colour choice by the way.

bikesgonewild said...

...again, excellent...wow, lotta points to ponder...

...as we know, early cross bikes were given higher bb's due to the nature of the terrain...

...i'll go out on a limb w/out confirmation & say i'd bet most cross courses have been designed to be incrementally faster, over the years...you may just have designed a prototypical cross machine for new age thinkers...

...pedals alone nowadays allow for greater lean angles, so you can better utilize that lower bb...

...you mention that "if a driver or rider perceives confident handling, greater speed seems possible & not unreasonable"...that's the old 'seat of the pants' thing & you're quite right, that can make all the difference in the world...

...gary fisher is one of the great 'seat of the pants' designers of excellent handling mtb bikes...i did some work for gary years ago & my personal bike was the first prototype 72deg. head tube angle mtb we knew about...race bikes at that time were typically 70deg...gary had a theory & like you, followed up on it...although it was radical for it's day, everything was so well thought out & configured, that bike handled beautifully...in tight fast corners, both ends of that bike would stably drift...

...anyway, i'm glad you stuck to your guns & built that cross bike...i'm sure it accomplished more than successfully proving your theory...congratulations...

gewilli said...

Why are things done they way they are?

It is a question no one can answer i fear.

Looking at some old measurements of my bike... it seems to fall into the not so dropped BB :( (carelessly measured with all the bits on the bike at ~6cm... or measured from the ground up with big commuter tires at 29cm)

But, it is in between my Paramount (lower) and my Klein (higher)...

And, what can i say... other than i'm getting all drooly about a speedvagen again with the super low BB and Sacha's special tubes...

...drool...

thanks for building and writing about it and giving us all something to dream about...

question i'm now wondering is:
how much BB drop does ATMO use?

guzzilla said...

The '08 Redline Conquest has a 6.5cm BB drop. When did Redline have the 7.5cm BB drop that you mention? The Jamis Supernova has a 7.4mm drop, so you are not alone.

Bolivar said...

these are great points...I've been considering a new cx bike - looking at the Gin&Trombones which has a 7 cm drop. Most of the other bikes I have been considering are around 6cm. I noticed a big difference in handling when I rode a friends G&T as a pit bike. I could lean a lot more into the corners like my road bike which gave me more confidence to enter turns at high speeds.

tjh said...

gewilli, just as an fyi, my speedvagen has 70mm of drop. I don't consider that 'super low' but compared to the 58mm drop of my steelmans, it's been nothing short of a revelation...and as for the super swagy tubeset...mmmmmm.

...end sacha lovefest...

bikenerd said...

Not that I'm any skilled 'crosser, but I'd tend to agree with your theory. My custom steel cross bike (Rock Lobster)has a fairly low BB (72mm drop) and it corners like mad. I didn't specifically request such, but Paul sure knows what works. I do whack the occasional pedal on an off-camber or a rock, but the full-time handling and stability more than make up for it.

~bikenerd

Colin S. Dees said...

It is funny you built this bike... I have just such a bike built nearly 30 years ago! My Gazelle built with Zeus internal fork crown and lugs in Italy is 60cm by 57cm, and has 8cm of BB drop.


I have no idea why such a bike would be built in that day, but with light/tight construction, and nary an extra hole or braze on, it's obviously a strictly race-only machine.

My finding was that before I adapted to the bike, coming from a tall, tall fixed-gear cross bike where I obviously pedaled all the time, was that I was clipping a pedal all the time; every rut, every corner, and coasting too much. Where the bike excels is what you mentioned... sand and snow.

Jason Musgrave said...

atmo you're right on the money on this one...

I did the same experiment when I built my cross frame for this season: 75mm BB drop. That bike really comes into it's own on as things break loose...

I'm a tall lad and ride 177.5's so I've occasionally whacked a pedal, but it's a worthwhile trade off as soon as things get sketchy...

An added benefit not mentioned is as the BB is lowered, the saddle is lowered relative to the ground. A bike with a deep BB makes remounts easier because you don't have to jump as high to get back on...

Anonymous said...

Padraig,
Thank you for praising Toby. He's done a great job with supporting local Jr racing in the New England area. His course has been described by some as "awesome". To build a bicycle that you get to ride on is a true joy in cycling. As to your discussion of BB drop, and thank you for using that term, rather then BB heightfrom the ground, I have to politely disagree.
You're looking at the frame as simple series of variables, like front-center measurement, fork rake, head and seat tube angle, and BB drop. If you alter one, then the frame will work "better". What is much more important when looking at a frame design is the "whole" picture, including TT length, chainstay length, seat tube length, BB drop, and front center measurement. If I may offer a suggestion to people that are considering doing a frame drawing, take a verticle line, and pass it through the center of the BB, from the TT. Draw a horizontal line from front hub center to the rear hub, and a two verticles through the hubs. Start with your fixed points, and the complete wheel circumfrences, with the tires, and work your way in to the more easily moved pieces. Your ultimate goal is to make a balance, sensible, real world design, that will offer any rider all the advantages that certain aspects of good frame design can offer them. You cannot change wheel circumfrence, the length of cranks, or where to put the braze-ons, and you have to take into account shoe size and tire clearance. Think about dropping a BB, it changes the tire clearance at the chainstays. And how much room will you have between the seattube and the tire, for wheel changes? I think that things like the stability, steerability and overall ridability of the a frame are also affected by the size, and shape of the tubes used with certain types of frame geometry. The finished size of the bicycle is the most important factor in doing a design, because what works for a 50cm frame will not always benefit a 60cm frame. Think about looking at a frame as two separate pieces of single tool, a front, forward of the center of the BB, and a rear, the aft behind the center of the BB. Now, try to make a frame that has the rider balanced between those two areas. My dream now is to take a design, look at it with software that shows me how it's going to respond, and changing my lay-up to change the modulous of the tubes, and the compliance in differnet angles of deflection.
So, my thought is that just lowering a BB without looking at a number of other factors will not always make a "better" riding bike. Otherwise we'll see the fancy people walking into shops, insisting that they get a "75mm BB drop on the fixie I'm looking at." Ughhh, fashion!
As an aside, riding one day, on a frame I had designed, drawn and built, I found out why the Italians always seemed to have longish front-center measurements. I had checked many dimensions on about 60 different frames to see what was being used by different builders back in the late 80's, as I was designing my own frame. On this ride I was climbing a large hill, and found myself weaving from side to side, making sharp turns at each edge of the crowned unpaved surface as I slowly made my way to the top. At each of the turns, I'd hit my toes on the tire, as I turned the bars, moving the front wheel off center, to clear the edge of the road. It was in the afternoon, and I was finishing up a 75 mile road ride, a little short of water, and this was the last hill going home. I was pretty upset with the builder, and cursed his lack of attention to a small, but significant detail. But, he built great wheels.
Tacissimo

Anonymous said...

I have read that e-richie uses 8 cm bb drop on his road frames, not sure if that is the same on his cross bikes...

Curly said...

Rutledge also rode the Tour of Italy... that alone makes him the authority on anything bikes

DYG said...

I do not know what the number is, but the BB on my Richard Sachs feels pretty low. I can do race starts with my ass in the saddle and one foot on the ground. Pedal strikes are never a problem on cross race courses with the bike. I am sold on the idea of a low BB and low center of gravity for a cross bike (and road bikes in general).

Anonymous said...

Great post on the low BB. I've always been a fan of a low BB since I read about Sachs road bikes over 20 years ago in the old Bicycle Guide. I've had a few custom frames made since then, between 75 and 80mm and have always loved the handling.

I just had a custom Hampsten Strada Bianca built (welding by IF) and worked with Steve Hampsten on the geometry. The Strada Bianca is the Hampsten Bros. "gravel road" bike, so it's very stable, and you can take it on rough roads while still having good comfort and handling.

We went back and forth between 75 and 80mm, and Steve settled on 75mm. The reason: 75mm is still low, this specific frame had a slack head tube and I was using (relatively) fat road tires, so the frame was going to have plenty of stability already.

I received it just before Christmas, and have only put in a couple of weekends worth of riding, but yesterday, I took it on dirt paths and icy gravel roads. The geometry, with the low BB contributing, handled the rough surfaces very well, despite the fact that I had road, not cross tires on it. The ride is wonderfully stable and reassuring.

Brian said...

Interesting series of articles.

My 2006 LeMond Poprad has 74mm of BB drop.

Brian said...

Sorry-- I forgot to add:

The Poprad has 74mm of BB drop (for all frame sizes) compared to 70mm for the steel Sarthe. This is on a 49mm frame.

The carbon Zurich (49cm) has 72.5mm BB drop and the top-of-the-line has the same.

Anonymous said...

I think this is a great discussion. I currently own Ridleys which have a very high bottom bracket that annoys me. I'm switching frames for 2008 and have considered going with something lower along the lines of the discussion here.

Some manufacturers list BB Drop and Some BB Height. Do these two numbers correlate? Will knowing one tell me the other? I'm just trying to evaluate several frames on the same basis. Thanks.

Andy A said...

I too am coming off a couple seasons on bikes with relatively high bb's: a Kelly and a Ridley Supercross. Infinitely dif't animals but the same when it came to the vague, high, slightly unstable feeling I got in a lot of situations. I thought this was the way it had to be on cross bikes ... until I got my Speedvagen. The sexy tubes and dropouts and special Sacha touches are the cream cheese frosting to the geometry which is where the real difference in performance lays.

One problem though is that in all its infinite Sexvagenness it still cant lift itself off the hook in the basement. Maybe Sacha and co. can work that out for next season's Gen. 2's.

Ron said...

I don't believe you Freddy :)

Good work!