Slack geometry, once and for all...

I don't really know much about cyclo cross frames but my take on slack geometry would be:-

More laid back seat tube / head tube angle.
longer chain stays
longer wheelbase
higher bottom bracket......

..when compared to a road bike.

A road bike is often describes as tight, responsive, stiff etc. Making the above changes would therefore make the bike feel "slacker", less twitchy, more stable over rough terrain etc. Mountain bikes also share these traits.

Incidentally, chain stays on a standard road bike do slope upwards from the b/b to the wheel axle to keep the b/b lower than the axle. MTB hardtails are close to horizontal to keep the b/b higher and to compensate for smaller wheels.

jimbeauxk said:

I bought a 59cm cyclocross bike a year ago from an online dealer. Got a fair deal, I don't have any issues with the bike. The problem is is that I'm 6'2" and fluctuate between 210 and 230 lbs., which is the reason I decided to go with the cyclocross setup, but don't know if I took the right size.

Much to-do is made of sizing down, and I could go blue in the face with the number of threads suggesting you size down the cross frame 1-2 cm from a standard road frame. I took a guess at the 59cm (which would indicate about a 62cm standard frame, no?), and I seem to be not so much cramped, but more compacted, with my knees in my gut when I ride. I'm considering a new road bike, or a longer/higher stem, or a longer seat post, or anything else I can dream of to help me strettttttch out. Maybe it's that I'm not used to being compact, and the bike fits me perfect??

Anyways, everything I ever read on the difference in frames always speaks of "slacker geometry" and higher bottom brackets. Do common-speaking folk normally know what is meant by slacker geometry? I have a degree in math, and I must have slept through that class. Is there an illustration of what is meant by this somewhere online? In simple terms, I would guess my seat tube is more vertical, as well as the steer tube, hence the rake compensation. Perhaps the top tube is shorter because it "rests" further down the seat tube? Should my exposed seat post be 1/2 the length of the seat tube?? I'm afraid I look like one of those circus bears on a bike. And on the bottom bracket, if you raised it higher, wouldn't the chainstay have to go from the horizontal position to an upward sloping position, given a fixed 700 wheel hub height?? Seems to me that every bike I ever see has a horizontal chainstay, unless that's an optical illusion to us normal people.

Anybody care to take a shot at defining slacker geometry? Should I continue to ride the bike until I get used to riding in the fetal position?

The wisdom on 'downsizing' a cross frame applies if you are riding cross with it. For road riding it doesn't necessarily fly. If riding road it's likely too small for you. If you want a properly sized road bike, get properly fitted and quit guessing or you will make another costly mistake. Slacker in simplest terms usually references seattube and headtube angles, and if 73* were considered 'normal', then 72 would be slacker. There is nothing about a compact bike or sloping toptube that makes it fit differently.There is no voodoo, it simply has to be the right size and correct geometry(usually seat angle and TT length) for your body type. All 6'2" riders are not the same, nor do they have the same fit requirements. More saddle time also helps as does flexibility and conditioning.

Interesting - a lot of the threads talk about cross bikes being twitchier. And I thought the rake on a cross compensated for an upright steering column, as opposed to a "laid back" angle. Goes to show you how little I know about the bike I bought...

darrenf said:

I don't really know much about cyclo cross frames but my take on slack geometry would be:-

higher bottom bracket......

..

Nope, that ain't part of it.

boudreaux said:

Nope, that ain't part of it.

And just to be clear, the difference in geometry across bikes (aero/tri bikes not included) is exceptionally small, controlling for overall size of the frame (smaller frames tend to get steeper seat tubes). Nearly all are between 73* and 74* seat tubes, and the BBs are within a few millimeters of height. The only real differences are in the top tube length (and even that's not much), and the slope of the top tube (which only really changes standover height).

Aztec said:

And just to be clear, the difference in geometry across bikes (aero/tri bikes not included) is exceptionally small, controlling for overall size of the frame (smaller frames tend to get steeper seat tubes). Nearly all are between 73* and 74* seat tubes, and the BBs are within a few millimeters of height.

Road apples.

boudreaux said:

Road apples.

I have a question- with all this talk of relatively small changes in goemetry making large change in riding characteristics- how come no one really talks about the big changes in geometry with different size bikes. When I look at geometry charts for a specific bike model, the seat and head tube angles change by 1 or even 1.5%, and the bottom bracket height may increase by 12 mm in a small size bike. What gives- don't these changes really change the ride of a smaller bike (I'm 5'6"😉 compared to a larger model of the same bike? How can I interpret the reivews of a bike when the reviewer rides a different size frame?

Jason

jasonsankar said:

How can I interpret the reivews of a bike when the reviewer rides a different size frame?

Jason

Who said that interpreting reviews is a useful exercise? The consensus opinion is to get on a bunch of different bikes and test ride them. I can vouch for this approach, I soon started to figure out which geometries felt good and which ones weren't doing it for me.

Randybaker99 said:

Who said that interpreting reviews is a useful exercise? The consensus opinion is to get on a bunch of different bikes and test ride them. I can vouch for this approach, I soon started to figure out which geometries felt good and which ones weren't doing it for me.

What an intelligent response. Preach it brother.

boudreaux said:

What an intelligent response. Preach it brother.

Yes, I wish I had heeded this warning. I think newer riders, as myself, tend to take some of the reviews from forums like this to heart and assume that bikes that get glowing reviews must be fine purchases. I'm now in a situation where I understand I was wrong, but I'd sleep better knowing why. Knowing what slack geometry is and how it affects a bike's ride may help me understand how to remedy it. Maybe by adjusting a component, I can simulate the size or geometry of a properly fit bike. Otherwise, I'll have a "hardly used" bike on eBay. Reading between the lines, I'm hoping to find out which is more cost effective.

jimbeauxk said:

Yes, I wish I had heeded this warning. I think newer riders, as myself, tend to take some of the reviews from forums like this to heart and assume that bikes that get glowing reviews must be fine purchases. I'm now in a situation where I understand I was wrong, but I'd sleep better knowing why. Knowing what slack geometry is and how it affects a bike's ride may help me understand how to remedy it. Maybe by adjusting a component, I can simulate the size or geometry of a properly fit bike. Otherwise, I'll have a "hardly used" bike on eBay. Reading between the lines, I'm hoping to find out which is more cost effective.

The bike you bought may be a perfectly good bike for cross or comuting,but not necessarily for road if that is it's primary use. You have a FIT problem. It may have NOTHING to do with so called 'slack geometry'.You really do not remedy it.Dollars to donuts you bought the wrong size bike.As I said, get fitted by someone that knows what they are doing,and quit guessing or you will throw more $$ down the toilet. In my not so humble opinion, reviews are wortheless, unless you are targeting a specific product like wheels for instance, and the majority of reviews are negative in that they point out faillures or design flaws.That's a red flag. The ones that start out with: 'I just can't say enough good about...blah, blah,blah,blah...might just as well be flushed.

boudreaux said:

Road apples.

Road apples road apples! Almost all roadbikes have 73* seat tubes. Almost all have the same BB height (or drop). Head tubes are similar angle as well, usually within 0.5*. Nearly all the variety comes from the top tube length and height/slope. You know this, so I don't need to tell you to check it out.

Note that I said "controlling for size" since smaller frames tend to get steeper seat tubes. Re: the poster who mentioned that, I would have to guess that's the method used to shorten the top tube since the wheelbase can only be shortened so much?? Just my guess.

Aztec said:

Road apples road apples! Almost all roadbikes have 73* seat tubes. Almost all have the same BB height (or drop). Head tubes are similar angle as well, usually within 0.5*. Nearly all the variety comes from the top tube length and height/slope. You know this, so I don't need to tell you to check it out.

Note that I said "controlling for size" since smaller frames tend to get steeper seat tubes. Re: the poster who mentioned that, I would have to guess that's the method used to shorten the top tube since the wheelbase can only be shortened so much?? Just my guess.

Road apples.

boudreaux said:

Road apples.

:-P Prove it. There's lotsa data out there you can cite. Trouble is, most of it supports ME.

Aztec said:

:-P Prove it. There's lotsa data out there you can cite. Trouble is, most of it supports ME.

I have better things to do besides proving the obvious.

boudreaux said:

I have better things to do besides proving the obvious.

I hate to flog a dead horse, but I guess that I did not ask my question very clearly. As I can only ever try out a small frame due to my being "altitudinally challenged," I wonder if small frames ride substantially differently than larger frames in the same model line.

If anyone has ridden small front wheel bikes like Terry bikes, is the ride different (assuming that the geometry of a small wheeled small frame would be similar to that of a larger sized standard frame)?