Why should I prioritize top tube length over stand over height on a CX bike?

Question

I purchased a 50cm CX style bike from an online retailer. After assembling the bike and filling the tires, I found only had around 1" of stand over in bike shorts. Since I will be commuting on the bike and using it for transportation around town, my preference would be for something with greater clearance over the top tube for the following reasons:

1. Saftey - being able to dismount quickly in an emergency situation
2. Clothing choice - I won't be wearing bike shorts when commuting or riding around town
3. Cyclocross - I do plan on riding the bike off-road, so more clearance would be better on uneven ground.

Now to my question: I emailed the retailer asking for assistance selecting a smaller size, but they "strongly recommend" sticking with the 50cm frame stating it would be "perfect" and that the top tube length is more important than stand over height.

What is the thinking behind this advice?

UPDATE

I kept the bike and am overall pretty happy with the fit. I've used it for commuting, long distance rides and a few races. It's also survived a collision with a bus and lived to tell the tale. Luckily, I wasn't seriously hurt. I can also attest that the stand over height didn't make a difference in that scenario.

Answer 1

Why should you prioritize top tube length over stand over?
Because top tube length affects fit and stand over does not affect fit.
When you are riding the bike stand over does not affect fit.

My CX only has 1" of stand over.
A big triangle is good for stiffness and shoulder carry.
A tall top tube means faster transition to shoulder carry.

I don't get your reasons

1. 1" versus 4" should not affect your ability to dismount quickly.
   You ride on the seat - not the top tube.
   In a controlled dismount the bike is stopped - 1" is a quick and safe stand over dismount.
   If you have to bail because a truck pulled in front of you then lay the bike down - don't stand up.
2. So your clothes touch the tube? What is the harm?
3. How is more clearance better on uneven ground?
   When you stop then don't stop over a hole.
   If you fall on the top tube an extra inch of clearance is not going to save you as your feet are not going to be under and you are going to be moving.
   Even with 2" of clearance falling on the top tube should always be your last choice.

Answer 2

Cyclocross bikes are race bikes intended for courses that are mixed terrain with a number of dismount and carry points. They typically have a larger front triangle so you can easily do a shoulder carry. Theses bikes will also have clearance for at minimum 32 mm wide tires plus some mud clearance. There is a design trade-off here between stand-over and the ability to carry. The geometry is also set up for quick turn ins and pedal clearance as these features are key for most cyclocross courses. And because it is raced focused they will typically have a longish top tubes relative, although not as long as a race road bike.

As such, with a cyclocross bike you will always be starting from point of limited stand-over height by design. While new riders worry about stand over, and it has been used by many as a cheap and easy way to assess fit, rarely is it actually an issue. Most people who get comfortable riding a bike will naturally lean it over when dismounting and starting, which lowers the effective stand over. Rarely does anyone stand over the the top tube with be bike perpendicular to the ground. Therefore, in fitting a cyclocross bike you should expect a tighter stand over fit than other styles of bike.

History: CX vs Gravel

In the past, because of the off-road capabilities, many manufacturers would add fender and rack mounting points so that they could also market the bikes for other uses such as commuting or light touring. The bikes were not designed per se for this, but could do the job.

Recently the trend of “gravel” riding specific bikes have freed up manufacturers to actually tune grave bikes to how most people were actually using cyclocross bikes (i.e., not for racing - but a variety of terrain and purposes). This includes a higher stack (i.e., more upright riding position), slacker geometry with a lower bottom bracket (i.e., more stable) and clearance for larger tires as cyclocross tire clearance directly relates to race regulations. This has also let many manufacturers focus their cyclocross bikes to be only competition focused now.

Answer 3

An inch is plenty.

When I stand over my bike on casual pants without shoes, the pants actually touch my top tube.

When I stand over my bike on cycling shorts and cycling shoes, I find the height of the frame perfect.

A good bicycle is designed to have minimal clearance between your pants and the top tube when standing over it. The reason for this is that frames are stronger that way.

So, the algorithm to select a frame is:

• Consider the length, selecting the frame by length.
• Verify you can at least barely stand over the frame in shoes and whatever pants/shorts you have for cycling. Remember to do this test with the tire size you're planning to use.

If you cannot stand over the frame, then and only then should you consider a smaller size and have some sort of strategy for ensuring length is ok (e.g. by installing a longer stem).

Answer 4

I kept the bike and am overall pretty happy with the fit. Prioritizing the fit over the stand over height was the right decision.

Having used the bike for commuting, long distance road rides, gravel rides, single track trail and a few CX and road races I can say the bike was adequate at everything and I didn't have any issues with the stand over height.

1. Commuting: The 1" height is plenty. I can unclip both feet and stand at a traffic light or stop sign if I need to. Keeping one foot clipped in works as well. Good brakes, not stand over height will help keep you safe on the road.
2. Mounting/Dismounting: There are no issues here because you mount the bike from the side, not the top tube.

If you do decide to prioritize the stand over height over the top tube height, you may find the bike isn't comfortable to ride. I purchased a used mountain bike and thought it felt too small. I checked the size guidelines and determined that the recommended size was one larger. The guidelines are there for a reason, and probably will work for most riders.