Ride the Puddles

Stuff Beginners Should Know (But Almost Never Do): Bike Sizing: CX and MTB

Posted by bikezilla on August 8, 2010

NOTE: The earlier version of this was just unedited notes that I was working from. It was intended to be saved as a draft, but was accidentally published. DOH! END NOTE

This time it was Bill Larson of Cycle Path in Portland Oregon that I was pestering.

Even if you don’t live in the Portland Oregon area you’ve likely heard of Bill, because he’s been included several times in articles at Cyclocross Magazine.


I’ve said before, to me, it seems that bike sizing is the first step in finding a proper bike fit. But it’s something most people know almost nothing about and it often seems like bike dealers and bike shop techs aren’t much better informed.

Bill Larson:

“I agree with you in it being the first step. Yup, fit is something that does escape many people, is very important, but is getting more attention and play in shops all around the US.

I see more shops taking a serious role in bike fit these days. The “mom and pop” shops that dont tend to progress with technology and trends as quickly are taking more interest in providing good fit services. The bigger shops that focus more on business, that talk about how many “units” they sell in a year rather than talking about all the awesome bikes they built and sold to happy customers, are getting more into fit services. I really think bike fit is coming of age and is being seen more universally as something that needs to be taken seriously and offered if a shop wants to weather the storm of these economical and informational times.”


When I bought my MTB the dealer used nothing but standover height (he said 1″ inch for MTB, 2″ for Road).

When I bought my CX the guy used God knows what method. His only input was “You looked a little stretched out on the 56 cm.” and then only after I’d asked to test the 54 cm.

Bill Larson:

“It was typically said that you needed 2″ of standover for mtn and 1” for a road bike. Any time riding off road you need more standover and typically a smaller bike to be able to man handle it easier. However, geometry for bikes has changed a LOT in the last 10 years and you really cant use standover anymore because top tubes on bikes slant down so much more than they used to, not to mention many other refinements.

Today bike fit is all about top tube length.

For decades these rules of thumb were the standards for fit and those people within the industry who didnt progress (that is, continued the learning process as the bike industry matured and got much more technical and specialized) still often use the same old techniques.

Its one thing to use these simple tactics to initially get someone out on a bike for a test ride, but in the end if someone is purchasing a bike from you, further steps need to be taken to assure its a great frame size and there is a proper fit.”


It seems like from shop to shop there is a lack not only in standards, but in the education of the techs and sales people. That isn’t helpful when the average person is entirely ignorant on the subject and expecting that the guy selling the bike is going to be an expert, and so trusts him.

Bill Larson:

“As I have said, things are getting better in terms of many more shops having fit knowledge. Like any industry, there will always be the simple shops and/or those that dont believe in technology, dont have time, interest, financial ability or the customer base that pushes them to invest in these great customer services. Because of this there will always be customers that wont have great experiences riding bikes and therefore wont get “hooked” on cycling.

It should be said though that there are PLENTY of resources for education and standards on fit. One just need put the time and interest in searching and making it happen.

It doesnt have to cost a ton of money. Obviously it can though.

Given the importance we see in fit, we put a lot of money and time in that direction. We use the Retul fit system which is an LED motion-based fit system. Its a great tool as its extremely precise in optimizing one’s fit.

You dont have to use something such as this to great a good fit, but it helps when you have a demanding customer base or one that is focused heavily on performance. You can get more info on Retul at

Furthermore, I have spent time going to fit schools, seminars, reading as much as I can find and even studying up on anatomy/physiology and sports science. There is a lot of bad info out there, there is a lot of good info. I think by learning as much as possible you take a broad spectrum and figure out what seems to make the best sense.

Many customers do tend to have a bit of fit info. This can be good or can be bad. It all depends on how accurate it is, but its a great step in getting a customer towards a more exact fit.

However, the responsibility should not be left up to the customer. It is not their profession and they should be able to rely on their professional bike shop for that service.”


What should we be looking for / at when we size a CX bike? A MTB?

Bill Larson:

“You gotta search out a great shop that you trust that seems to know what they are doing. It may not be easy depending on what city you live in.

In Portland, customers are a bit more lucky as its a very progressive, bikecentric city that places a lot of importance on bikes as a way of life. It doesnt mean each shop necessarily is giving out the best info, but the chances are much greater and any shop that has put any effort whatsoever into learning about a professional level of fit will be much better than one that hasn’t.

Weeding these shops out shouldn’t be too difficult. One could just go into a shop and ask:

— what kind of fit system they use,

— who the fit tech is,

— what accreditations they have, etc.

If they have put some good effort into this, it should show.”


Is there an accreditation service?

Bill Larson:

Serotta has a fit school. Retul has a fit school. CronoMetro has a school. BikFit has a fit school. Slow Twitch has a more triathlete-specific fit school. These are just a few off the top of my head.

There are lists of fit data that give benchmarks of angles used in fits. I’m sure if one were to do a simple Google search, he or she could find plenty of this kind of info. So, there are plenty of references and sources for professional knowledge and experience.”


What’s the proper toptube height vs standover for either style of bike? Is this even a valid method of finding the proper size bike for either CX or MTB?

Bill Larson:

“This would not be a proper method for finding bike size anymore. You obviously need standover, but the standover doesn’t correlate to bike size accurately. As I said, top tubes slope down more these days, so standover height is inherently much lower than it used to be.”


Or should be be looking at something else, like top tube length?

Bill Larson:

“Top tube length is king!

Bikes used to be square geometry which means if you rode a 56cm bike it had a 56cm top tube length and seat tube length. So, you could have a bike that was the proper length, but you may have had no standover. So, you’d downsize to get some standover and you had to compensate for the short top tube length with a long stem.

People would also use layback posts to get the cockpit longer even though they may have needed a straight-over seatpost so that their knee was positioned over pedal spindle in the three o-clock position of the crank revolution.

Now with sloping top tubes you can get all the standover height you need even though you might need a long top tube length for a long torso.”


I know this is a complex subject, but what are the most basic things anyone interested in buying a bike should be aware of in sizing their new ride?

Bill Larson:

“It is complex subject, but I will try and give you some advice that could be useful even though it might require some tools and may not be easy walking into a bike shop to purchase a bike:

1. Extension angle of the leg is of utmost importance. When seated with foot at bottom of crank stroke there should be between a 30 and 40 degree extension angle.

That would be the angle if you were to extend the femur out past the knee cap and use the tibia and femur as your angle arms.

A glorified protractor with long arms (called a goniometer) is very useful in facilitating this.

2. From here you’d check the flexion angle, or the angle at the 12 o-clock position of the crank. here you’re ideally looking for a 110 degree angle using the femur and tibia as your arms for the goniometer and taking the angle in front of the shin, not behind the leg. Given you have a proper extension angle, if you get a flexion angle in the range of 115 degree, you could ideally use a shorter crankarm length.

If you’re getting a flexion angle in the 100-105d range you could ideally use a longer crankarm to bring extension/flexion into a 35ish/110ish range respectively.

3. The next part of fit is to get the knee over pedal spindle in the 3 o-clock pedal position whereby you’d drop a plum bob down to accomplish this.

You can move the seat for/aft to accomplish this.

4. At this point you address upper body fit criteria.

There are many different ranges in upper body based on what a person desires in a fit. Comfort? Performance? A little bit of both?

The benchmark for back angle on a road bike or cross bike is 45 degrees, 50 degrees on a mountain bike (MTB), 20-30 degrees on a tri/tt bike.

You choose a proper back angle from these benchmarks based on where your intended purpose falls.

If you want a more sporty road fit, you can set yourself up more towards the 40 degree back angle.

Time trials are very short as opposed to triathlons, so going 20 degrees or lower on a TT bike is common, while on a triathlon bike people trend 20+ in back angle.

Back angle is derived by taking a horizontal line (parallel to the ground) at the top of the femur and making another line from that point to the acromion process, which is the most bony tip of your scapula located where your shoulder meets your arm. There are other things that go into fit, but this nails things in a nut shell.”


If toe overlap is a problem do we have the wrong size frame or a frame with the wrong geometry? Or is toe overlap an acceptable problem we should just learn to deal with?

Bill Larson:

“It is rather acceptable.

Cross bikes tend to have big issues because they have bigger tires and shorter top tubes on average than road bikes.

Its very common as you get down into the very smallest of sizes. People need a given size top tube, but at some point if the top tube goes shorter, the rider will have massive toe overlap issues.

So, you get around the 50-52cm range and with many brands you won’t see top tubes lengths shortening much as you go smaller in frame size.

This sucks as its hard to get a bike short enough for someone, but its seen as a lesser evil to toe overlap. So, people then tend to go with VERY short stems or get a fork with a funky rake which can make a bike handle a bit funny.

Ultimately this is where custom geometry could come in and help a bunch, but it costs a lot of money too.

My wife is 5’2″, so she is a great story here.

We had Moots custom build her a road bike and cross bike. We had the head tube angle cocked back half a degree which kicks the wheel more forward and the top of the steertube more rearward thus effectively making the stem closer and wheel further away.

You obviously wouldnt want to mess with this too much or you’d end up with a funny handling bike, but she doesnt have any issues with hers.

Additionally, we had the seat tube angled forward by 1 degree which brought the seat closer to the handlebar while keeping the crank the same distance away from the tire.

Again, this is something that can mess up a bike’s handling if taken to extremes, but she loves the bike.

Additionally, Moots figured out between the two of these alterations how small we could go in top tube length combined with intended crankarm length and size of Samantha’s shoes what we could run to where she wouldn’t have toe overlap but end up with as short of a top tube as possible.

Samantha has never had better fitting, better handling bikes in her life! Of course there is a cost to this, but if it positively changes a bike geek’s experience on a bike like it has done for Sammy, then its obviously worth it.”


Is there a fitting system that you recommend once the proper size frame has been found?

Bill Larson:

“There are many adequate systems out there, but LED based 3-dimensional motion-capture systems are awesome.

They place LED’s on each necessary pivot point of the body; metatarsal, heel, maleolus, epichondral of the femur, greater trochantor of the femur, acromion process, elbow, and wrist.

The unit that reads these LED’s and plots them out on a computer not only sees these points on a x/y axis, but also knows how far away these points are, so we get a knee tracing as your leg goes up and down through its pedal stroke. So, we can see what kind of angle your pedal stroke has, we can see how much your knee moves medial/lateral through its stroke. This can help us weed out knee/forefoot issues that some people have where we can try and fix problems with shoe/pedal shims, longer pedal axles, etc.”


How do we arrive at the proper frame size / top tube length for CX? MTB?

Bill Larson:

“It’s about achieving the correct upper body angles once extension angle in the leg and knee-over-spindle has been acheived.

This obviously isnt an answer that helps the consumer address frame size very easily just walking into a shop, but a very well-versed shop will be able to look at a customer and have a good idea of frame size, if not a choice of one of two sizes.

After that its about setting you up on the bike and making the right adjustments to optimize the fit and building a bike that is proportionally correct in its cockpit part dimensions.

Ideally building a bike that is well balanced in terms of weight distribution and part sizing. If you got to put a 130mm stem on a 52cm road bike to get the upper body in an ideal position, obviously the person is probably better suited to ride a 54cm with a 110mm stem….”


— Once we get the basic numbers, are there any modifying factors like age, fitness level, flexibility and medical conditions that we should make adjustments for?

Bill Larson:

“Definitely. This is all asked early on in the process though.

Can you reach your toes when bending over? Your age? Will you be racing? Are you a recreational rider? Are you somewhere between race and recreation? Have you had knee, lower back, upper back, shoulder or neck injuries? Any leg length discrepancies that you know of?

A fit tech will take these answers into consideration in terms of how low he/she sets the back angle.

This info will help decide if we need to spend more attention addressing biomechanical issues with the knee or forefoot where we might install cant wedges or still one leg to make it the same length as the other.”


— Are we looking for top tube numbers that are long or short in comparison to the equivalent numbers for a road bike?

Bill Larson:

“You will typically setup the cockpit of a CX bike shorter. Therefore, either the top tube ends up being a tad bit shorter, or you make up for it in the stem length or bar reach.

A shorter reach allows you to handle a CX bike better offroad. It gives you a better posture to lift the front end up, bunny hop, corner in slick sharp turns and handle bumpy terrain.

MTB top tubes are generally longer or at least as long. That, and/or an MTB bike ends up with a longer stem. MTB benchmark for back is 50 degrees making it a more upright fit than a road bike, but MTBs don’t have a handlebar that extends forward another 3-4″ out to their shifter.”


For riders at the smaller and larger end of things, what should they look for as an acceptable range for head tube and seat tube angles in order to avoid purchasing a poor handling bike that technically may be the correct size?

Bill Larson:

“We all have different ideas of what make a bike handle well. I think a shop that does a great job of providing a good fit for a test ride will give a customer a wonderful opportunity to see if that bike has the attributes he/she is looking for. Since we all have different fit needs based on flexibility, injury, etc this question can’t be answered simply or with one answer for all.”


Is there some factor or factors commonly overlooked when sizing one type of bike vs another that we should be aware of?

Bill Larson:

“Getting a bike too big or small are bummers we all have heard about. So, this is where you need to find a good shop and put some faith in them once they have earned your trust.

If you’ve had some experience in riding bikes, have bought a couple/few in your time, you probably have some input towards what made your past bikes good or bad. Feel free to offer that info up when discussing the size bike you’ll be getting next.

Many people are in between two sizes of a make/model they feel they absolutely want. So, weighing the pluses/minuses of going up or down in size need to be weighed and a good shop employee can help with that.

All brands are not created equal size wise. dont get married to a particular frame size. If you owned a Cannondale before and are buying a Moots now you may not be getting a 56cm Moots just because thats the size of the Cannondale. They derive measurements from different spots on the bikes. This is why good fit always goes back to getting those fit dimensions dialed in once you’ve found a frame size or two that appear to be the ticket.”

Reference the Road Bike Sizing article, to get an idea of how to reach the starting point for your ideal top tube length.

Here’s the contact info for Bill and for Cylepath:

2436 NE MLK Blvd
Portland OR

PHONE: 503-281-0485




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