How to understand frame sizes

[Al in CT]

It is hard to understand to interpret frame sizes and reflect it to real world people. In addition to the huge variations in the seat tube, the seat post has a lot of variation, as well as the pedal crank arm.
I would love to see the measurement taken from max seat height to the lowest pedal position. For example on the bikes I ride, I need this distance to be 38-39". My pant inseam is only 34", and I can't even remember my frame size because it is so irrelevant (lol).
Most bikes are reporting frame sizes around 20" for the larger bikes. And since I can't be there to check out all these bikes, I don't know what to do!

 

[Nova Haibike]

Frame sizes with detailed numbers is provided by the mainstream manufacturers, i.e. Trek, Giant, Specialized. In other words, the brands you would find in a bike shop. Such things are often not provided by online or small sellers, because they often have very little to do with the design and the bike or bikes they offer only come in one size.

 

[Al in CT]

Frame sizes with detailed numbers is provided by the mainstream manufacturers, i.e. Trek, Giant, Specialized. In other words, the brands you would find in a bike shop. Such things are often not provided by online or small sellers, because they often have very little to do with the design and the bike or bikes they offer only come in one size.

I've looked at all the manufacturer's websites for the bikes I am looking at. I also just now checked out Giant's site per your comment and confirmed that frame size is the distance from the top of the seat tube to the center of crank diameter. They then say that frame XL is good for people 6'3" to 6'7". The seat tube length is 23". My present bike needs a dimension of 38" from seat to extended pedal. There is almost no correlation between frame size and people requirement. My point being that a person of a specific height may not necessarily have the same leg length as another of the same height.
They should be willing to go the storeroom and take that measurement for me.

 

[Nova Haibike]

All the measurement are provided: seat tube length, standover height, stack, reach, etc. They give a range of fit.

23" is a big frame. Add 7" for the crank. So you only need to have the seat be 8" above the seatpost collar to get 38". If you feel like there is not enough standover, go one size down. The caveat is when you get a smaller bike, the handlebars will be closer and lower. That can be customized of course. Many shops will offer some kind of credit to swap out parts.

 

[Al in CT]

Thanks Nova. I agree that 23" is an adequate frame. (my Diamondback Wildwood has a 22" frame, whereas my Cannondale ST300 has a 26.5" frame).
7" cranks are an estimate, as they can vary from bike to bike. Longer cranks provide more torque, but shorter cranks provide more rpm. I looked up that cranks can vary from 165 to 180mm (6.5" to 7"). Both my bikes measure at 7". But again, looking at the dimensions of the bikes I am looking at, they don't specify frame size. Ride1up, for example, uses minimum and maximum seat height, but their website doesn't say whether it is from the seat to the pedal or the seat to the ground (the picture is terrible).

I guess I am just asking Court and other reviewers to make this measurement during their reviews to show people the actual seat range.

thanks
al

 

[Nova Haibike]

175mm is the de-facto standard size for mountain bike cranks; certainly for the larger sizes. 170mm would be on smaller or cheaper bikes. 180mm is highly unusual; I believe only road cranks are offered in such a length. I have worked in the business for four decades and do not ever recall seeing a 180mm crank.

The picture on Ride1Up's site shows the pedal-to-seat distance; notice the lower line (the one marked "total length") is above the tire contact point

 

[indianajo]

If you click specs, a lot of vendors have a full set of dimensions on their website. I was looking at biktrix today for a 55" tall person. 3 sizes of frame available.
My yubabike left had a full set of dimensions, came in 2 sizes, and I bought it because my legs are rather short, but I carry 80 lb groceries/supplies. fit fine when the box came in from California. Nobody in town sells small bikes but single speed rim brake pink bikes with a Holly Hobbie logo on the seat. I'm male and rim brakes are obsolete in the rain, they don't work. Bike left has 24 speeds, not one.