Sizing Trek 7100

[kantkawianning]

Hello,

My wife recently purchased a Trek 7100 "Lowstep." She is 5'7" with a 33.5" inseam (measured for biking, not clothing).

She was sized at the LBS on a 16 inch frame. The other frames sizes available in this version are 14 and 18.5 inches.

I'm not sure this is the right frame size, but I could be wrong.

The shop set her up with the saddle low enough for her to completely plant her feet on the ground when seated, which is fine, since she is just getting back into biking. However, with this setup, her knees rise way past parallel during an upward crank rotation. She hasn't complained of discomfort yet, but she hasn't ridden it much. My thought is that this may eventually cause knee problems, but I'm no expert. Is this normal on a hybrid/comfort bike?

I have the same exact inseam, though I am 6' tall, and when I rode it in this configuration, it didn't feel right to me.

I raised the seat for her, so that when seated in the saddle only her toes touch the ground, but her knees still rotate a little past parallel.

Being a good husband, I didn't want to interfere with her shopping, but I did notice two things: they searched the shop and an offsite warehouse for a 18.5 in this bike for her to try out, but could only find the 16. Part of me felt they were simply trying to sell stock on their floor. Secondly, after waiting 30 minutes for the "Bike Prep," I realized when we got home that the tires had not been filled. This lapse in basic service, and the subtle "desperation" of the sales assistant to make a sale causes me to wonder: did she get the right sized frame?

I should add, my wife did try the 18 inch frame at another shop, but she felt less comfortable in it than a Navigator 100 frame (I forget what size it was). My guess is that the added height of the wheels scared her a bit.

So, I know the geometry on a hybrid/comfort bike is different, but is this high knee rotation normal? Or would she be better off on a larger frame with a lowered seat post?

Any input would be gladly received,
Patrick

 

[60ish]

I am going to guess that she has the correct size. My sister is 5' 10" and she had to step down to a 16" bike after riding an 18" for years and being uncomfortable. I will tell right off that bike stores very frequently sell women bikes that are one size too big (that's why he was looking for the 18"..he'd have sold her that one if he'd had it. You're lucky.); that's because of a woman's long inseam for their hieght. The problem then is the reach to the bars as women have short arms compared to men. Now for some fine points of sizing. To see if the bike is the right size do the following. This setup may initially make her "feel" unsafe, and she may want the seat lower, but eventually she will raise the seat to this level. Another thing, the bottom bracket on a mountain bike is higher than a road bike; so she'll sit higher on the bike from the "get-go". OK the general sizing information. 1. make the saddle level and put the saddle rails in the middle of the seat post clamp. Hopefully the seat post is straight and does not have a setback at the top. If it does have a setback, bring the saddle forward until the middle of the rail is just behind the center line on the seatpost. 2. raise the seatpost until she can pedal the bike with her HEELS (shoes on) just reaching the pedals without dropping her hips side to side. You can hold the bike for her while she pedals backward slowly. 3. From this position she should reach or nearly reach the center of the handlebar with her middle finger by placing her forearm between the front of the saddle and the stem. If she can't reach, she will need a shorter stem, but don't go below 80mm; the bike will get very twitchy to steer. Saddles differ in length so this measurement is a little questionable. Another way is that her line of sight while sitting in this position with her hands on the bar should pass through the stem and the front hub. Another way is to watch her hide, the angle between her spine and upper arm should be LESS than 45 degrees with her elbows slightly bent. Her feet will not touch the ground at this point and she may want to start with the seat lower, but the seat is too low if her feet are flat on the ground, a BMX or mountain bike downhill position. The lower the seat the more strain on the knees; so if she can start with lowering the seat until her tippy toes touch; she'll feel better at the end of a ride and she'll be able to go faster. Please let me know how you make out and if the bike is the right one after-all.

 

[60ish]

On one of the sizing points, I should have said that in a riding position the angle between her spine and upper arm (elbows slightly bent) should be about 45 degrees, but not more than 45 degrees.

 

[kantkawianning]

60ish,

We'll give your fitting suggestions a try and get back to you on the results. The seat post is, in fact, set back about an inch. The handlebars, however, have an adjustment for moving it aft and fore, so this may eliminate any problems in reach.

One thing I'm a bit confused about: when you suggest to place forearm between saddle front and handlebars, do you mean while she is standing next to the bike, touch her elbow to the front of the saddle and try to touch the bars with her middle finger? I'm a little confused what handlebar stem height has to do with that. Also, because these are moustache handlebars, should she be trying to touch the invisible line between the two grips, or the top of the stem?

Finally, if after making all these adjustments, her knees still rise above 90 degrees (parallel), is the frame too small?

Thanks,
Patrick

 

[60ish]

It is so difficult to tell someone how to do all this. It's so easy when you're there; so please don't hesitate ask questions. The seat cannot just be adjusted front to rear without considering the relationship of the final seat position to the bottom bracket; that's why I suggested starting with the seat center aligned with the seat tube. You want her knee-cap to be in a perpendicular line with the front of the pedal axle, with the seat in the position and height I suggested as a starting point. This means she cannot just move her seat forward until she reaches the handle bar. You can use a weighted string to check this out. Her knees will not rise above 90 degrees if the seat is at this height. But as I said, she initially may feel more secure riding with the seat lower, but lower it after your finished with all this.
Yes, I did mean that she should stand next to the bike a place her forearm between the front of the seat and the center-top (at the stem) of the handlebar once the seat is at the correct height. No, don't draw an imaginary line between the bar ends for this. It's a rough way to see if the stem length (not height) is correct, ie the reach. The other two methods may be a little better as I don't know the length of her saddle. For a beginning rider the seat should be about level (or a little lower) than the center (at the stem) of the handlebar (not the ends) when you finish all this. It sounds like you have some adjustment capability in the stem to correct for stem height and stem length. You could try these suggested measurements on yourself and your bike, assuming you're comfortable, and see what I'm trying to get at. www.coloradocyclist.com has a good fit kit on their web site. Good luck.

 

[kantkawianning]

60ish,

Thanks for the help. It appears the bike is the correct size. It would have been nice for the LBS to have gone through some of this, and done a thorough fitting...but who needs them!

As you mentioned, the "correct" seat height is well above her comfort level, but she can work her way up to it. Her reach to the bars is adequate at the "correct" seat height (using the forearm gauge you suggested), and she can still acheive a relatively upright position (well past 45 degrees). Finally, at this seat height, her knees do not rotate past 90 degrees.

Thanks again for the help and for providing peace of mind!

Patrick (and Melanie)