Hisamatsu

I'd be willing to get the PRO if it fits better, but whichever fits that is cheaper with decent quality spec

AnthonyG

From what I can make out both Luath24 models fit the same but its hard to say exactly as they aren't publishing the frame geometry specifications I want to see. I like the 145mm cranks for that size bike and I'm eyeballing the geometry. One interesting feature to note is that the Luath models have zero setback seatposts. 25mm setback seatposts are kind of standard and I'm recommending that you find a 35-40mm setback post for that bike to fit your girlfriend better. What I'm noting here is that it should be relatively straightforwards to extend the handlebar to saddle distance REARWARDS which is what you need for better comfort and balance on the bike.

I suspect the seat tube angle on the luath models is 73 degrees which is not as relaxed as I recommend but better than the 75 degree angle that used to be standard on such a bike not that long ago.

Here's what you eyeball. Draw an imaginary line up from the centre of the Bottom Bracket (BB, Bearing the cranks run on) and asses how far behind this line the tip of the saddle is. Some small bikes have the tip of the saddle even with or even in front of this line. This is ridiculous in my view. It places a lot of weight forwards and makes riding very uncomfortable. The aim here is to get a good 4-5cm of setback from the line to the tip of the saddle if you can. Mt custom racing bike that I built around 650c wheels has around 7cm of saddle setback. The seat tube angle is 71 degrees.

Giant had some 24" wheel cyclocross/Mountain bikes that looked interesting but they aren't publishing the specifications I want to see. You could possibly test ride them if you see them.

What I'm looking at when making my recommendations on fit are. 1. Short cranks. 140-145mm is good. 150mm is getting long. Definitely no longer than 150mm. This will be one of the key problems your GF is having with adult size bikes.
2. A relaxed seat tube angle which moves the riders weight rearwards and off their hands/shoulders/arms. Again the small adult bikes have ridiculously steep seat tube angles which pushes the riders weight forwards. They do this so that on paper they can state a short top tube length.

Its a serious fudge. Small wheeled bikes allow for a short top tube length AND a reasonable seat tube angle. CHeck out the seat post diameter from the start and see if its possible to buy a long rear set seatpost in the size thats on the bikes your looking at.

Hisamatsu

https://www.diamondback.com/haanjo-trail-24

My gf tried out this bike at the shop today. She felt much more comfortable. Still deciding whether to get it or not.

AnthonyG

Its definitely going to be an improvement over an adult bike. I believe that the Luath24 will be better again but it comes down to how much effort you are prepared to go to to try them, and how much you are prepared to spend.

The Diamondback has a 155mm crankset which is WAY better than 170mm but still long for her inseam. The seat tube angle on the Diamondback is 74 degrees which is better than 75+ degrees but still a little steep.

linberl

I think you need to compare geometry, frame size, as well as wheel size. Surly has their full geometry for each frame size available: Straggler 650b | Bikes | Surly Bikes. A slightly bigger wheel - with a more proportional frame like a 38 - may or may not be better for the rider. Do the math and check the geometry. It's not just about wheel size, it is about proportional fit on those wheels. Get the islabike geometry info and match both to the rider's dimensions.

Hisamatsu

Was almost about to get the diamondback, but she went around playing with other bike's shifters/brakes and fell in love with the ultegra ones. She didn't like the stiffness of the clari,sora,tiagra,105 since her hands are smaller and can't reach as well.

LOL, she is making this harder. The diamondback has claris 8spd. I could get used ultegra parts and swap out, but that opens a new can of worms. Prob need new hub/wheel. The ultegra crank doesn't come shorter than 155 length?

Starting to look like islabike pro line will make her happy the most, but too bad it is out of stock.

AnthonyG

It can be a bit of a hassle yet ANY off the shelf bike is less hassle than a custom bike which is what I have. Yes the Clari and Sora shifter are difficult for small hands so it always perturbs me that these shifters are fitted to youth bikes. Just another example of the disregard that the bike industry has for youths/small people. The Shimano Tiagra, 105 and Ultegra levers should be pretty much the same however. A few years ago when I bough levers they were the same. Its a matter of fitting wedges or adjusting the lever travel. Also individual cabling on each bike becomes an issue. If the cabling is fitted poorly which adds friction then this becomes an issue. Also the size and curvature of the bars has a great impact on how the levers perform for small hands as well.

I'd be very surprised if you could get Ultegra cranks in anything shorter than 165mm. When it comes to short cranks just be thankful that you can get ANYTHING. I had to have my short cranks engineered.

Hisamatsu

I have a feeling she was playing around with an ultegra bike with hydraulic disc break or something and it felt super nice compared to the mech disc she tested prior.

AnthonyG

I've been resisting commenting on the Surly frame geometry as I know you guys love Surly's and its just going to start a flame war.

I have just assessed the frame geometry linked above and its exactly what I expected from a large wheeled bike. The smallest properly proportioned frame was the 52cm. Everything smaller than 52cm is fudged with steep seat tube angle and short seat tubes.

This is what the Industry does. They fudge the geometry of large bikes so that short people can somehow fit on them but is not at all comfortable to ride these bikes any distance.

A bike designed properly for small people NEEDS smaller wheels. This is a fact. There is no way around it.

For the record. A standard 26" Mtn wheel has an ISO size of 559 and there is a wide range of tire sizes to suit. Fitted with a 25mm tire which are available is quite a small wheel and even with a 38mm tire its not that big a wheel. 650B has an ISO size of 584 which is bigger and there are only big tires for that size making it a big wheel. The 24" wheel size that I recommended and the OP is looking at has an ISO size of 507 and even with big tires its the right size to make the bike work properly for small people.

OK, in defence of the 650B (ISO 584) wheel it is smaller than a 700c (ISO 622) and typically with 700c wheel bikes they only work properly from the 54cm size. Still, its only one size better and the more common 26" MTN (ISO 559) wheel can do better still but its still not small enough for people around 5' or even shorter.

Hisamatsu

Just to update. We returned the Diamondback and got her a Luath finally; she is liking it so far. Took a while since Islabike was hammered with orders for the upcoming summer seaon and out of stock on their pro line.

AnthonyG

Its good that she likes it. The next step is to move the saddle back as far as you can. The further back the saddle goes the less weight will be on her hands/shoulders. Buy another seatpost with more setback at the clamp.

fietsbob

Bike Friday is probably too much of an investment, but a folding bike with smaller wheels solves

a common problem big wheels on small bikes creates, that is toes and the front wheel overlap.

And can touch , in slow speed turns.

Hisamatsu

Which seatpost would you recommend? Current islabike one doesn't allow for much adjustment with setback

AnthonyG

Whatever fits. Find out the current seatpost diameter. Assess how much length is required. Buy a new one that meets the size requirements and has a rear-set clamp.

mickw

Assuming she can standover fine with the current frame , take a look at the stem she has.
Mine was a 115mm 15 degree stem and I changed to a 90mm 15 degree stem , this made a big difference in reach.
A swept back bar might be something to try , reach wise and hand position might suit her better.
Mick