Road Cycling - Opinions needed to a unusal question on wheel size?

[FONT=Arial Black]I'm a MTBer new to roading. I'm short rider and looking at closeout on a NEW 47cm Jamis Ventura full Tiagra setup, steelie frame, clip pedels for $650.00 QUESTION is, It has 650c frt wheel and 700c rr wheel. What do you all think of this setup. Test ride nice, but wondering about accurate handling and why did Jamis choose that setup? :)

With the size of the frame (very small) I would bet it has to do with geometry and stand-over height issues. In that line the 47cm is the only one with that set-up.

Do you have to go that small? If you get up to 50cm you might be able to use 700c on both ends.

The 650b (?) is used on the front end to get normal steering geometry without raising the handlebars or risking toe overlap with the front wheel. With normal steering geometry allowed by the 650b, the handling should have been correct for a road bike.

A 650b has about 10% less circumference than a 700c, and that's why there is a 700c on the rear end. You can shift to a higher gear, but if you've topped out on the 11T or the 12T, you will have no place to go to to make up for the 10% gearing deficit imposed by using a 650b. The only place you are likely to run into this problem is going downhill fast.

In short, it's a good setup if you can tolerate the two different tire sizes and tube sizes.

I have been told that in an emergency, you can fold a 700c over on itself and use it in a 650b tire.

It's a better setup than two 700c's and the bad front end geometry needed to make you safely fit.

Terry makes a lot of their women's bikes with that setup. It avoids kicking the front wheel while turning and pedaling. Seems necessary in a frame that small.

Hi,
how tall are you? 47cm is a size for people who are somehwere around 5 feet tall.

650/700 wheels look cool IMHO (known as funny bikes, for obvious reasons), but to be honest 650cs front and rear make more sense unless you want serious a aero advantage (TTs etc). There's no real reason not to go with a funny bike except having different tyres and tubes, but by using slightly larger chainrings you can overcome the gear disadvantage as mentioned by SDS. If your heart's set on this Jamis, by all means go for it, but look at some 650-wheeled bikes if you're not sure.

Hi,
how tall are you? 47cm is a size for people who are somehwere around 5 feet tall.
Thanks for asking. I'm 5'2" I test rode it yesterday. It felt ok, but the shop changed out the headstem to a 120mm.
At first it felt as though I was over the top to much till the stem change. The shop owner said if I was down with my hands on the lower bars, the handlebars should block the view of the front hub. So that my face is not past the handlebar if looking at my riding posture from the side. Any tips?

A stem that large on a bike that small would seem to indicate you need a slightly larger frame. I would think that a 90 or 100 stem on a frame below 50 ctc would be more appropriate.

Hi,
only one. try a few bikes in the 47-50 size and see how they work for you. 47 sounds a little small, but Jamis likes a long top tube. With that size frame, it's hard to geometry right with 700c wheels.
Don't rule out bikes with 650c wheels.

At 5'2" it is likely that you can fit well on a slightly larger frame unless you have a special disqualifying feature like short legs. This is also suggested by the relatively long 120mm stem.

I probably know about eight women cyclists who are 5'2", and they are indeed on everything from a 47cm (?) women's specific Trek with 650b wheels and jacked up handlebars (long tall stems), to 52cm road bikes with 700c wheels, Serottas and LeMonds, etc. And probably none of them have exactly the same fit.

My suggestion is that you go to another shop and tell them that you are looking at another bike, but that you are not sure about your fit, and see what they say. A good shop on a weekday will be happy to show you what they have and figure out your fit. Weekends they are a little too busy to do that well.

Some disadvantages are imposed when you go to oversize chainrings. Because front derailleur capacity is a fixed value given in teeth (Shimano's nominal limit for a triple front derailleur is 22T, which is why the Ultegra triple is 52-42-30, for a 22T difference), as the rings get larger the ratio difference gets smaller (ex:4 is twice as big as two, but 8 is only 1/3 greater than six). So if the big ring gets larger the small ring has to get larger too, and then if you need a particular low gear you will have to use a wide-ratio cassette (ex: 11-34 instead of 11-21), which then means that the shifting pattern to get the next nearest gear can be quite complex, which is a real disadvantage when the roadie next to you is only clicking to the next cog on his cassette with no ring changes. Road bikers are always in search of small improvements of efficiency, which is why they like the narrow gear splits of the tight cassettes.

The point is that the road rings and the derailleurs are optimized for use with a particular wheel diameter (700c), and deviating from that wheel diameter will impose a disadvantage at high performance levels. This is only a small disadvantage, but there is no point in not avoiding it if avoiding it is costless.

I just checked the fit on the back of one of my tandems, which was last used by a woman who is 5'2". She's done Paris-Brest-Paris, and zillions of other miles, so I think she has a clue....

When you put a person on a tandem, you ideally will have the same size crankarms as their single, and transferable fit measurements. So....with our tape measure (you have to take one of those shopping), measuring from the center of the bottom bracket, parallel to the seat tube, to the top of the saddle, it's 27-1/2". I remember fitting women of 5'2" as much as an inch shorter. From the center of the seatpost, parallel to the ground, to the center of the handlebars, is 21-3/8". I remember fitting women of 5'2" as much as 1/2" longer. This leaves handlebar height to be considered, and for an initial approximation I like to see the handlebars as high as the top of the round part of the seatpost. You should be able to generate a set of numbers for you and carry them around and verify that the bikes match with your tape measure. As long as you first verify that the bottom bracket heights are the same, it's a little bit easier to get identical handlebar height by measuring vertically off the ground to the center of the handlebars.

What do road bikes look like when they are fit properly? I'm a little hesitant to talk about this because I'm seeing young people today with their handlebars very low in comparison to the top of the saddle, and a lot of seatpost showing, even on road bikes with horizontal top tubes. In the old days we'd see about a fist's worth of seatpost showing (road bike, not criterium bike), and if that was the case we would expect the top tube would be the right length to put the handlebars in the right place with a stem in the middle of the available range.

I'm not picky about standover height. I will go down to zero on my bikes, but the general rule is that you have to have an inch. Again, if you've got a lot more, that might suggest the frame is too small. The exception is the "compact geometry" road frame with the sloping top tube, where obviously you will have more standover height.

Aside from all the other good suggestions, perhaps the bike shop is having a hard time selling the 47cm BECAUSE of the odd 650/700 wheel combo. Looks funny as well as making tire buying a PITA.

Some makers (like Bianchi, Lemond,...) make a 49cm frame that may just be your ticket. How much clearance do you have when you standover the frame?
You should check out the fit calculator at http://www.wrenchscience.com. One thing you want to make sure is to get a properly sized frame.

I agree with BlastRadius. Do an online fit calculator. The reason for this is that when you go into a LBS, you have no idea if the guy in there has a clue about bike fit. I just went through the buying process. There are more people out there that don't know what they are doing than you can imagine.

If I had walked each shop and out with the recommended size bike. I would have everything between a 54cm (Puh-leeze) and a 59cm!!! The fit calculator will at least give you an idea where to start looking.

Here is another one:http://www.competitivecyclist.com/za/CCY?PAGE=FIT_CALCULATOR_INTRO

Do you have a long torso/short legs? You may have to use some creativity in getting the bike to fit.
A 12cm stem seems way too long. The critical dimension on a bike frame is the top tube length. On small bikes, you should be able to set the bars comfortably using an 8-10cm stem. If the stem is proportionately too long, the steering can get sluggish.

The dual wheel size design means that you have to carry 2 sizes of inner tube as spares. It makes sense for a small frame to use small wheels. It also makes sense to use small cranks, so check this. 170mm is for the average size male. You will probably want a 160-165 size.
Check out the fitting guide at Peter White Cycles. This is far more sensible than most guides, esp for big/small people and non-racing use.

For short legs/long torso, you want a bike which is low but long. Compact style frames are designed to be low in height , but normal in length. If you get a compact frame a bit larger than recomended for your leg length, then you will still have plenty of standover, but some extra length.