Comparing bike sizes

Nairnster

I am 5'10 with a 31" inside leg.

Currently I ride a Medium Giant Defy 2 with geomoetry as below




I am thinking of purchasing a Ribble 7005 Ultralite Frame, and wondered what size was the closest match . I was looking at the 54.5 cm, but not sure if this would be too big. Can anyone compare the geometries and give me some advice?

drlodge

I would look at 2 dimensions: effective top tube length, and seat tube angle.

Your Giant has 54.5cm top tube length, I assume this is effective TT length (measured parallel with the ground). It has a 73 degree seat tube angle (STA), this is quite relaxed. Now, lets also assume you want the same overall "reach" to the bars i.e. same top tube length from the saddle and same bar stem length.

The Medium ribble has 52.5 eTT but 74 deg STA (Add 1cm for each deg of angle), this gives 53.5cm equivalent.
The Large ribble has 54.5 eTT but 73.5 deg STA so overall 55cm equivalent.

So the large ribble is closest, but you could use the medium ribble with 1cm shorter stem and get the same overall size. What stem do you currently have? FWIW, I'm about the same size as you, my Rourke has 54.5cm eTT with 73.5 STA and 11cm stem. So we're both in the same ball park.

Nairnster

Thanks for that explanation, was sort of arriving at the same decision, but more by luck lol.


I use a 100mm stem, so making 64.5 cm reach.

So am I right that with the large Ribble, I would need a 95mm stem to get the same, but a 90, or perhaps the 100 I have would do? Or would that extra 5mm be a push, or hardly noticeable?

Thanks

mfin

drlodge wrote:

The Medium ribble has 52.5 eTT but 74 deg STA (Add 1cm for each deg of angle), this gives 53.5cm equivalent.
The Large ribble has 54.5 eTT but 73.5 deg STA so overall 55cm equivalent.

Look at the Ribble table, it's actually 535mm ETT for the Medium, 550mm ETT for the Large... hence the starting point for the maths is wrong it appears.

Also, OP needs to be careful, Large will need about 15mm less stem and saddle going back on the rails causing about 5mm of this, HT will need up to 25mm more spacers than the Giant too, dropping to a Medium would need 45mm more spacers... these figures are not exact, they are just gut maths (for example i'm not taking into account integrated vs external headsets that have more stack, just taking into account enough things to see if either suits)... but gut reaction is that it appears to me neither Ribble will suit as the large one would need a shorter stem than would be sensible for the bike's size and the medium would need far too many spacers.

Note that either one with it's spacers will bring the bars back a few mm which will need compensating for too.... anyway, I think it's all irrelevant as this model of Ribble is just too far adrift from the Giant.

Nairnster

Gut maths doesn't really explain it for me, sorry.

Can you say how you arrive at needing 15 mm shorter stem, and so many spacers?

Incidentally, I perhaps should have said I am not overly concerned about having the same sportive position on the Ribble, just which one would be the best fit based on my current bike dimensions. Or at least an idea as to which one.

Thanks

mfin

Nairnster wrote:

Gut maths doesn't really explain it for me, sorry.

Can you say how you arrive at needing 15 mm shorter stem, and so many spacers?

Incidentally, I perhaps should have said I am not overly concerned about having the same sportive position on the Ribble, just which one would be the best fit based on my current bike dimensions. Or at least an idea as to which one.

Thanks

Well, it's too difficult to say (especially when you start saying you would ride in a different position), you are either looking at being a LOT lower on the medium, or running a BIG stack of spacers and possibly a slightly longer stem and having your saddle moved back on the rails 10mm to negate the steeper seat angle... OR... you are looking at going for a shorter stem on the Large one (which I think will be too short for the size of bike) and still riding lower or having more spacers.... if you are already running spacers on the Giant there is no way you'd want the medium, and the large has big compromises....

The ribble is not the same ballpark of geometry as the Giant, hence why I would dump the idea, UNLESS the setup on your Giant is way wrong in the first place. Have you had a bike fit? How many spacers do you have under your 100m stem on your Giant?

Nairnster

So you are basically saying the Ribble couldn't properly fit someone of my height, in any size.

No I have not had a bike fit, and don't intend to.

The Giant will of course be differently set up, as it's a sportive geometry. Not looking to recreate the position as already stated, looking for advice on which frame is the right size going off the frame I currently ride.

If it's not possible to work it out, rather that people would say.

I understand I would have a larger saddle to bar drop on the Ribble, that's a given due to the fact it's not a sportive I would think.

Nairnster

Again though, could you explain why the stem would need to be 15mm shorter on the large?

mfin

Nairnster wrote:

So you are basically saying the Ribble couldn't properly fit someone of my height, in any size.

No I have not had a bike fit, and don't intend to.

The Giant will of course be differently set up, as it's a sportive geometry. Not looking to recreate the position as already stated, looking for advice on which frame is the right size going off the frame I currently ride.

If it's not possible to work it out, rather that people would say.

I understand I would have a larger saddle to bar drop on the Ribble, that's a given due to the fact it's not a sportive I would think.

No, there is one that will fit someone of your height, but some people ride in completely different positions to each other... for instance, there isn't a single Giant like yours I could buy as none of them could I get low enough on, and some people will not fit well on the Ribble, I'm not trying to be clever here or argumentative.

There is no such thing as sportive geometry really, that really is a term used for marketing, the most it really tends to mean is 'tall headtubes', limiting a lot of people to higher bar positions.... some people it doesn't limit them as they will ride a bike with a higher bar position as it is correct for their flexibility and body proportions. ...the Ribble does not have tall headtubes, if anything they are going the other way and this model is on the short side. (get some people on this and they will need a lot of spacers, you would if you wanted your bars at the same height)

I wasn't suggesting you go for a bike fit, I have never been for one myself, but what I do know is the exact measurements to the mm which I need to transfer onto any new bike from years of experience, (my own decided bike fit if you like that I know is bang on), this is completely regardless of how they 'market any particular bike'... but it sounds like you don't know what the measurements are for you and don't wish to know them or transfer them from bike to bike, in which case, the whole sizing issue becomes a bit irrelevant as the starting point is your body, your flexibility, your measurements and always will be, you are the constant.

I would suggest noone on here could pick the right size of the two for you, but I'm sure a lot of experienced people will know what I mean by saying the Giant vs the Ribble here are quite chalk and cheese in geometries and dimensions.

Saying "Not looking to recreate the position as already stated, looking for advice on which frame is the right size going off the frame I currently ride" I'm afraid is over simplified and I bet that if anyone suggests any size based on what you have said you will be at the mercy of fluke as to whether it fits you well or terribly, this will affect your enjoyment, your fatigue on the bike, the bikes handling for you, backache, etc etc etc. So, sorry, in this case the bikes are too different to each other, and hence the problem.

Nairnster

Fair enough, I don't really Have any issues when it comes to flexibility, and actually previously to the Giant, my old road bikes were more of the ilk of the Ribble.

I basically got the Giant to try it as 'sportive' was the buzz word.

I thank you for your input, and do understand what you are saying.

I guess if I hadn't mentioned the Giant, and just asked which size for my height, the answer may have been easier. Though of course it's still not simple, as there is no substitute for actually trying a bike, this just isn't an option for me here really.

Thanks once again.

drlodge

Yep I read the wrong number, but for saddle-bar reach, the medium is best. But as has been said above, the head tube length is also an issue as it would be for me.

My Rourke is based on a 54cm CAAD8 frame, same 54.5cm eTT but with longer head tube. I recently rode a Synapse (steeper STA) and had to push the saddle back some, and I had all the spacers under the stem. When I look at carbon frames to give me the same reach, I note the head tube is substantially shorter and hence I'd struggle with the drop and this is what Nairnster is also coming up against.