compact & semi-compact frame sizing

redmenace1
redmenace1 Posts: 50
edited January 2008 in Workshop
hi folks

looking at buying a new bike over t'Internet and would welcome some advice on the issue of sizing of compact versus a trad frame.
I currently ride a 56cm trek, and am looking at a Van Nicholas Zephyr or simiilar.

Having done a bit of bike sizing the recommendation seems to be going towards a 54cm rather than a 56cm which I am comfy on currently.

Obviously I can't do a test ride either so if anyone has good experience of the sizing issue of 54cm compact V traditional 56cm frame, please let me know your thoughts.

any help appreciated

Mike in Dublin :?
«1

Comments

  • aracer
    aracer Posts: 1,649
    The most important dimension on a frame is the toptube length. Simply measure what your current one is (if you think the fit is right), and get the size which has the same. The only other important dimension is the headtube length - this needs to be short enough to get your bars as low as you want, but preferably no shorter than you need so you don't need lots of spacers under your stem. Unfortunately bikes are sized by the seattube length, which given the availability of long seat posts is largely irrelevant.

    (I suppose other dimensions such as angles, chainstay length and wheelbase are important, but they tend to be fairly standardised).
  • redmenace1
    redmenace1 Posts: 50
    Ok thanks for that!
    Anyone agree or disagree, as on the basis of Aracer theory, I should be staying witha 56cm??????????? :?:
  • John.T
    John.T Posts: 3,698
    Yes. Top tube length is the important one. Real for standard, virtual for compact.
    My Fleet.
  • Down the Road
    Down the Road Posts: 949
    Get properly measured. Then you can get the right size.
    Racing is life - everything else is just waiting
  • cannonfodder
    cannonfodder Posts: 183
    Yes aracer is right. Assuming the same seat tube angle - as if you have to slide your seat forwards or backwards slightly to adjust for this, you will either reduce or lengthen the effective reach. 1 degree is about a centimetre on the top tube.
  • aracer
    aracer Posts: 1,649
    Good point, cannonfodder - the actual important dimension is horizontal BB to head tube, since that removes seat angle (which you can adjust be saddle setback). Unfortunately not that easy to measure that!
  • redmenace1
    redmenace1 Posts: 50
    thnaks folks advice duly noted!!! :D
  • top_bhoy
    top_bhoy Posts: 1,424
    John.T wrote:
    Yes. Top tube length is the important one. Real for standard, virtual for compact.

    I'm looking at the differences also. Current model is a compact and looking at a semi-compact. Is it still the top tube length which is the main issue and what is meant by "Real for standard, virtual for compact"? Will selecting by top-tube not possibly result in the standover clearance being insufficient (ie frame too large) and is there any other criteria to consider?

    Cheers
    Click here to view Top_Bhoys Pedal Force:
    Click here to view Top_Bhoys RC2:
  • jhop
    jhop Posts: 369
    Don't Trek measure their frames from centre to top rather than centre to centre hence comparisons with other manufacturers need careful checking?
  • Garybee
    Garybee Posts: 815
    jhop wrote:
    Don't Trek measure their frames from centre to top rather than centre to centre hence comparisons with other manufacturers need careful checking?

    That's just one of the many reasons why there's no point goint by the frame size in the brochure. As others have said, virtual top tube length in conjunction with the seat tube angle is the important one.
    Hypocrisy is only a bad thing in other people.
  • meagain
    meagain Posts: 2,331
    "what is meant by "Real for standard, virtual for compact"? "

    Actual HORIZONTAL c of seat tube to c of head tube (usually along the top of the errr top tube). Virtual, horizontally from where the c of the seat tube would be (i.e. the saddle post) to the same place on the head tube. Usually near enough by eyv altho I suppose could use a spirit level!

    Well, that's how I do it!
    d.j.
    "Cancel my subscription to the resurrection."
  • John.T
    John.T Posts: 3,698
    With todays frames getting the correct TT length is most unlikely to not give enough stand over height unless you have very long arms and torso and short legs. Even less of a problem with a compact.
    My Fleet.
  • giant_man
    giant_man Posts: 6,878
    Why not just go to a dealer who stocks VN, such as the excellent Paul Hewitt, get measured properly and get the right size - first time!
  • top_bhoy
    top_bhoy Posts: 1,424
    giant mancp wrote:
    Why not just go to a dealer who stocks VN, such as the excellent Paul Hewitt, get measured properly and get the right size - first time!

    Because Paul Hewitt et al aren't in Oz :lol: More seriously, the RC2 set-up I currently have is perfect for me and I don't really need a new bike but the current RS2 Pedalforce group buy looks appealling as I could have a carbon frame and fork for about 350 GBP (including USPS P&P to Oz). I'm thinking its a matter of translating the RC2 geometry frame onto the closest RS2 frame and afterwards, tweaking the set-up to my personal set-up using different stem and saddle lengths. I thought that this was one of the advantages of having compact and semi-compact frames?
    Click here to view Top_Bhoys Pedal Force:
    Click here to view Top_Bhoys RC2:
  • Garybee
    Garybee Posts: 815
    Top_Bhoy wrote:
    giant mancp wrote:
    Why not just go to a dealer who stocks VN, such as the excellent Paul Hewitt, get measured properly and get the right size - first time!

    Because Paul Hewitt et al aren't in Oz :lol: More seriously, the RC2 set-up I currently have is perfect for me and I don't really need a new bike but the current RS2 Pedalforce group buy looks appealling as I could have a carbon frame and fork for about 350 GBP (including USPS P&P to Oz). I'm thinking its a matter of translating the RC2 geometry frame onto the closest RS2 frame and afterwards, tweaking the set-up to my personal set-up using different stem and saddle lengths. I thought that this was one of the advantages of having compact and semi-compact frames?

    I can't see any sizing advantage in compact frames. As the seat tube angle is a set figure, and your saddle height is predetermined by your leg length and pedalling style, then your saddle will end up in the same position regardless of the angle of the top tube. I think there may be some advantahes of sloping top tubes but I can't see how sizing is one of them.
    Hypocrisy is only a bad thing in other people.
  • top_bhoy
    top_bhoy Posts: 1,424
    I thought instead of having say, 50,52, 54, 56,58,60cm increment frames, manufacturers could replace these with by S, M, L, hence cutting down on no. of different frames made allowing cost savings. Tweaking was then done using the stem and saddle sizes to make up the differences. Is my understanding wrong?
    Click here to view Top_Bhoys Pedal Force:
    Click here to view Top_Bhoys RC2:
  • John.T
    John.T Posts: 3,698
    This was what Giant thought when they first introduced compact ultra sloping frames but they found it did not work out that way in practice. General practice for road bikes is to have a stem between 100mm and 130mm. Going beyond these limits can usually be felt in the way a bike steers and also does not look 'right'. Most manufacturers now make about 5 sizes in compact frames.
    Garybee. Just to add to the confusion, on some frames the seat tube does not meet the centre of the BB so the effective seat angle differs with seat height. The original Trek OCLV 5200 was like this and the new Madone seat mast does not quite line up with the seat tube. It is not a problem as there is plenty of room for adjustment on the seat rails.
    My new Madone 5.2 Performance fit is my first sloping TT bike. It rides great but I do not notice the TT when I am riding.
    My Fleet.
  • Garybee
    Garybee Posts: 815
    Top_Bhoy wrote:
    I thought instead of having say, 50,52, 54, 56,58,60cm increment frames, manufacturers could replace these with by S, M, L, hence cutting down on no. of different frames made allowing cost savings. Tweaking was then done using the stem and saddle sizes to make up the differences. Is my understanding wrong?

    Sizing is just as critical with a compact frame as it is with a traditional geometry one. There does seem to be a bit of a myth that is propogated that they are more adjustable but they aren't. If you think about it, the position of the BB and the head tube is static and the saddle position is predetermined by your leg length and the seat tube angle.

    If a shop tells you that they are more adjustable they either don't understand or they are spinning you a line to try to sell you a frame/bike.
    Hypocrisy is only a bad thing in other people.
  • Garybee
    Garybee Posts: 815
    John.T wrote:
    Garybee. Just to add to the confusion, on some frames the seat tube does not meet the centre of the BB so the effective seat angle differs with seat height. The original Trek OCLV 5200 was like this and the new Madone seat mast does not quite line up with the seat tube. It is not a problem as there is plenty of room for adjustment on the seat rails.
    My new Madone 5.2 Performance fit is my first sloping TT bike. It rides great but I do not notice the TT when I am riding.

    Yeah, I was avoiding that to avoid even more confusion. I don't mind either generally (sloping or trad. geometry) but at 5'10" if i were to buy a giant (or other steeply sloping frame) then I would struggle to fit a bottle on the seat tube. On the bright side, sloping frames allow you to do the 'Lindus Gerdermann tuck' :D .
    Hypocrisy is only a bad thing in other people.
  • John.T
    John.T Posts: 3,698
    I am just under 5' 11". My new Madone is set up like a compact. It is a 56cm frame but measures 48cm C to C. I measured 27cm from top of TT to top of saddle. I can still get a 750 bottle in OK. I got this size because I wanted to stay with a 120mm stem to match my other bikes and this gave the TT I wanted. I can do the LG tuck but don't want to get stuck under the saddle so I don't think I will bother.
    Looking at The Trek website I could have fitted on 5 different sizes from 54 to 62cm. The virtual TTs would be from 53.8cm to 59.8cm. The stems would have been from 140cm to 80cm. The 55.7 TT on the 56cm is OK with a 120cm stem. It also has the most trail of any of the sizes so I hope will be a bit more stable with Tri-bars fitted. It certainly is no trouble at 45mph hands off. It was a bit of a toss-up between the 56 and a 58 Pro fit. In the end it came down to stem length and the colour. Plus my test ride was on the 56.
    My Fleet.
  • meagain
    meagain Posts: 2,331
    It all sounds very complicated! Why not start from the *old* rule of thumb of inside leg minus 10" (though I think -11" more in keping with modern design) equals seat tube c to t , "virtual" if a sloper, and work from there?
    d.j.
    "Cancel my subscription to the resurrection."
  • Garybee
    Garybee Posts: 815
    meagain wrote:
    It all sounds very complicated! Why not start from the *old* rule of thumb of inside leg minus 10" (though I think -11" more in keping with modern design) equals seat tube c to t , "virtual" if a sloper, and work from there?

    Some manufacturers build frames with longer top tubes. My Ribble frame (Dedacciai) is like this, so i had to buy a 52 where I would usually ride a 54. This kind of thing is why you can't really go by the seat tube.
    Hypocrisy is only a bad thing in other people.
  • meagain
    meagain Posts: 2,331
    That's why I said "start from"!
    d.j.
    "Cancel my subscription to the resurrection."
  • Garybee
    Garybee Posts: 815
    Jesus you can't have a discussion round here without someone trying to start an argument. Grow up.
    Hypocrisy is only a bad thing in other people.
  • John.T
    John.T Posts: 3,698
    meagain. Top tube length is the most important measurement on a frame to get a good fit. You can only realistically have 5cm of stem length adjustment and about half that fore and aft at the saddle. With modern seat posts you can adjust seat height by more than 20cm. You actually have more of a problem with handle bar height as 5cm is about as much spacer as you want under the stem. Coupled with a flippable stem this will give about 8 to 10cm of adjustment.
    Your method puts me on about a 61cm (24") frame. I actually use a 56cm (22"). This would give me an 80mm stem to get my preferred position which I do not want..
    My Fleet.
  • meagain
    meagain Posts: 2,331
    Sorry. I bow to everyone's expertise. Knowledge has clearly moved on and as usual I am left wallowing in the wake of those with greater understanding than I.
    d.j.
    "Cancel my subscription to the resurrection."
  • John.T
    John.T Posts: 3,698
    Don't worry about it. I had to re-learn a lot when I re-started riding again after a 35 year lay off. I was just amazed at how good modern stuff is. Back in the early 60s I did indeed ride a 24" frame but times have changed.
    My Fleet.
  • Garybee
    Garybee Posts: 815
    For those that have been riding a long time (20 yrs +), do you think it would be fair to say that people are fussier nowadays and less willing to 'make do' with something close?
    Hypocrisy is only a bad thing in other people.
  • John.T
    John.T Posts: 3,698
    I think we were just as fussy. The kit just was not as good. Still state of the art for the time. My bike was almost a Tom Simpson replica. Campag 5sp, Mafac racer brakes. Stronglight cranks with 52/43 TA adapter chainset, 13/23 block (cassette for the youngsters). My winter bike is better than this was.
    My Fleet.
  • Pirahna
    Pirahna Posts: 1,315
    Don't forget head tube length. With compacts it's possible to buy frames that are either very tall or very low at the front.

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