# Look 595 sizing question-help in making the right decision



## 03Vortex (Jun 17, 2003)

Hello all. I am about to make a final decision on a new 2010 595 and I want to be sure that I am making the correct decision on frame size. I have gone through a fitting at my LBS and while I trust their judgement, I would also like the opinion of others (including C-40 who hopefully will read this and provide his thoughts as well). Basic measurements for me are 32in cycling inseam and overall height is about 5'9" I do not currently have the remaining measurements handy.

I currently ride a 2003 LS Vortex that is dialed in just right which I am looking to duplicate on the Look. It is a standard non sloping 55 frame (C-T) with a 55.5 TT, 73 STA, 73.5 HTA, and 141 integrated HT. Standover height is 79.2, Saddle height is 72; tip of saddle to center of bars is 55.5 and I currently run 25MM of spacers (2 in drop) with a 110 Deda Newton stem (standard 84 degree that is not flipped).

My shop recommends the size M 595 which has a 54.5 TT (sloping frame), 73.75 STA, 73 HTA, and 148 HT (which is also integrated). Standover is 76.7 beacuse of the sloping frame of course.I intend to use an identical saddle on the Look as I do on the LS which is a Fizik Aliante. 

I recognize I will need to adjust my overall reach with the appropriate stem length but my more immediate question is if the size M frame is correct to enable me to duplicate what I feel is a perfectly dialed in LS? Also, if anyone has any thoughts on this frame in general, I am all ears.

Thanks and if I am missing any critical information, I will attempt to supply it.

03vortex


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## toonraid (Sep 19, 2006)

You said saddle height is 72 - if that's measured from center of BB to top of saddle then it seems a bit low for someone with a 81 cm (32ich) inseam - I would have expected it to be a tleast 75cm! Did they measure you up on a fitting frame or just took your dimensions? If they measured you up on a rig did they also measure knee angle at maximum extension? This should be somewhere between 25 & 30 deg theory behind it is that you have more power at 25 than 35 - Tri frames is a little different as they don't want to use their running muscles and therefore set theirs around 30 - 35 deg - it looks like you might have your saddle to low (around 35 deg) and therefore have some uptapped power in your legs - won't be immediate as your muscles will need some conditioning period so will have to gradually raise your saddle, 5 - 10 mm at a time.

If this is the case then you will be better off to hold fire on your order till you find your new sweet spot as raising your seat by something like 2 or 3 cm will also affect your saddle to bar drop by same margin - yu are already on 25mm worth of spacers (max recommended is 30mm (i.e. same as steering tube diameter) and given that 595 has a slightly taller head tube of 6mm you will only be able to raise your bars by around 11mm (6+5) leaving you short by nearly 20 mm in the new position.

If however you are happy with your current set-up knowing it could be less than ideal then all you need to do on Look is fit a 120 stem (instead of your current 110) with 20mm spacers under the stem to replicate the same position.

As for 595 - it is one of the best there is out there and it has lugged construction so great value too. I have one and it is a great allrounder - there are stiffer frames out there, there are also more comfortable frames out there but none that can optimize the 2 side by side but if you found that you will be happier with a higher set saddle position and the increased saddle to bar drop causes too much lower back pain then perhaps a good idea to check out the 585 optimum in Large as it will give you a taller head tube enabling you to reduce the saddle to bar drop deficit.


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## C-40 (Feb 4, 2004)

*thoughts...*

Nothing wrong with your saddle height. Sounds right to me. I have an 83cm inseam with 73cm saddle height. A common formula would also suggest 72cm.

I would suggest a small size, except that you don't use very much saddle to bar drop, so the medium would probably be the better choice. I use a much larger 9-11cm drop, so the small size is the better choice.


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## toonraid (Sep 19, 2006)

C-40, not sure if you are also familiar with the Holmes method (measuring knee angle) as opposed to the Le Mond way (0.883) - for me Lemond way comes up to a Saddle to BB height of 78.5 (89 x 0.883) where as using a goniometer at 25 deg (ala Holmes) brings it up to 82 which is a heck of a difference. So over the past year I have been gradually increasing my Saddle to BB height and am on 81.5 now - would be interesting to see if others have had different results from the two methods - here is some lit if you are interested to look into it including a very interesting article on changing positions of team CSC (Schlek & co)!
http://www.bikeradar.com/fitness/article/technique-how-to-get-your-seat-height-right-14608
http://www.cyclingnews.com/features/the-right-fit-shaping-saxo-bank
http://www.cyclingnews.com/features/the-right-fit-shaping-saxo-bank-1


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## 03Vortex (Jun 17, 2003)

C-40, do you agree with "toonraid" that I would need a 120 stem? Based on our prior exchanges (which I posted in another forum when I wasn't initially getting any responses in this forum), you had thought I would need the same 110 stem length. Which would it be for me to achieve the same reach that I now have?

Also, given I do not go with as much drop as you do, the medium frame I believe to be the better choice for me.

Thanks....


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## C-40 (Feb 4, 2004)

*thoughts...*

Most experts recommend something closer to a 30 degree bend at the knee. The amount of heel rise that feels natural for a rider greatly affects the saddle height that would create that bend. If you set the saddle so the foot is horizontal with the leg fully extended (no bend at the knee) at the bottom of the stroke, all it takes is about 3cm of rise at the heel to create that 30 degree bend. The difference between a 25 and 30 degree angle would require only about a 1cm height difference, not several centimeters. That can easily be proven with an oblique triangle calculation. For example, a leg that measures 90cm from floor to hip joint, with lengths of 40 and 50cm would require a third side length of 87cm to create a 30 degree bend at the knee joint. Changing the third side length to 88cm changes the angle to 24.3 degrees. 

I don't use an extreme rise of the heel, but I don't pedal anywhere close to level footed either. I find that setting the foot horizontal at the bottom of the stroke is a touch high for me. I also use a shoe and pedal combination with a very thin stack height. If I used some other pedal and shoe, my saddle height might be 1cm, higher.

As for using a goniometer, it would mean nothing unless you can figure out the exact foot angle while pedaling. A slow motion video would do a lot better job.


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## C-40 (Feb 4, 2004)

The .75 degree steeper STA on the LOOK adds about 7mm to the reach, so the reach should only be 3mm shorter than the Litespeed. That's not enough to make a 120mm stem necessary. The suggestion to use a 120mm stem overlooks the difference in the STA.

The idea that you might want a larger frame due to increasing saddle height isn't wise either, IMO. The medium is plenty large. 

I'm old, but I've increased my saddle to bar drop from 9cm to 11cm and no had any issues with it. You might be surprised how much lower you can go if you don't go down more than 5mm at a time over a period of several weeks. I dropped mine 15mm all at once when I switched from an 84 degree stem to a 73 and moved the 5mm spacer on top of the stem to below it. Later I moved that spacer back on top to get the full 2cm of additional drop.


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## toonraid (Sep 19, 2006)

C-40, I think you have based your calculation of horizontal difference between various seat tube angles on full length of seat tube which is mis-leading. The horizontal top tube length of both frames has already been provided by the manufacturer's geo table by taking into account their respective STA's so I think it would be more logical to measure the horizontal difference between the two from there after - i.e. from the point of HTT so instead of using 72 you would base the calculation on 12 and with an angle of 0.75 you will get less than 2mm instead of 7mm - do you agree?

As for measuring the angle - agreed its very hard, I use stickers marked with a cross and stick them to my hip joint, knee bone & heel bone and then rig up my camcorder to lap top and video while cycling - on playback I use freeze frame (pause) at max foot extension and use a screen geniometer software ($30) its not so hard once you get the hang of it and a lot of fun.


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## C-40 (Feb 4, 2004)

*no...*

The formula for calculating the difference in reach due to changes in the STA is (cosA-cosB) times the c-c frame size. The seat height of 72cm is NOT used. I have no idea where you got the 12cm value from, but that is not the number to be used in this calculation.

Using angles of 73, and 73.75 with a 53cm c-c frame you get 6.6mm. The difference in the stem length required to get the same reach on both frames would only be about 3mm, not 10mm.

Reach is defined as the horizontal distance from a vertical line through the BB, to some agreed upon point at the head tube. Cervelo uses the top-center of the head tube rather than the HT/TT intersection point that I used. 

When comparing the reach difference, the point at the head tube must be at the same height for both frames. The value from the calculation would not change much if a slightly larger height, like 55cm was used. The answer would change from 6.6mm to 6.9mm. My formula calculates the difference in the seat tube setback at the TT centerline. The value is added to the TT length of the frame with the steeper STA, then the difference of the two lengths is figured. In this case, it's (54.5 +.7) compared to 55.5. The difference is 3mm.


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## toonraid (Sep 19, 2006)

C-40 said:


> The formula for calculating the difference in reach due to changes in the STA is (cosA-cosB) times the c-c frame size. The seat height of 72cm is NOT used...


I don't think you understood what i was trying to say - perhaps my fault for not going into it in detail and using some random numbers as an example so l will compare reach dimensions for size S and M in look 595 OP can replace his old bikes dimensions with either size S or size M to find difference in reach between the two.

Your reach/horizontal position on a bike is determined by the distance between your saddle (actually sit bone might be a more appropriate term) and handlebar so when looking at changes in STA and its effect on horizontal reach it is important to take note of its effect at saddle height and not C-C of seat tube coz as saddle height is raised it will move away from the bar. So saddle height MUST be used to calculate reach.

On the other hand HTT distances are already provided in Look geometry tables - it is the prime parameter for calculating reach and of course it is calculated by Look based on the frame geometry for each size - so changes in STA are already taken into account at HTT height (i.e. dimension A1 on Look geo charts) so to simplify when comparing 595 size S vs M the difference in reach is 15mm at Seat tube height of 510mm for S and 530mm for M which is the relating A1 dimension on the chart however to calculate the true reach difference betwen the two frames one needs to look at the effects at the required saddle position which in this case is 720mm. Look already provide a reference - i.e. dimension E which is the horizontal distance between seat tube at A1 and a vertical line going through BB so all we need to do is calculate same dimension at saddle height which I call E1.

Size S = 720 x cos STA (74.5) = 192.41 = E1
Size M = 720 x cos STA (73.75) = 201.48 = E1

Now all we need to do is add this dimension (E1) to the horizontal BB to HT distance (B-E) for each size;

Size S reach = B - E + E1 = 530 - 136 + 192.41 = 586.41
Size M reach = B - E + E1 = 545 - 148 + 201.48 = 598.48

size S would be 720-510=210 x cos STA (74.5) = 56.12 
size M would be 720-530=190 x cos STA (73.75)= 53.17

So according to above calculations the difference in reach would be 12.07 mm based on a reference point which is at the junction of center line of HT and TT center line so further calculations need to be made based on differing HT angles and no of spacers used but it would be minimal as we are talking about an angle of 1 deg and perhaps 20 - 30 mm of spacers and perhaps more practical to dismiss. However one needs to take into account the KOPs position as they had on size S he will need to move the saddle forward by 9.07 mm i.e. 201.48-192.41 (E1sizeM-E1sizeS) in which cas the difference in reach would be reduced to 3mm (12.07 - 9.07).

I hope i managed to make it clear this time.


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## C-40 (Feb 4, 2004)

*still wrong....*

Nice of you to clear this up, but your calculations are still wrong and you're not comparing the OP's Litespeed 55cm (c-t) Vortex to a size M LOOK 595, which is what he asked for and what I provided. You suggested a 120mm stem length to him, rather than his current 110mm, but I know for certain that the reach on the 595 is not even close to 10mm shorter.

Your calculations have lots of errors. For one thing, you're figuring the reach at two different stack heights. Also, there is no need to know the saddle height of the rider to calculate the reach difference between two frames. Frame reach is not affected by the rider's saddle height. To make a valid reach comparision, it must also be done at the same stack height. 

Perhaps you should go to the cervelo website and look at their frame geometry drawings. Even though companies like Cervelo, Trek and C'dale list the reach of each frame, there is a pitfall to comparing these lengths. If the reach is not at the same stack height, a correction of 3mm for each 10mm of stack height difference is required. This can be seen if you compare a 51cm R3 model to a 54cm. The reach difference is listed as 10mm, but it is not at the same stack height. To correct that, multiply the 21mm stack height difference times cos73 to get 6mm. The real difference should be 16mm. I think of it as measuring the reach on the 51cm at the same 380mm stack height as the 54cm frame. That reduces the reach on the 51cm by 6mm and makes the true difference 16mm. For some reason, there's a 1mm error between this reach comparison and a TT length comparison, even though the STA and HTA are supposed to be 73 degree on both frames. Probably just some roundoff error.

http://www.cervelo.com/bikes.aspx?bike=R32010#G

If you want to calculate the true difference in reach, you must calculate the seat tube setback for each frame at the SAME height, and correct the TT length of one of the frames to that height. Then you take the difference and get the correct difference in reach. Saddle height is NOT involved. That's the beauty of the reach concept - you can figure the difference in a bike's fit without any need for the rider's saddle height.

Here's how the comparison that you tried to make between the two LOOK frames should be done:

First calculate the seat tube setback at the same 53cm height for both frames, I get values of 148mm and 142mm. Correcting the TT length of the 51cm to account for measuring at a 53cm height is next. It's very small, at only 1mm, but I'll include it, making the corrected TT length 52.9cm. Next, subtract each setback from the TT length. I get 545-148 = 397 and 529 -142 = 387. The difference is 10mm.

The only other thing not considered with this reach comparison is the difference in stem position at the actual bar height. If that's included, then the predicted stem length difference is reduced by about 1.5mm to 8.5mm.

I normally use a much simpler method to make this type of comparison. I just take (cos73.75 - cos74.5) times the average c-c frame size and add that value to the TT length of the frame with the steeper STA, then take the TT length difference. If this is done with the 51 and 53cm LOOK frames, I get 54.5 - 53.7 = 8mm. That's close enough to produce the same prediction that one size shorter stem will make the 53cm fit about the same as the 51cm. It also agrees almost perfectly with the more complicated method.

Your idea that the two LOOK frames have a reach difference of only 3mm is WRONG. The reach concept can be a bit tough to understand, but it is correct. The only other calculation that someone might want to make is the difference in seatpost setback required to get the saddle in the same position. It does not alter the fit of the frame just because one frame use more seatpost setback with a steeper STA to produce a given saddle setback, relative to the BB. If two frames have the same reach, at the same stack height, they will fit the same, with the same stem length. If they don't have the same reach, then the stem length is altered to make the combined reach plus stem length the same on both bikes. One frame could have a 72 degree STA and the other a 75 degree, but only the seatpost setback needs to be different. As long as the saddle nose (and rider's sitbones) are in the same position behind the BB, it make no difference how that is achieved.


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## toonraid (Sep 19, 2006)

I would love to get to the bottom of this but perhaps we are going about it the wrong way, lets start with principals and see if we can agree on them first - but one parameter at a time.

1 - Definition of reach - for me reach is the horizontal distance between your sit bones and center of handle bars, type of saddle, saddle position, seat post set back, type of bar, stem & Saddle height all effect your reach on the bike as it alters the horizontal distance between your sit bones and your hand position on the bars.

Can we agree on this?


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## C-40 (Feb 4, 2004)

*no...*

The concepts that you are trying to use produce incorrect results. One by one, the major brands are going to the relatively new concept of frame reach, as defined by Cervelo. What this concept does is insure that what you are calling the reach (from the sitbones to the bars) is accounted for, but without needing any knowledge of the rider's saddle height or how far behind the BB the sitbones are. It CAN be done. Until you get this through your head, you'll continue to make incorrect calculations. Read this tech presentation from Cervelo. Maybe it will help. http://www.cervelo.com/slideshow.aspx?id=5

If any two frames are being compared, it is always assumed that the rider will want the same saddle setback or sitbone position on each frame. Just because one frame has a different STA than another does not force a rider into a different position, it only moves the seatpost and might require a different seatpost setback. That's why the only dimension of concern is the "reach" from the vertical line through the BB to the point at the head tube. This point must be at the same vertical height on both frames. If it's not, then corrections must be made to make the two points at the same height. 

There is a much older, but nearly as accurate rule of thumb for comparing two frames of similar size, but with different STAs. Just add 1cm per degree to the TT length of the frame with steeper STA, then compare the TT lengths. The difference will predict the difference in the stem length needed with little error. This method still requires no knowledge of the saddle height or setback and it is accurate. It's accuracy can be improved by using the formula (cosA-cosB) times the c-c frame size rather than 1cm, since that's where this rule of thumb number came from.

For example, if two frames have the same 55cm TT, but one has a 74 degree STA and the other a 73 degree, you know that the reach on the frame with the 74 degree STA is about 1cm greater and will require a 1cm shorter stem to produce the same fit with saddle at an identical height and setback, once the saddle is moved to the same position. What confuses a lot of people (including me until a few years ago) is the idea that the saddle height is needed to figure the amount that the saddle must be moved. It is NOT needed. If the frames are the same size, you'll get a more accurate answer using a single calculation: (cosA-cosB) times the c-c frame size. If the c-c frame size is 55cm, then the result would be 9.2mm. That's nearly identical to 10mm. Notice that this calculation does NOT require a saddle height. If you choose some example saddle height and go through your calculations, but come up with a number like 3mm instead of the 9.2mm, you're wrong.


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## toonraid (Sep 19, 2006)

Sorry I did not find a single sentence in those 4 paragraphs stating why I am wrong in my assumption - just that I am wrong and a link to Cervelo.

My aim is to get to the bottom of this and do it one step at a time without compounding or complexing the issue so I will make my previous question even simpler;

*If I raised my saddle height by a few centimeters would it alter my reach if all else remains the same?*


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## C-40 (Feb 4, 2004)

*certainly...*

Of course the reach will increase, but if you are comparing two frames and do the same on both, then both have the same increase in reach. What you are overlooking is the fact that every rider has only ONE saddle height and ONE saddle setback amount that will be the same on both frames being compared. All you've been doing with the saddle height information is figuring out where the top of the seatpost will sit. That makes no difference, as long as the saddle ends up at the same location on both frames, relative to the center of the BB. That might require a post with a different setback but it does not change the fit of the frame. 

What you need to figure out is horizontal distance from the center of the BB to the exact same point at the head tube for both frames - that's the difference in reach. That will also tell you the stem length difference required to make both frames fit the same. The difference in seatpost setback should not be intermingled with this calculation. The seatpost setback difference is simply (cosA-cosB) times the saddle rail height (not the actual saddle height), where A and B are the two STAs. 

I've had frames with very different STAs and used a zero setback post on the frame with a 72.5 degree STA to position the saddle exactly the same as my other frame with a 74.5 degree STA, and a 25mm setback post. Measuring from the saddle tip to the center of the bars, the frames also measured the same, with the same stem length. The TT lengths were much different, but what was the same is the REACH. The same reach requires the same stem length. The saddle on these two bikes could be moved up, down, forward or backward, but as long as the same movement was done on each one, they both have the same fit.

Read the instructions below and try making the drawing I have described. It proves with no doubt that saddle height and setback are not needed to compare the fit of any two frames. It merely assumes that the saddle will be in exactly the same position, relative to the BB on both frames. 

Here’s a graphical method that might convince some skeptics that the concept of frame reach will predict the difference in the fit of two frames without any knowledge of the rider’s saddle height. I drew up a representation of two frames on graph paper. Using graph paper makes it much easier to make the drawing and measure the reach. In this example, I am defining reach as the horizontal distance from a vertical line through the bottom bracket to the head tube/ top tube intersection point. Cervelo uses a slightly different point at the top-center of the head tube, but the same principles apply. The defined point at the head tube can be set to any desired vertical stack height, but that stack height must be the same on both frames under comparison.

1) First draw a vertical line of a convenient length. I made mine 3 inches long, with each end at a horizontal grid line on the graph paper. Mark a point at the lower end of the line to consider the center of the bottom bracket. 

2) Draw a horizontal line intersecting the top of the vertical line. Mark this line TT for top tube.

3) Next, draw a line that represents one seat tube centerline, starting at the center of the bottom bracket. Rather than worry about a specific angle, I drew mine so the upper end of the line was two squares behind the vertical line and horizontal line intersecting point.

4) Next draw another line representing another frame’s seat tube centerline. Make this line have more setback. Instead of two squares, make the setback four squares for an extreme difference.

5) Extend both of the “seat tube” lines further up, to represent a saddle height. The amount makes no difference – just make the lines the same length and some reasonable amount above the TT line. 

6) Next, draw a horizontal line representing the saddle and end the line behind the vertical line through the center of the bottom bracket. Consider the end of this line to be the saddle tip, with some amount of setback, behind the center of the BB. If the saddle tip setback and height are the same on both frames, the sitbones will also be in the exact same position.

7) Draw two points or short vertical lines along the TT line to represent the ends of the top tubes on each frame. Make each mark the same distance from the starting point – the intersection point of the TT line and the seat tube line. That will represent two top tubes of the same length. Notice how one of these points or vertical lines is much closer to the vertical line through the center of the BB. The amount should be the same as the difference in the seat tube setbacks. On my drawing, this is two squares on the graph paper. Two squares is the difference in the reach of the two frames that I have drawn, but both have the same TT length.

8) Draw a vertical line extending up from the end of each of the top tubes. They should be the same two squares apart, just like the seat tube setbacks. Make the lines long enough to extend higher than the saddle height line.

9) Measure from the point representing the saddle tip to each of the vertical lines just drawn. The difference is still the same two squares and the same as the difference in the reach. You could move that sadle tip point anywhere you want, up, down, forward or back, but the difference between the reach of these two frame will remain the same - two squares.

To make these two "frames" fit the same, the stem on the frame with the steeper seat tube angle must be shorter, or the stem on the frame with the slack STA made longer, by the difference in the reach - whatever amount the two squares on the graph paper represents. Then the horizontal distance between the saddle tip and the handlebar centerline will be the same on both frames, with the saddle in the same position relative to the bottom bracket. No knowledge of the actual saddle height or saddle setback was required for this analysis. The saddle height or setback can be anything you want, as long as it's the SAME for both frames being compared.

I’ve also stated that the stack height must be the same to make a valid comparison in reach. To convince yourself of that, draw an angled line from the end point of either top tube to represent an extended head tube. The further up the line is extended, the closer it gets to the vertical line through the center of the BB and the shorter the reach. If you compare reach values that are calculated at different stack heights, then you will not get a proper prediction of the stem length difference required to make each frame fit the same. A correction that is accurate enough just requires an increase in the reach difference of 3mm for each 10mm of stack height. If you look at two frames with that have a 10mm difference in the published reach (from the manufacturer), but one has a 20mm taller stack height, then the real difference in reach is about 16mm. A good example is the 51 and 54cm Cervelo R3 frames. Since these two frame have the same STA and HTA, the difference in the TT lengths should match the difference in the reach. When two frames have the same STA, all you really need to do is take the difference in the TT lengths to get the difference in the stem length required to make the frames fit the same, but it good to know that a proper reach comparison produces the same answer (within 1mm). I suspect a little roundoff error somewhere.


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## C-40 (Feb 4, 2004)

*more...*

If you have two frames with the same STA and HTA, but different TT lengths, how do YOU figure the difference in the fit? I just subtract one TT length from the other and that tells me the difference in stem length required. I don't need to know the rider's saddle height - do you?

If one of those frames was modified to increase the seat tube setback, and the TT was also extended to meet the seat tube, so the head tube did not move, would the fit of that frame change? The answer is NO. Why? Because the reach did not change. If the BB remains in the same position relative to the head tube, the only change needed to make this modified frame fit the same is a seatpost with less setback, or perhaps just moving the saddle forward on the same seatpost. If you get the saddle back to the same location relative to the BB, the fit will remain unchanged, with no change to the stem length. It makes no difference how the saddle gets back to it's original location. I also don't need any knowledge of the rider's saddle height to figure this out.

Perhaps what's missing from the analysis is any mention of the seatpost setback. It's very easy to figure that, but most people inquiring about frame fit are more concerned about stem length. To figure the difference in the seatpost setback, I most often just use an average 12mm per degree, but if you want exact, it's the same (cosA-cosB) times the saddle rail height. At least with the LOOK frames in question, the seatposts have variable setback so the chances of any problem with the seatpost are small.


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## C-40 (Feb 4, 2004)

*about your mistakes...*



toonraid said:


> I don't think you understood what i was trying to say - perhaps my fault for not going into it in detail and using some random numbers as an example so l will compare reach dimensions for size S and M in look 595 OP can replace his old bikes dimensions with either size S or size M to find difference in reach between the two.
> 
> Your reach/horizontal position on a bike is determined by the distance between your saddle (actually sit bone might be a more appropriate term) and handlebar so when looking at changes in STA and its effect on horizontal reach it is important to take note of its effect at saddle height and not C-C of seat tube coz as saddle height is raised it will move away from the bar. So saddle height MUST be used to calculate reach.
> 
> ...


1) The E1 dimension just tells you the difference in the seatpost setback required. It's valuable info, but has no effect on the fit. That's simply (cosA-cosB) times the saddle rail height, not the top of saddle height. A and B are the two STAs.

2) Your second calculations of "reach" are wrong because they are not at the same height. The setback of one frame must be calculated as if it is the same frame size as the other. It doesn't matter which one is chosen as the standard, but one must be chosen. In this case, it's a bit more complicated because the head tube angles are not the same. The error will be small, so I'll neglect it. The 136mm value should really be 142mm, to be at the same height as the other frame (cos74.5 x 53) =142. There is a 6mm error. The reach should be 530-142= 388. Once again, forget about those saddle setbacks. The idea is to figure out the stem length difference between the two frames. If you would have figured the setbacks at the same height and skipped the seatpost setbacks, you would have had the right answer: 397-388= 9mm. That's the difference in the stem length needed.


3) Your last calculation is not needed, period. You're already done once you get the difference in the BB to HT lengths (the reach difference).

I could make two other vary small corrections. To get an exact setback for the smaller frame, it should be (sin73.75 x 53)/tan74.5 = 141. That's 1mm less. The difference in the HTA also reduces the reach by another 1mm at bar height, so the really accurate difference becomes 7mm. 

The old rule of thumb thumb would have figured an 8mm difference with one simple calculation (9mm per degree times the .75 degree STA difference, added to 530 equals 537. Subtract that from 545 and you get an 8mm difference. That's close enough.


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## C-40 (Feb 4, 2004)

*bike CAD...*

You might also want to try drawing the two look frames on bike CAD. I did it and got excellent results that match what I've been posting. If drawn properly, you'll get the same front-center for both bikes, that is published by LOOK - 580mm.

http://www.bikeforest.com/CAD/bcad.html

I just drew the frames up as having horizontal TTs, since that is how they are defined. Getting the right amount of head tube length, plus 20mm for the headset top is a little tricky, but if it's all drawn properly, you can get a horizontal distance from the center of the BB to the center of the bars that's very close to my calculations. It will be a little different, but that's because the measurements are at bar height, not the top-center of the head tube. For the reach dimensions to be valid, both most be at the same vertical height from the BB.

Once again, no knowledge of the rider's saddle height or setback is needed to predict the difference in stem length required. As long as the same saddle height and setback from the BB are used on both frames, the numbers would just cancel out in any calculations.


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## toonraid (Sep 19, 2006)

Sorry I haven't had a chance to reply - past few days have been very hectic and so haven't had the chance to sit down and do some drawings and calculations - thanks for reminding me about bike cad - will have a look and get back to you in the next day or 2.


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