Difference in reach with diff ST angles is?

[tuscanyswe]

Anyone know how to calculate this or knows of a good site that can help?

Im trying to compare a frame with a 73.5 st angle with 73 ht angle to another frame with 73 st angle and 74 ht angle.

What would the difference in reach be at a saddle height of 78 with no spacers and 18 cm headtubes?

Thanks for any leads!

[vqdriver]

punch it in here

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

[cmg]

steep head tube angle, sounds like a track bike. type in "calculating reach, seat tube angle" in google and see the opinions. Difference is probably less than 1cm in reach. But keep in mind that the saddle is going to be in the same place in relation to the bottom bracket (horizontal and vertical dimension) on either bike. What does that mean? the 73.5 sta bike will have a shorter reach to the bars if both bikes have the same lenght top tube. draw your self an extreme seat tube angle diagrams with similar lenght top tubes then place the saddle in the same place in relation to BB.

[Peter B]

Assuming the same TT length on both frames, and that you hold saddle setback constant, the 73.5* STA frame will have ~1/2cm greater reach.

[jlwdm]

73.5 sta will have the longer reach.

Jeff

[tuscanyswe]

Again this forum shows it infinite visdom.

Thanks every1.

Silly me i thought the 73 would have the longer reach.

The 73 frame has a 1cm longer effective toptube. This means that they will have almost identical reach with same seatpost/stems. Thats what you are saying right? :)

[Peter B]

The 73* frame with 1cm longer TT will have ~1/2cm longer reach than the 73.5* frame assuming you hold saddle setback (relative to BB) constant on both (and that you use identical stems and bars).

[happycampyer]

Bear in mind that the bb drop will affect all of these calculations. If one has a lower bb drop, then its "effective" ht height is taller.

[Dave]

The saddle height makes no difference. I would assume that the head tube, with the headset and spacer is about the same on both bikes.

The formula to use is (cosA-cosB) times the c-c frame size. Roughly, it's 1cm per degree for the STA difference. The steeper STA will add about 5mm of reach.

For the head tube, the same formula is used, but the lenght to multiply by is the distance from the HT/TT intersection point to the center of the stem extension. Each degree that the HTA is steeper will add only 1-2mm of reach.

The net effect of the .5 degree steeper STA and 1 degree more slack HTA would then be about 4mm more reach.

[tuscanyswe]

Dave how can saddle height not make a difference?

If stem is fixed at a certain height and you pull out more seatpost wouldent one of the frames saddle extend more towards the rearwheel creating a greater reach than the other?

Thanks!

[tuscanyswe]

Hmm i must be dumb. But i cant understand how the 73.5 angle makes for a longer reach. Just looking at this picture makes me think its the opposite, what am i missing or not getting?

[Pegoready]

Are we to assume you have your saddle relative to the BB in the same spot on both bikes? So, the saddle may be centered on the rails on the 73* bike, but pushed towards the back of the bike on the 73.5* bike. If so, the 73.5* bike would have a longer reach, no?

[tuscanyswe]

I guess that would be a good assumption considering you would want that but i did not factor this in for this example. So just using the same seatpost and stem the 73 sta would have a longer reach than the 73.5.

Guess thats what i wasent getting. Thanks again.

[Dave]

It's always assumed that the saddle is in the same position relative to the BB on both bikes. Having a steeper STA should never force a rider to adopt a more forward position. The steeper STA requires the saddle to be moved further back and increases the reach.

I've setup bikes to have the same saddle position with anywhere from a 72.5 to 74.5 degree STA.

The saddle height only affects the amount of seat post setback required. The same formula is used as before, but the saddle rail height would be the multiplier. The saddle rail height is NOT used when calculating reach. See the drawing linked below for the definition of reach. Cervelo uses a slightly different vertical reference point - the top-center of the head tube. Noting wrong with that as long as it's consistent. Trek and C'dale also publish reach and stack values to this definition. Keep in mind that reach values can only be compared at ONE stack height. If frames have a different stack height then the frame with the shorter stack should have 3mm added to it's reach for each 10mm that the stack is shorter. A good example of this is seen when comparing the 51 and 54cm R3 frames. The real difference in reach is about 16mm, not 10mm, if it's assumed that the head tubes, with the headsets and spacers would be the same. If a rider is trying to pick between these two sizes, the head tube length difference should also not force a rider to adopt an unwanted position.

http://www.cervelo.com/en_us/bikes/2010/R3/geometry/

[mister]

I guess that would be a good assumption considering you would want that but i did not factor this in for this example. So just using the same seatpost and stem the 73 sta would have a longer reach than the 73.5.

Guess thats what i wasent getting. Thanks again.

if you keep setback the same which most people would do.
then as the seat angle gets more slack the headtube moves closer to the rider.
reach would be shorter because the seat would have to move forward to keep setback the same.

[tuscanyswe]

Ive never fully understand what you all are saying before but ofc it makes perfect sense now.

Mostly buying out of the box frames i never paid much attention to angles, test rides usually was enough.

Love it when i learn something new and especially vital so thanks!

[happycampyer]

Björn,

Not sure if my earlier comment made sense, so I'll elaborate just in case.

Another factor to consider is the bottom bracket drop of the two bikes. If one has, say, 7cm of bb drop and the other 8cm, and yet both bikes have 18cm head tubes with no spacers, your saddle-to-bar drop on the bike with 7cm bb drop will be roughly 1cm lower (assuming that you have the same saddle height and setback on both bikes—which you should as others have mentioned—and use the same stem/stem angle).

So, even assuming that the angles of the bikes were identical, if the bike with the lower bb is just right, then you will need a spacer or a stem with a different angle to get to the same saddle-to-bar drop on the bike with the higher bb. Conversely, if the bike with the higher bb is just right, then the one with the lower bb will feel to high unless you change the stem.

Bill

[tuscanyswe]

Thanks for elaboration. Makes sense.

Only difference is the 73.5 sta frame has a 1cm shorter toptube. For me its perfect, they have 17.5 / 17.7 headtubes and 7cm bb drop.

In other words if i use the same componets but with .5 cm extra setback seatpost on the new frame (73.5 sta) it ends up having .5 shorter reach. Just perfect as i got abit to play with left on handlebar reach and stem should i need to.