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good-bad attributes to steep seat tube angles(25 posts)

good-bad attributes to steep seat tube anglesrollinrob
Apr 16, 2003 8:46 PM
Hi there,
I am on a quest to find a perfect steel bike and have it down to two.
Bike one has a relitivly steep seat tube angle at 75 degrees. The headtube angle is 72.5. The bike is 51cm c-t with a 52cm toptube. Is this more of a crit bike or is this commen. What type of ride should I expect?

Bike two has a 73 degree seat tube angle and a 72 degree head tube angle. This bike is 51 c-t and has a 52 cm top tube. Again what type of ride should I expect.

I will be purchasing one of these bikes off the net in a couple of days so I will not be able to ride them. I know I should but this is the cheapest, best way for me to get a new frame.
I generally ride 30-50 mile rides with a couple of centuries a year.

I have gone to a couple of bike shops and most said I probably would not be able to tell the difference. However one shop told me the bike with the steeper seat is better to climb with.
I lust after both these bike....
thanks for any input..
Steeper angle -> longer effective top tubegeeker
Apr 17, 2003 1:31 AM
Whichever bike you buy, you'll be moving the saddle fore-aft on the rails to put your knee in your preferred position relative to the pedal spindle. If bike 1 has 2 degree steeper sta than bike 2, your saddle will be close to 2cm (probably a little less; I'm too lazy to do the math) further back on bike 1. That's a pretty big increase in effective top tube.

I'm not sure one could estimate the geometry's effect on ride without test-riding identically set up frames. Your current bike setup should give you an idea of preferred effective top tube length. Maybe choose the new frame's geometry to match this.
your wrong!the bull
Apr 17, 2003 3:34 AM
The bike with the steeper STA will have a shorter top tube.

The two bikes are completely different.
Make sure you know what your doing before ordering a bike you cant be fitted on!
actually the top tube would be the said length!the bull
Apr 17, 2003 3:55 AM
because it is moving forward as the sta gets steeper right?
The effective top tube length moves forward with sta angle!
wrong on both counts....C-40
Apr 17, 2003 4:47 AM
When comparing two frames with different STA, you must add 1.2cm per degree to the stated TT length of the frame with the steeper STA to account for moving the saddle back to produce the same rider position relative to the bottom bracket.

An example: A frame with a 74 STA and 54.3cm TT is will have the same effective TT length as a frame with a 73 STA and a 55.5cm TT. Both frames will require the same stem length to produce an identical reach to the bars with the rider in the same position relative to the BB. The frame with the steeper STA will have the saddle positioned 1.2cm further back on the seatpost and if all other things are equal, 2% more weight will be placed on the rear wheel. The frame with the steeper STA will also have a shorter wheelbase.

The formula for calculating the exact amount of difference in the effective TT length is calculated with the formula: saddle height x (cosA-cosB). The value will range between 1.0 and 1.4cm per degree depending on the saddle height, with 1.2 being a good average value. Saddle height is measured in the traditional method along a line through the center of the seat tube.
Ok, so here's a question...Nessism
Apr 17, 2003 6:45 AM
...Why will "The frame with the steeper STA will also have a shorter wheelbase"?

In your example, both frames will have the same front center (assuming the same head tube angle, fork rake and chainstay length). This will make the wheelbase the same, no?

Ed
yes, there are TWO errors...C-40
Apr 17, 2003 8:38 AM
Glad someone reads this stuff to keep me honest. I wrote too quickly this morning.

In this example, since the difference in STA is canceled out by an equal difference in TT length, the wheelbase would be the same. These bikes should have the same fit and weight distibution.

All that's really different is the position of the saddle on the seatpost.
This should be posted permanently,MXL02
Apr 17, 2003 7:34 AM
rather than that stupid "idiots guide to bike fit". What is amazing is how few experienced people (LBS owners/mechanics) understand this concept. Nice post...thanks
...whats really amazing isSteve_0
Apr 17, 2003 8:16 AM
how few people apparently paid attention in 9th grade geometry.
Hmm?the bull
Apr 17, 2003 9:17 AM
Moving the saddle back with a steeper sta kinda defeats the reason you made it steeper right.It seems to me you would make the sta steeper to move the saddle more forward than it was in the 73 angle in the first place.So now that there are all these realitive lines and effective lengths It starts to get me confused.Are they doing this to achive more weight over the rear wheel?I honestly think all this crap does not make much of a differance when your going to be using a laid back seat post on a 74 where on the 73 you were using a seat post with no offset.Furthermore once I understand(or think I understand) how a bike is measured another company measures theirs different.

STILL CONFUSED
Tommy
next timelaffeaux
Apr 17, 2003 10:33 AM
Next time be a little less agressive with the "you're wrong!" posts if you're not sure of your answer either.
reasons for steep STA...C-40
Apr 17, 2003 12:01 PM
One of the reasons to use a steep STA is to permit the chainstays to be as short as possible.

Another reason might be to shift your position forward to accomodate short femurs.

Ideally, the saddle should be relatively well centered on the seatpost. If you have the saddle all the way forward or all the way back, then the STA is not optimum or the seatpost design is not the best choice for use with that STA. It's not unusual to read a post from someone who can't get the saddle back far enough when a Thomson straight-up post is used. All that's needed is a change to one of the more tradiional posts with more setback.

Builders have settled a pretty narrow range of STA for a given size. Rarely is there more than 1 degree difference for an equivalent size. I can fit on the majority of stock 53-55cm frames by changing the stem length by 10mm and moving the saddle 1.2cm or less.

The most extreme differences are between most small LOOK frames that have a 72.5 STA and most other brands that typically use 74-75 STA. You can find some effective top tube differences as large as 2.5cm. The newest LOOK frame geometry (KG 461 and 486) has increased the STA to 73.5-74 degrees on the small size frames.
here is some advice!the bull
Apr 17, 2003 3:40 AM
73 is the standard sta (seat tube angle)
steeper angles are seen on bikes like time trial bikes.
If you are going to be in the drops alot a steep sta might help.
If your current bike has the seat far forward on a seatpost without any setback you might benifit from a steeper STA.
re: good-bad attributes to steep seat tube anglestarwheel
Apr 17, 2003 4:21 AM
The frame with the steeper seat tube angle will fit longer across the top by about 2 cm. That's because you will have to move the saddle back to keep your knees over the bottom bracket. A steep seat tube angle is good if you like to spin at high cadences or for time trials where you are riding in a more forward position with aero bars. For general use, I would think a 75 angle would be rather steep unless you are specifically looking for a time trial bike. A seat tube angle of 73 is pretty standard and I would choose that frame, if I were you, unless you have some unusual requirements or need the extra top tube length.

I have two frames, one with a 74 seat tube angle (relatively steep) and the other with 72.5 (slack). Both frames fit fine the way I have them set up, but I had trouble finding a saddle that fit the bike with the 74 angle. The problem with the 74 frame was that most saddles would not move far enough back to position my knees over the BB. I eventually found a saddle and the bike fits fine now.
Agreed, 75* STA is steep for a century bike. Go 73*!Spunout
Apr 17, 2003 4:38 AM
Sounds like a Tri-bike.
Tiny bikes= steep STAMR_GRUMPY
Apr 17, 2003 6:13 AM
Most road bikes that have a 73, 73 1/2, or 74 STA in the normal sizes (56 to 60 cm), have a 75 or 76 STA in the tiny sizes (50 to 51 cm). Anything less than a 54 usually gets pretty steep. Check out some major manufacturer's web sites.
if a 51 cm is tiny, what do you call a 47 cm frame? (nm)Becky
Apr 17, 2003 6:36 AM
Micro.MR_GRUMPY
Apr 17, 2003 6:39 AM
I'm sure you have 650 wheels, but what is your STA ?
Micro.Becky
Apr 17, 2003 6:52 AM
Right on the 650s, STA is 74 degrees.
GripesMR_GRUMPY
Apr 17, 2003 7:01 AM
It really gripes me that people who ride tiny bikes don't weigh anything. Micro riders are even worse. They have a negative weight.
Micro.littleMoots
Apr 17, 2003 4:21 PM
I guess I'm another Micro rider. I ride a 43 cm! I used to ride a Trek 2000 (STA=76) but moved to a semi-custom bike to get a better fit. I have a really long femur so a slacker STA was necessary for me to get my saddle back enough. Moots used a 74.5 STA on my frame and now I finally, finally, have a bike that fits. Fit is everything!
Micro.Becky
Apr 18, 2003 5:23 AM
Wow....I thought my bike was small! MIcro riders unite :)
I'm of average height, but my torso is way short and my legs are longer than many women my height...hence the micro frame with small wheels in order to shorten the TT.
I agree- fit is everything!
are we even considering rider size here?rufus
Apr 17, 2003 6:29 AM
a bike that small will need a steeper seat tube angle in order for the smaller rider, and hence, shorter legs, to keep his knee in the proper position over the pedal spindle. granted, a rider with longer legs proportional to torso will need to move the saddle back further with a steeper seat angle bike than one with slacker angles. but if he doesn't have a rediculously short torso, he'd probably be better off on a bigger bike.

but if the rider has short legs, he would have to do the opposite, in order to achieve the proper knee over pedal spindle position. that's where a steeper seat angle comes in. it lets him achieve the proper position without having to move the saddle all the way forward.

i guess what i'm trying to say is that he wouldn't necessarily need to shove the saddle back at all on the steeper angled bike, and may even need to shove it forwad on the other one. it all depends upon his leg length, and his ability to achieve the proper positioning.
Good point, rufus...Fredrico
Apr 17, 2003 10:30 AM
That explains why small frames have steep seatubes and larger frames have shallow seat tubes, because of femur length of the rider. Longer levers need more saddle setback, short levers less setback.

Notice the 75 degree seat tube bike has a 72 degree steering tube? That puts the front wheel far enough out to maintain a 39 inch wheel base, to eliminate toe-overlap, and maintain fore-aft balance.

I'd put my money on the steeper seat-tube and shallower head-tube frame. Way up over the crank, you can get up an hellacious spin, pedal fast all day. A shallow headtube will make the bike track nice and stable. Fast and sure footed, that's a great combination.
To address the issue of which climbs better...greg n
Apr 17, 2003 9:16 AM
No one seems to have addressed this. However, in reality, it really isn't an issue. Yes, in theory, a steeper seat tube angle (given all other factors, i.e. fore/aft position, head tube angle, etc. are exactly the same on both bikes) is better for climbing because it will put you over the pedals more. But, like everyone stated, you'll move your saddle position fore or aft to accommodate the seat tube angle, so in actuality, your saddle will be in the same relative position over the pedals regardless of the seat tube angle. Not to mention that typically, you slide forward on the saddle when climbing to compensate for the amount of grade.

Bottome line is get whichever bike is most comfortable for you. If this bike is going to primarily be a racing bike, I'd get the 75/72.5. If it's more all-purpose, get the 73/72.