Bike Forums (https://www.bikeforums.net/forum.php)
-   Framebuilders (https://www.bikeforums.net/framebuilders/)
-   -   Frame geo/design curiosity ???'s (https://www.bikeforums.net/framebuilders/925696-frame-geo-design-curiosity-s.html)

 thook 12-10-13 07:28 PM

Frame geo/design curiosity ???'s

Howdy,

I'm saving up for a custom frame. It'll still be a while off before I'm ready 'cause saving is going slow.<<<**** vehicle repairs!!>>> So, in the meantime, I've been trying to educate myself on frame geometry to better understand how to get what I'd like in terms of performance, comfort, and visual appeal.....in that order. I suppose I could rely totally on a builder to account for that, but something in me would feel more comfortable in knowing exactly what's going on in the process of the design.

So, one question comes up for me that I haven't been able to get answered in my web reading. What would happen if the head tube angle were steeper than the seat tube angle?

Hypothetical....

Let's say a HT angle of 73* and a ST angle of 72*. Let's say the fork has a rake of 40mm with a wheel of approximately 675mm's in diameter. This would give a trail in the high 50's....close to 60mm's. I read that's a pretty neutral amount for handling. (I'm just giving factors that might be pertinent to my question here. Not really sure if it's important.)

Reason I ask is I don't really see any frames designed where the ST is slacker than the HT. Most frames I see have a HT and ST angle the same or the ST angle is a bit steeper than the HT. The latter particularly on smaller frames. I can understand why frames are being designed this way due to reach, wheel clearance, wheelbase, etc., but I believe I'd do well for comfort/center of gravity with a slack ST angle of at/around 72*. Also, it would reduce the effective reach for my height and body geometry relative to my usual saddle position. Eg., I could have a TT of 54 or 55cm's that would perhaps give the same effective reach of a smaller frame with a steeper ST and a TT of perhaps 53cm's.

Anyway, what do you say? Is there a problem with a slacker ST angle vs HT angle?

I have the same opinion about my best ST angle, about 71.5 or 72. I tend to like the saddle back pretty far. I don't think or don't know if my femurs are long or what, but I do have more upper body weight than I should and to feel in balance, even with improving core strength, I like to be pretty far back. By finding your ideal saddle position (at least for today's riding) you can calculate your ideal seat tube angle assuming a zero setback post. You can then modify that spec based on assuming a seat post with 15, 20, or 25 mm setback.

The main effect of seat tube angle is front-back weight distribution relative to the BB axis.

 corwin1968 12-10-13 09:10 PM

Are you using BikeCad to render out your design? I've spent hours on hours working on my own hypothetical custom bikes and it's a blast and very easy to use once you get the hang of it.

 thook 12-10-13 09:57 PM

My current ride has a seat tube angle of 73*. The seat post has 25mm of set back and I have the saddle at the furthest backward adjustment. I still find myself on the back end of the saddle to feel balanced with the weight off my hands when I'm cruising for long periods.

After trial, error, suffering, expense, and much web researching I finally....finally....understand that point; weight relative to BB axis. My hands and wrists used to go numb to the point I could not WAIT to get off the bike after a long ride. That was on a bigger frame, though. It's not that bad since I went to smaller one.......though, even this one is still a bit too long.

Anyway, I've got a pretty strong core. I do a lot of lifting and handsplit my firewood atleast half of the year. Still, it's hard to hold oneself up when riding long distances if you're not balanced. After about 30 miles, I start wishing I hadn't ridden out quite so far...hahaha! Can't do anything about it right now, though. I have the frame I have. But, I have a lead on a frame with 54cm square geo and a ST angle of 72.5*. Ain't quite there for me, but it'll be better, hopefully. My current frame has a 55cm TT and I figure I need to come off about an inch of the total current reach. So, switch the 54cm frame, use an 80mm stem (current is 95mm), and possibly the only compact bar I could find with 70mm reach. My current is at 80mm's. That would come to more than an inch off, but not much. I'm not how much 1cm will make a difference. I suppose I could lower the bars a little if I feel to cramped on that theoretical set up. Right now, I have the bars at saddle height.

Speaking of calculating, do you have a resource you like to use for that? I've use Leonard Zinn's calculator for frame fit. I'm probably gonna retake some measurements and review.

Oh, I don't suppose you have any comments on the ST angle being slacker than the HT angle? I'm wondering since I don't see anyone doing it, there might a good reason. But, then again, maybe not.

 thook 12-10-13 09:58 PM

Quote:
 Originally Posted by corwin1968 (Post 16319805) Are you using BikeCad to render out your design? I've spent hours on hours working on my own hypothetical custom bikes and it's a blast and very easy to use once you get the hang of it.
No, I'm not. I don't have that. I've seen it mentioned here and there, but I've never acquired it or used it.

 Scooper 12-10-13 10:00 PM

My custom Waterford RS-22 (Road Sport) has a 72.5° STA and a 73° HTA. Trail is 59.9mm, and the chainstays are 432mm long.

My main goal was comfort on longer rides, but without sacrificing much in the way of performance. The trail and wheelbase make it pretty stable, and I can spend all day in the saddle without feeling fatigued. I've done the week-long AIDS/LifeCycle ride from San Francisco to Los Angeles on the bike (the route is 435 miles) twice and plan on doing it again in 2014.

http://i32.photobucket.com/albums/d7...rawingONLY.jpg

 thook 12-11-13 04:13 AM

You're a tall fella, aren't you? ;) Nice low bottom bracket, too!

So, is that the design Waterford came up for you or did you come with it?

 Mark Kelly 12-11-13 04:23 AM

It's fairly common to have the HT steeper than the ST for large bikes so the front centre doesn't blow out too much.

The reverse is true for small bikes to prevent excessive toe overlap.

 unterhausen 12-11-13 07:15 AM

the seat tube angle isn't all that important except in its role in fit and weight distribution. Small frames are always a compromise, pay no attention to them unless you want to design a small frame.

 Andrew R Stewart 12-11-13 09:32 AM

I think that any bike that doesn't have a lower HT angle (DT/HT) other then 60* will handle like crap.

OK that's a joke but I did work for a guy who believed this so every bike he designed was around this joint angle. The other builder and I finally came to terms with the reality of a lower HT angle that worked with the rest of the design(s) and just didn't manage to say anything to the boss, for a while.

Posters are correct in that the ST angle doesn't matter except to allow the seat to be well positioned. Different seats or seat posts will have different tube angles to attain a centrally (on the seat rails) post clamp and also end up with the same hips/feet relationship. Andy.

 Scooper 12-11-13 12:51 PM

Quote:
 Originally Posted by thook (Post 16320213) You're a tall fella, aren't you? ;) Nice low bottom bracket, too! So, is that the design Waterford came up for you or did you come with it?
Thanks. I'm 6' tall with long legs, and the geometry was a collaborative effort between me, the fitter, and Waterford. I took a lot of cues from my '72 P15 touring Paramount.

http://i32.photobucket.com/albums/d7...psa9ec85fb.jpg

 thook 12-11-13 05:19 PM

I appreciate all the input, gentlemen. Yeah, I guess when I finally have the opportunity to get with a frame builder, I can request a more relaxed seat tube. And, just because I tend to be super curious, I'm real curious to know what kind of ST angle I'll need now. That's my thing....I've got long legs, too. For my height, anyway.

On this morning's web searching adventure, I found this! Whooaa!

http://www.cyclofiend.com/ssg/2010/s...ermain0510.htm

What a beaut', eh? Steep head tube, relaxed seat tube. :)

 thook 12-11-13 05:21 PM

Quote:
 Originally Posted by unterhausen (Post 16320378) the seat tube angle isn't all that important except in its role in fit and weight distribution. Small frames are always a compromise, pay no attention to them unless you want to design a small frame.
What would you call a small frame? I'm looking at something....most likely...in the 53cm/54cm range for myself.

 Andrew R Stewart 12-11-13 05:51 PM

Quote:
 Originally Posted by thook (Post 16322337) What would you call a small frame? I'm looking at something....most likely...in the 53cm/54cm range for myself.
Isn't any bike smaller then the one you ride a small bike?

I built a number of 45-50cm bikes, some with 520 or 571 wheels. I'll just say that the frame angles are not the only aspects that are different. Andy.

 thook 12-11-13 06:07 PM

Quote:
 Originally Posted by Andrew R Stewart (Post 16322430) Isn't any bike smaller then the one you ride a small bike? I built a number of 45-50cm bikes, some with 520 or 571 wheels. I'll just say that the frame angles are not the only aspects that are different. Andy.
Well, yeah. But, he said small frames are always a compromise and pay no attention to them....etc. So, I wondered if he might clarify his thoughts on that for me. I suppose I could have said just that. ;)

On that note, would you mind elaborating on what you're saying? I'm really wanting to understand all this.

 unterhausen 12-11-13 06:13 PM

Quote:
 Originally Posted by thook (Post 16322337) What would you call a small frame? I'm looking at something....most likely...in the 53cm/54cm range for myself.
I think the temptation to compromise starts showing up with anything smaller than about 50cm. If you see a slack head angle and a steep seat tube, the designer almost surely made compromises due to the size.

 Andrew R Stewart 12-11-13 06:47 PM

Quote:
 Originally Posted by thook (Post 16322464) Well, yeah. But, he said small frames are always a compromise and pay no attention to them....etc. So, I wondered if he might clarify his thoughts on that for me. I suppose I could have said just that. ;) On that note, would you mind elaborating on what you're saying? I'm really wanting to understand all this.
Well it's hard to pack 35 years of learning into a few words but- First and foremost is the fit for the rider. Pick you method of deriving this. Once you know the contact points and their relationship with the crank (arm length, Bb drop) you can begin to see how the other components will fit, wheels first. At the same time the steering geometry is created. I'v come to understand that with less rotational mass of a smaller (then 700c) wheel the steering wll feel quicker or less stable (depending of the glass full/empty view). So I'll soften up the steering when using 571 or 520 wheels. I don't like to have head tubes much shorter then 100mm's as the headset starts to be levered a lot when the bearings are close together and the adjustment gets more sensitive too. This complicates the bars/seat height difference so a custom stem might also be in order. The BB height comes into play here too. Many times the rider is much lighter and less strong then more average sized ones. So tube diameters are often less then bigger frames will use. I do try to not go too light on wall thicknesses though. If a hard to fit person gets a bike that really works well for them they're unlikely to want to replace it in a few years. (Unlike the model that the majority of the bike business seems to follow. "Let's see, it's Friday so it must be time for a new bike... the magazine said so.") I'd rather my bikes be more durable then light and a small bike with small parts will already be light. Gearing needs are another area that sometimes are not dealt with well. 170 crank arms on sub 50cm bikes, the assumption that slightly built and short riders want to push a compact crank, and chain stays that are so short (because they can be) that the chain run is noisy are common mistakes I see. Handle bars that are built on the need of stiffness (31.8 stem clamps) that a 110lb rider will never appreciate. There are more but I'll end with the locating of cable stops and braze ons that are too close to tube ends, cable adjusters and rack mounts which won't allow the brake cable to exist too. Andy.

 thook 12-11-13 06:48 PM

Quote:
 Originally Posted by unterhausen (Post 16322484) I think the temptation to compromise starts showing up with anything smaller than about 50cm. If you see a slack head angle and a steep seat tube, the designer almost surely made compromises due to the size.
Okay. I understand what you're saying. I'd read an article by Grant Peterson on seat tube angle and he pretty much said the same. Surly bikes....like the Cross Check....is a good example. Peterson's article is pretty much where I first started understanding the problem I've been having fitting on bikes. I went out one day for a long ride and just said eff it. Even though I knew I'd be compromising the reach on my bike, I slammed the saddle back as far as it would safely go and wouldn't you know it? There it is! My center of gravity! I could feel what people were talking about and being balanced over the bike. So, after that "dawning discovery", when I started looking at off the shelf frame geo's, everything was in a different light. I then realized I couldn't find anything of what I needed. Unless, of course, I got a frame with a short top tube and paid upwards of \$70 or more for a seat post with a lot of set back. More than an inch. That problem was/is only exacerbated by needing a certain post size for whatever frame. Specifically, my current vintage one.

 thook 12-11-13 06:57 PM

Quote:
 Originally Posted by Andrew R Stewart (Post 16322555) Well it's hard to pack 35 years of learning into a few words but- First and foremost is the fit for the rider. Pick you method of deriving this. Once you know the contact points and their relationship with the crank (arm length, Bb drop) you can begin to see how the other components will fit, wheels first. At the same time the steering geometry is created. I'v come to understand that with less rotational mass of a smaller (then 700c) wheel the steering wll feel quicker or less stable (depending of the glass full/empty view). So I'll soften up the steering when using 571 or 520 wheels. I don't like to have head tubes much shorter then 100mm's as the headset starts to be levered a lot when the bearings are close together and the adjustment gets more sensitive too. This complicates the bars/seat height difference so a custom stem might also be in order. The BB height comes into play here too. Many times the rider is much lighter and less strong then more average sized ones. So tube diameters are often less then bigger frames will use. I do try to not go too light on wall thicknesses though. If a hard to fit person gets a bike that really works well for them they're unlikely to want to replace it in a few years. (Unlike the model that the majority of the bike business seems to follow. "Let's see, it's Friday so it must be time for a new bike... the magazine said so.") I'd rather my bikes be more durable then light and a small bike with small parts will already be light. Gearing needs are another area that sometimes are not dealt with well. 170 crank arms on sub 50cm bikes, the assumption that slightly built and short riders want to push a compact crank, and chain stays that are so short (because they can be) that the chain run is noisy are common mistakes I see. Handle bars that are built on the need of stiffness (31.8 stem clamps) that a 110lb rider will never appreciate. There are more but I'll end with the locating of cable stops and braze ons that are too close to tube ends, cable adjusters and rack mounts which won't allow the brake cable to exist too. Andy.
Wow! Thank you very much. I'll have to read over that a couple more times and think about what you just said. But, after a brief reading, there certainly is more in designing than I'd realized. But, then I'm not a frame builder, either. I thought at one time I'd like to build one or two of my own. But after realizing the time, effort, and expensive of just learning (I'm usually short on time and funds due to other aspects of life, anyway) I gave up that idea. Figured it be easier and just as rewarding to let some other artist do it...haha!

I envy frame builders, a bit. I know it's work, but I'm not afraid of that. Just seems like it'd be a lot of fun..... as much fun as building things can be. And, I do like building things.

 Canaboo 12-11-13 09:05 PM

I made this bamboo frame with a ridiculously laid back seat tube to prove a point and made the HT steeper to compensate for the old steel fork that has an almost chopper like rake to it.
When the seat is moved into the into the right position it's hard to really notice any quirks in handling etc. because of those features.
It does seem like there is considerable leeway in a frame geometry that can be compensated for by what you actually mount on the frame as far as the fork and seat go.
http://i399.photobucket.com/albums/p...5/CIMG5929.jpg

 Andrew R Stewart 12-11-13 09:28 PM

Quote:
 Originally Posted by thook (Post 16322584) Wow! Thank you very much. I'll have to read over that a couple more times and think about what you just said. But, after a brief reading, there certainly is more in designing than I'd realized. But, then I'm not a frame builder, either. I thought at one time I'd like to build one or two of my own. But after realizing the time, effort, and expensive of just learning (I'm usually short on time and funds due to other aspects of life, anyway) I gave up that idea. Figured it be easier and just as rewarding to let some other artist do it...haha! I envy frame builders, a bit. I know it's work, but I'm not afraid of that. Just seems like it'd be a lot of fun..... as much fun as building things can be. And, I do like building things.
Well, I'm no frame builder (as defined by earning my keep) but i am some one who is collecting a body of knowledge. As so many of my bikes have been on the small end of the bell curve i am sensitive to them. Also having wrenched professionally longer then I've practiced building I also am sensitive to poor design as it relates to servicing the bike. Andy.

 MassiveD 12-11-13 10:06 PM

On performance bikes (vs comfort), my design priority for the seat tube is bike fit. Then structural efficiency, and detailing. Most bikes that are to be ridden with a forward posture, but not to the extent of a time trial or tri geometry, have seat tubes at around 73 degrees. The reason for that is that on average men's frames that accommodate rider position, with normal seats, and normal posts, 73 degrees is the sweat spot. It has nothing to do with balance over the BB, or working in concert with front end geometry. HT geometry and ST geometry are related mostly by coincidence of the requirements of the bike. They occur together, they don't work together.

Also, 72 degrees is not a significant change, but it can be useful. As far as I know, the main reason for post set-back is to allow the wheel to be tucked under more. Some bikes don't need that, and some seats are not designed for it. So in my case I use a 72 degree ST and a zero set-back post with my seat, to get the geometry I would have with a 73 degree ST, normal set back post, and normal seat. But at the end of the day, what makes any of this work is my knee over pedal measurement, and subsequent tuning.

When making up a design, I start with real things. My bike fit, the type of bike I want to build, the specific components I will use. I'm not just riffling on stuff I saw in a catalog. For one thing, the numbers in catalogs are often not real.

 MassiveD 12-11-13 10:30 PM

"Also, it would reduce the effective reach for my height and body geometry relative to my usual saddle position. Eg., I could have a TT of 54 or 55cm's that would perhaps give the same effective reach of a smaller frame with a steeper ST and a TT of perhaps 53cm's."

I can't quite make sense of this. If you tip the ST back, all else equal you increase reach. If you steepen the relative angle of the HT, you increase reach. If you are after a comfort posture (sorta a misnomer), where you are more upright, then you will probably be raising the bars, since putting the ST back will create a deeper torso angle, otherwise. Raising the bars, increases relative reach. ?

When I put the ST back to 72, it doesn't change reach because I am adjusting the post and the seat.

When you swing a 24 inch seat post back from 73 to 72 degrees, it moves back and down .4" on the horizontal, and .13" on the vertical (24" from center of BB). It isn't a ginormous change.

 thook 12-12-13 12:15 AM

Quote:
 Originally Posted by MassiveD (Post 16323094) "Also, it would reduce the effective reach for my height and body geometry relative to my usual saddle position. Eg., I could have a TT of 54 or 55cm's that would perhaps give the same effective reach of a smaller frame with a steeper ST and a TT of perhaps 53cm's." I can't quite make sense of this. If you tip the ST back, all else equal you increase reach. If you steepen the relative angle of the HT, you increase reach. If you are after a comfort posture (sorta a misnomer), where you are more upright, then you will probably be raising the bars, since putting the ST back will create a deeper torso angle, otherwise. Raising the bars, increases relative reach. ? When I put the ST back to 72, it doesn't change reach because I am adjusting the post and the seat. When you swing a 24 inch seat post back from 73 to 72 degrees, it moves back and down .4" on the horizontal, and .13" on the vertical (24" from center of BB). It isn't a ginormous change.
You probably can't quite make sense of it because I'm not always good at illustrating verbally. I am still trying to put all this together in my head. Also, I'm certainly not a frame builder, but I'm not sure agree entirely with what you've said. I don't think comfort exclusively means raising the handlebars. I already have them at saddle height as it is. I really believe my problem has been weight distribution. When I move back on the saddle end, the weight comes off of my hands a great deal. Of course, it will.

If the saddle position relative to the BB doesn't change, yet you tip the ST back while the HT angle remains a constant, the top tube is going to move backwards latitudinal thereby drawing the handlebars backwards towards the rider. I don't mean to imply while also steepening the HT angle. If I were to have a custom frame, I'm just saying I'd like to have/keep a steep'ish HT angle. Most frames that might otherwise fit me tend to have somewhere around 72* or 71*. My current frame has one of 73*, best I can tell. I used an angle finder and compared it another frame with a known geo.

Grant Peterson explained in the article I mentioned that when you change the angle even by one degree, at a certain height....I don't remember that height, either.....but, it was right about where I do have my saddle height currently...... reach has effectively changed by one cm. Is he correct? I assumed he would be since he designs frames. I guess that's what I've been going by. And, it's probably why most of his frames have a slack seat tube. Even with small ones.

Sam, the owner/designer of Singular Cycles, also explained this to me once in a conversation I had with him when I queried him on the med. size Peregrine. He said by my account of measurements, it should fit me quite well because of the difference in ST angle compared to my current frame. Particularly since it also has a 54cm TT. Problem is with 29er tires it has a SOH of 32.75 inches. Right up there in the neighborhood of too close for me, though! With riding shoes on, my PBH is only a tad over 33".

Lastly, I read somewhere (not by Mr. Peterson or Sam) that a change in HT angle wouldn't change reach much. Atleast, not compared to a change in seat tube angle. Something about the angles' point of origin starting from different horizontal planes? Man, that was a while back. I'm probably totally wrong about that.

Edit: Well, when I look up there at that frame chart, I guess I was right. They do have different points of origin.

 busdriver1959 12-12-13 04:35 AM

The part that confused me and some others was the change in seat tube angle without the seat following it. I think most would use seat tube angle to put your butt where it needs to be with the seat roughly centered on the rails. Reach is achieved with top tube and stem length. Changing reach with ST angle is a little bit backwards.

All times are GMT -6. The time now is 09:04 PM.