Geometry and You or "Angles- How Do They Work?"
 

I've always labored under the understanding that a higher angle is a steeper angle, and therefore more "aggressive."

I understand that steering with a more aggressive angle will be more precise and more "twitchy."

What are the other characteristics of shallower or steeper angles?

For example- my 1984 Schwinn Voyageur SP is stated to have a head tube angle of 72 but a seat tube angle of 75 (which seems really steep to me- even compared to most "race" bikes). I've always had a hard time getting any other saddle to work on that bike, other than what came with it. I eventually swapped out the seat post for a more adjustable Superbe Pro. I'm not sure what else that 75 STA does for the ride of the bike, or my pedal stroke other than making it difficult to find a saddle/post combo that works.

For the record, the seat angles on my Miyata 1000 is 72, my 620 is 73.5 and my 720 is 73.

Not really enough info to comment on. Size of the frames can mean much as to why things are.
Also, do not believe published angles. You have to measure yourself and accurately to really know. Including making sure your datum is level.
That all expressed, there is an evolution in design over time that can be observed.
Head tube angle is btw, a guide until you include the fork rake. Trail can be derived from that. Don't forget bottom bracket drop too, all of this can and do influence a bikes handling and performance.

Actually, this came about because I started putting together a spreadsheet of published geometries for my bikes- to compare, and kind of see how much shorter chainstays or TT or whatever... the 75 really sticks out- but some of the later Schwinns are like that; and that explains why an ordinary seatpost doesn’t have the adjustment to level out the saddle- you’re either sliding forward or using the stock, banana shaped saddle.

The following uses examples from road racing bikes, but they should illustrate the issue - especially since "aggressive handling" more often comes up with road bikes.

Really, the angles themselves aren't aggressive, it's their effect on wheelbase that is. And this is mainly true head tube, not the seat tube.

Supposing the same seat tube angle and same top tube length, a bike with a 73 HTA will have a front center or wheelbase about 1cm shorter than a bike with a 72 HTA. Both head tubes originate at the top, and angle down from the top, so the net effect is seen at the front dropouts.

Short wheel base bikes will turn quicker than longer wheel base bikes for a given lean angle - the contact patches two wheels being closer together sit on a smaller circumference circle than a long wheel base, and that reduces the turn radius of that circle. So when you lean modestly you get more change in direction with a short wheelbase.


Steep seat tube angles were at one time used to maintain tire clearance on extremely short chain stay bicycles, but this can be accomplished other ways, like by curving the seat tube around the tire. These days, steep seat tube angles are generally found on smaller sized bikes, and are mostly used to artificially shrink the top tube, because steeper STAs lean toward the head tube. Some companies have called BS on this practice at various times - Cannondale in the '80s and Cervelo today just use 73° for all road seat tubes regardless of size.


Regardless of HTA, steering feel is largely controlled by rake, which can make up for the actual steering angle by keeping the trail numbers consistent across size ranges. This used to be easier with metal forks, but molded carbon generally forks require a different mold for each fork rake, so many brands simple have 1 to 3 fork rakes that have to serve the range of HTAs for that model, making trail less controlled than it used to be.


The reason we use the HTA angle range you see on road bikes - about 70 to 74.5° - is largely because of toe overlap preventing the angle from getting too steep, and wheel flop making shallow angles feel less controlled. More wheel flop makes steering accelerate away from center, and at some point that starts to feel bad. If you were building someone 7 feet tall a bike, you could make the TT long enough to allow for an 81° HTA, and then the fork would have zero rake to produce normal trail.

This is my experience. Add in tire size/pressure and it tells pretty much the whole story. Personally I don't buy into the laterally stiff vertically compliant line.

This is going to be a good thread.

Can someone define a few terms?

Agressive
Twitchy
Low trail

I admit to not understanding a LOT about bike design. I own a Trek 660 that just seems to be magic, wonder why?

I think wheel-flop has a much larger effect on handling than trail and is felt much more than any other metric. A 3mm difference in wheel flop between two bikes is very obvious when riding, 3mm of trail, rake, wheelbase or really anything else does not create nearly the same feedback.

This i-bob post from a few weeks ago has thrown me for a loop, so much of bike geometry is based on iterative development and not on a mathematical expression of design. Or ex post facto rationalization.

Are you assuming fork offset will remain constant? There's no reason to do so, especially since we're in C&V where steel forks are the norm, rather than in the world of carbon, where forks only come in a very limited range of offsets. Put a 2 1/2" (63 mm) fork rake on that 73 degree head angle and the front center and wheelbase are going to be a lot longer (and trail a lot less) than if it had a 43mm.

Simplified summary:

"Trail" is the factor that most influences handling, and as noted above, trail is determined by the interplay of head angle and fork rake.

Head and seat angle and top tube length determine rider placement on the frame and weight distribution between saddle and handlebars.

Chain stay length determines wheel clearance, overall wheelbase, and also clearance for cargo panniers.

Taken together, the frame is a system where no one measure can be changed without influencing others, and what might be best for a particular rider depends on their body, riding style, and expectations.

To complicate matters again....

Different saddles allow more or less fore-aft adjustment, and that goes for seat posts as well. Older saddle designs, like Brooks, tend to assume a seat post angle around 73°. If the frame builder makes the seat tube angle steeper, most riders will want to move the seat back to compensate... which is often hard to do.

Steep STA and slack HTA typically indicate a frame around or under 52cm. For everyone weighing in, typically ridden frame size is a must, otherwise we might as well be trying to build a house in outer space.

I personally ride a 63.5cm (25") frame with a range of 63-65cm.

Many vintage Trek models are pretty good at keeping identical (or very similar) HTA and STA as the frame size gets larger. The bikes are proportionally larger, or at least have proportionally larger wheelbases.

It seems on small frames, the thing to avoid was toe overlap while maintaining a desired trail number. On large frames, some companies (Ciocc, Masi, etc), made the HTA steep to keep the wheelbase closer to that of say a 57cm frame--to have the 57cm frame be a sort of "sun" that the other frame sizes would work to be close to in some regard.

For high/medium/low trail, I'd categorize it as this:

High trail: mid-60s mm and up
Medium/sweet spot: 56-61mm or so
Low trail: mid-50s mm and below, with very low trail once you get into the 30s.

STA never bothers me much because I can usually achieve my needed setback amount. HTA and resulting trail amount make their characteristics known soon enough. I have bikes in all three trail length categories, and all feel good for different reasons.

*** What no one talks about, but that which I think is critical as it takes into account both fore/aft rider weight on a bike AND steering feel is: ***

Where is the front-most point of your hand/palm in relationship to the front axle??? The 'pocket' or webbing between one's thumb and index finger is by and large the forward most point of the hand (fingers not on brake levers) when riding on the hoods. From that forward-most point, draw a vertical line to the ground. Where is that line in a fore/aft relationship to the front axle?

As the distance behind the front axle increases, the rider's weight it presumably further back, making the front end light no matter what. As the "hand plane" goes further forward of the front axle, a taming effect on the steering is had (more rider weigh, more lateral effort to turn the front wheel). It's almost like you "guide" the front wheel. So a more "alert" front end is calmed, yet the bike can still maneuver quickly.

My ideal "hand plane" to front axle setback is usually around 1cm. I like a medium to lighter steering feel, and definitely notice a "slower" steering attitude when my "hand plane" is forward of the front axle.

I won't delve into hand/hood height that is much higher than the top tube (a la Technomic stem) vs. a low height. I do have ideas on that, but will echo what [MENTION=20548]JohnDThompson[/MENTION] said and that is the frame is a system and no one change happens without influencing the other.

I made a post a bit earlier, that vanished. I will expand on:
Seat tube angle, WAS directions from the builder to where to place the saddle, or their world view on where the saddle should be placed, by whatever thinking was at play.
Often that can be worked around. (Way back that was done with "7" shaped seat posts. Never really understood that- why not take a direct route?, but I digress)
On very small frames the distortion of seat tube angle was done to "make the numbers" of top tube length most often, sometimes coupled with a slack head angle to help the top tube measure and provide a UCI acceptable front center dimension. Not sure about the recent versions of the UCI rules on dimensions of bikes but there are a number of key ones from decades back, max bottom bracket height, wheel base dimensions (to prevent recumbents), front "center" dimensions to essentially prevent "toe clip overlap". Race bikes were type formed and this infected almost all. I also think the UCI has an issue with short people.

The Eddy Merckx bike company a while (10-15 years?) ago announced a 500,000 Euro research grant for exploration of analyzing the how and why of well mannered and safe racing bicycles. Never heard any results.

In the early 80's the FCI shared its research of road race bike design with Italy's top builders. The result was a further of the practice that Gios was already onto of short top tubes, long stems and more weight on the front wheel. Pinarello from my review really bought into this afterward.

To illustrate the point, yes of course the same rake was used. And many bikes do use the same rake, despite varying HTAs. But even a change in rake that is proportional to the change in angle will affect wheelbase by only a few mm per cm of HTA wheelbase change.

The example also assumes the same fork length, same head tube length and is only an approximation of the change in wheelbase per degree.


Chain stay length also determines weight distribution on the wheels. Long chain stays shift the weight toward the front wheel.

Head tube angle doesn't really change rider position because we measure HTA starting at the top of the head tube. But changing HTA (even with a refinement in rake to hold trail) will change wheel weight distribution if it changes the front center.

This is true, but there is no situation where you would get a 3mm wheel flop and 3mm trail change at the same time, so it is a misleading statement. If you change a particular road bike's fork offset (rake) by 5mm, the trail will change by 6mm, but the wheel flop will only change 1mm. And if you leave rake alone and change HTA by 1°. the trail will again change significantly - 7mm - while flop only changes by 2mm. The two measures are not proportional.

There are mathematical expressions, but the post from the guy from Schwinn seems to assume that neutral trail is not a universal number, but is proportional to wheel size. So of course his example of two different wheel sizes yielding two different rides despite the same trail - the trail was only "the same" as an integer, not the same in proportion to wheel size.

On road bikes we say that anything below 56mm is low trail and everything above is high trail. But that's only true with a 700c wheel and a common racing tire size like 25c. If you build a 650c bike and you also want "neutral trail", you aren't going to find it at 56mm. Trail will be centered elsewhere, and a comparison of high trail between two different size wheels will be in proportion of change, not a fixed number of mms.


This was especially illustrated to me when I rode a bike that substituted a long fork with a roller blade wheel at the end. An 8cm wheel can't possibly have a trail of 5.6cm - trail has to be in proportion to the wheel diameter. The bike rode fine and didn't display any obvious weirdness. I would imagine any offset or trail was less than 3mm.

[QUOTE=RiddleOfSteel;20150186]

For high/medium/low trail, I'd categorize it as this:

High trail: mid-60s mm and up
Medium/sweet spot: 56-61mm or so
Low trail: mid-50s mm and below, with very low trail once you get into the 30s.

---
I will admit that I am ignorant, so educate me. What is trail? What part of the front end trails and what leads?

If I I look at two bikes, one with a fork that is almost straight and a second with the old school curve, which one is low trail?

A straight fork to my mind will be twitchy, and a long curved one "slower" steering.

Here is Dave Moulton's explanation of trail.

Google has many fine illustrations of trail.

Fork curve is not rake. You can have a curved fork with no rake and a straight blade for with lots of rake. Rake or offset is the amount of change from the steering centerline. Again, google.


Forks don't, on their own, do anything for handling.

Thanks, that helped.

Pretty easy to get lost at his site.

Thanks!
I did not mean to hijack this thread.

You didn't hijack the thread, but it is easier if posters don't have to define common terms that are easy too look up, what you look up will be more complete.

https://www.bikeforums.net/data:imag...AAAElFTkSuQmCChttp://blog.artscyclery.com/wp-conte...-and-trail.jpg
https://www.bikeforums.net/data:imag...AAAElFTkSuQmCC

Straight forks can have a lot or a little offset. It's the offset that matters, not how the tip of the fork gets there.

Even within the relatively narrow (~56-59cm) range of frame sizes that I would consider making the effort to get fitted to, I firstly look at frame angles in terms of their effect on fit, both directly and indirectly.


Directly, because the seattube angle positions the entire length of the toptube fore and aft, though doesn't define the saddle's position by one whit. So seattube angle must be "weighed" against the toptube length for getting started with fit.


Indirectly, the headtube angle may affect what range of stem lengths will allow desired steering behavior with the rider in or out of the saddle, which are two very different things. Head angles of frames within my size range are usually paired with a compatible fork rake dimension that makes any handling changes due to stem length changes somewhat predictable. Among the very large number of builds that I have fitted myself to, generally a steeper headtube angle gets along well overall (while riding both in and out of the saddle) with longer stem lengths, while shallow headtube angles may require a cm's-shorter stem in order to handle safely while riding off of the saddle.


All of the above, though far from comprehensive, is sufficient for me to predict if a frame (within my familiar size range, or a little larger) will justify the effort of a build, in that it can be made to handle properly with a stem length that I expect to need to get fitted comfortably. I've literally been through the "full" range of headtube angles, multiple times, even at the extremes from 69-degrees up to 76.5 degrees, and no longer do I get stuck with something that I don't want to ride due to fit and handling considerations.
And given the range of diameters and styles of handlebar and steerer attachment over the past 60 years, you might guess what my handlebar stem inventory looks like!


In one recent case of a larger (59cm) 1979 Fuji professional with a 75-degree HT angle, the expected "twitchiness" of this bike with it's original 9cm stem surfaced initially, but with a change of tires/rims (from 22mm tubulars to 25mm clinchers), and installing a longer chain to allow moving the rear axle rearward, I quickly adjusted to the bike's still-lively feel and it soon became a favorite ride. The stock stem was unique and rare, so any change there would have compromised the bike's originality further, so I gave the rather short, catalog-spec 9cm Nitto "Crystem" (with narrow bars) a go, and was just able to make it work as Fuji intended.

Avoid frames designed by committee.