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Let's Discuss Frame Geometry

Old 10-03-20, 11:22 AM
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Moisture
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Let's Discuss Frame Geometry

It should be apparent to most of us, that frame builders often go through great lengths to put together a frame which can seemingly do it all. Things such as head tube, seat tube angle, chainstay. seatstay. and top tube lengths all play a dramatic role in terms of how comfortable the bike feels for the rider, how well he is able to balance his weight between the rear/front axle, as well as the overall balance achieved between handling and stability. For example, the head tube angle tends to be on the slacker side (68-71 degrees) for mountain bikes, which results in slightly less precise steering, but improves stability. Road bikes tend to have a more aggressive head tube angle (about 74 degrees) which sets the fork further in front of the frame, increasing steering precision, albeit at the expense of overall stability. Seat tube angles on mountain bikes tends to be angled fairly aggressively backwards to increase the amount of weight towards to rear axle, setting the rider backwards for a more stable angle of attack during climbing scenarios. However, angle the seat tube too far backwards, and this will compromise the amount of weight the rider places over the handlebars, causing a light front end which may result in the rider flipping over. chain stay lengths tend to be increase due to the introduction of larger 27.5/29" rims. This helps to increase stability at speed due to a longer wheel base, and helps balance the weight better to prevent the handlebars from lifting up during climbs. A shorter chainstay can help to improve liveliness on technical trails. Furthermore, you may have heard of the terminology trible, or double butted frames. This essentially means that the diameter and shape of the tubing differs according to the amount of load being placed onto each respective tube. For example, the down tube and chain stays tend to take the most stress from riding, especially at the welding points. Consequently, this area of the frame tends to have thicker, stiffer tubing in place. The top tube and seat stays are usually a different shape. to allow for a little bit of forgiveness, so that vibrations and small imperfections in the terrain can be more or less absorbed before they reach the contact points of the rider.

After studying the frame geometry of my bike, a 1998 GT Zaskar LE, it is apparent to me that the engineers strived to achieve a good overall balance between polar opposites, light weight versus overall strength, stability versus handling precision, climbing versus descending, etc. The bike clearly excels on long, tough climbs, where i feel like the overall geometry and stiffness of the frame results in a high percentage of my efforts being converted directly to forward momentum. I never have to get out of my saddle no matter how steep the grade is. I just snatch and lower gear, keep up a good cadence at the cranks, and power my way up. However, with all this in mind, stability on steep hills do not seem to be affected.

With GT's triple triangle frames, the seat stays are welded to both the top tube and the seat tube. This obviously results in a more rigid frame, without needing to result in extra tubing stiffness. The end result in a punishingly stiff frame which also happens to be very light. my XL frame (23" seat tube), fully equipped, weighs in at just 21LB. That is very impressive for any aluminum mountain bike in my opinion. By allowing the top tube and seat stays to intersect further up in the frame, this results in a long top tube and chain stays without making the entire wheelbase of the bike ridiculously long. You are able to easily shift your weight forwards or backwards depending on what sort of riding you are doing. The end result, is a very well balanced geometry that strikes a great balance. The riders weight is set further back without necessarily impacting front end stability.
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Old 10-03-20, 02:15 PM
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TLDR; this guy really likes his ancient hardtail, despite having a rudimentary understanding of frame geometry and how it affects the way a bike performs.
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Old 10-03-20, 03:24 PM
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Thread complete in a single post. Impressive.
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Old 10-03-20, 05:15 PM
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Originally Posted by HD3andMe View Post
TLDR; this guy really likes his ancient hardtail, despite having a rudimentary understanding of frame geometry and how it affects the way a bike performs.
You must not be very familiar with the way a forum is supposed to work. I even titled this thread beginning with, "lets discuss." It would have been better to avoid commenting all together. Nobody asked you to summarize.
Originally Posted by Fahrenheit531 View Post
Thread complete in a single post. Impressive.
I think i'd have to agree. Hopefully you two don't summarize what the rest of the users on here behave like. I'm still hoping someone can input something more useful.
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Old 10-03-20, 05:25 PM
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Originally Posted by Moisture View Post
You must not be very familiar with the way a forum is supposed to work. I even titled this thread beginning with, "lets discuss." It would have been better to avoid commenting all together. Nobody asked you to summarize.
You must not be very familiar with how forums work.

My reply is part of the discussion. You don't get to decide whether people summarize your vapid wall-of-words, or not.
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Old 10-03-20, 07:08 PM
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Welcome to the forum. This is a good place to discuss frame geometry, but I'm not sure your OP leaves much room for discussion. I think the assertion that a triple triangle is significantly stiffer that a regular diamond frame is probably false. It's mostly a cosmetic choice.
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Old 10-03-20, 07:29 PM
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Originally Posted by Moisture View Post
You must not be very familiar with the way a forum is supposed to work. I even titled this thread beginning with, "lets discuss." It would have been better to avoid commenting all together. Nobody asked you to summarize.
You could learn a thing or two as well
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Old 10-03-20, 07:51 PM
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Originally Posted by Fahrenheit531 View Post
Thread complete in a single post. Impressive.
But where's all the argument & bickering?

*edit* Never mind, it came afterwards.
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Old 10-03-20, 08:10 PM
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Originally Posted by Moisture View Post
You must not be very familiar with the way a forum is supposed to work. I even titled this thread beginning with, "lets discuss." It would have been better to avoid commenting all together. Nobody asked you to summarize.


I think i'd have to agree. Hopefully you two don't summarize what the rest of the users on here behave like. I'm still hoping someone can input something more useful.
Um, ok. What about the choice of frame material? If you use more compliant material for the tubing, can you or do you need to adjust the HT or ST angles? And what about the tube thickness, which is another variable. Another thing I've found is that wider tires at lower pressure can make up for overly stiff geometry and frame material.

There you go.
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Old 10-03-20, 08:13 PM
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Alooooooominum

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Old 10-03-20, 08:34 PM
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Frames and Framebuilding (1981) What to Tell Your Framebuilder
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Old 10-03-20, 09:58 PM
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Originally Posted by Darth Lefty View Post
Alooooooominum

(runs away)
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Old 10-03-20, 11:09 PM
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My tour frame maker was famous for that style. I said NO thanks. It's ugly as sin and makes the rear rack attachment lousy too.
I drew a full size plan of a harp shape frame and that's what he made. I actually wrapped the seat tube and made a BB/ frame pouch with CF.
It has proven to be indestructible in 5 crashes. LOL.
The fork had the wrong trail, so it was a fail. It didn't want to turn. Later I was glad it broke 2 times in Vietnam. LOL. It now has a tandem one to hold up my drum brake.
It has 70/ 70 angles, 46,5" wheelbase and huge chain stays of 19 3/4".
The tubes are solid as a rock. But it still shimmies bad when loaded, only if not holding the bars.

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Old 10-03-20, 11:44 PM
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Originally Posted by Moisture View Post
1998 GT Zaskar
TLDR: NORBA geometry makes an effective 26” XC bike. Who knew?
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Old 10-04-20, 08:30 AM
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OP- a lot of current mountain bikes tend to have a more vertical seat tube angle. 74 and 75 degree seat tubes are common.
Others do have slacker seat tubes. Its a varied range since mountain bikes are so segmented in design.
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Old 10-04-20, 10:21 AM
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There are several drivers of mountain bike design that have changed them a lot in the last ten years. I think the most popular events right now are enduro racing, where the riders must pedal the course for the entire event, but the uphill parts are liaison stages. They are only timed going downhill. So they require bikes that are fast downhill and comfortable uphill. The final event of this year's Enduro World Series was 1500 m vertical in 58km... but only 8.5km of dowhill in four stages. They resemble downhill bikes but are not quite as extreme. This style of bike has proven pretty popular with regular schmucks who get to ride on Saturday or after work, and shorter travel bikes for bouncing around in the woods are now less-extreme versions of the same approach. Even trail hard tails now have slacker head tubes, more reach, and a little more fork travel.

XC racing bikes, which were once the big-tent category, have meanwhile become really dedicated racing machines with no compromises for versatility. Some of them are still hard tails and some have a clever bendy frame, some are full suspension, not everyone is on droppers. But I think nearly no one is buying this style of bike any more unless they want to go racing with it.

A second trend is bikepacking. The bikes for this are laid out like really conventional flat bar bikes for seated pedaling. What's changed from road touring bikes is the way the bike is loaded. Some of the camping gear is taken from through-hiking (a minimalist form of backpacking) and some of it, especially the triangle and seat bags, is now carrying back to road touring bikes.

And of course the another big fad is e-bikes, which has taken the heavy-duty enduro hardware and used it to absorb more speed on the level and climbing.
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Old 10-04-20, 10:47 AM
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Too many variables to summarize with HA & SA. Stem length and bar width impact how responsive, or non-responsive, any bike feels. And that is just one more piece of the pie. Slant top tubes, BB to axle, etc. all figure in the equation.

John
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Old 10-04-20, 01:27 PM
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Originally Posted by Moisture View Post
It should be apparent to most of us, that frame builders often go through great lengths to put together a frame which can seemingly do it all. Things such as head tube, seat tube angle, chainstay. seatstay. and top tube lengths all play a dramatic role in terms of how comfortable the bike feels for the rider, how well he is able to balance his weight between the rear/front axle, as well as the overall balance achieved between handling and stability. For example, the head tube angle tends to be on the slacker side (68-71 degrees) for mountain bikes, which results in slightly less precise steering, but improves stability. Road bikes tend to have a more aggressive head tube angle (about 74 degrees) which sets the fork further in front of the frame, increasing steering precision, albeit at the expense of overall stability. Seat tube angles on mountain bikes tends to be angled fairly aggressively backwards to increase the amount of weight towards to rear axle, setting the rider backwards for a more stable angle of attack during climbing scenarios. However, angle the seat tube too far backwards, and this will compromise the amount of weight the rider places over the handlebars, causing a light front end which may result in the rider flipping over. chain stay lengths tend to be increase due to the introduction of larger 27.5/29" rims. This helps to increase stability at speed due to a longer wheel base, and helps balance the weight better to prevent the handlebars from lifting up during climbs. A shorter chainstay can help to improve liveliness on technical trails. Furthermore, you may have heard of the terminology trible, or double butted frames. This essentially means that the diameter and shape of the tubing differs according to the amount of load being placed onto each respective tube. For example, the down tube and chain stays tend to take the most stress from riding, especially at the welding points. Consequently, this area of the frame tends to have thicker, stiffer tubing in place. The top tube and seat stays are usually a different shape. to allow for a little bit of forgiveness, so that vibrations and small imperfections in the terrain can be more or less absorbed before they reach the contact points of the rider.

After studying the frame geometry of my bike, a 1998 GT Zaskar LE, it is apparent to me that the engineers strived to achieve a good overall balance between polar opposites, light weight versus overall strength, stability versus handling precision, climbing versus descending, etc. The bike clearly excels on long, tough climbs, where i feel like the overall geometry and stiffness of the frame results in a high percentage of my efforts being converted directly to forward momentum. I never have to get out of my saddle no matter how steep the grade is. I just snatch and lower gear, keep up a good cadence at the cranks, and power my way up. However, with all this in mind, stability on steep hills do not seem to be affected.

With GT's triple triangle frames, the seat stays are welded to both the top tube and the seat tube. This obviously results in a more rigid frame, without needing to result in extra tubing stiffness. The end result in a punishingly stiff frame which also happens to be very light. my XL frame (23" seat tube), fully equipped, weighs in at just 21LB. That is very impressive for any aluminum mountain bike in my opinion. By allowing the top tube and seat stays to intersect further up in the frame, this results in a long top tube and chain stays without making the entire wheelbase of the bike ridiculously long. You are able to easily shift your weight forwards or backwards depending on what sort of riding you are doing. The end result, is a very well balanced geometry that strikes a great balance. The riders weight is set further back without necessarily impacting front end stability.
Can you use smaller words or maybe some pictures?
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Old 10-04-20, 02:27 PM
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This is arguing over old fashion DF frame geometry. A degree here, a degree there. Big deal!!!!!!!!! How about crank forward bike frames, now there is something to actually talk about that is different.
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Old 10-04-20, 02:41 PM
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True story- my wife just walked by as I was beginning to read the OP. She pointed and said "That's a lot of words, are you really going to read that?" I didn't, but I gather it's about vintage 26" XC MTBs. May appeal to a limited audience, not much new to be said at this point.
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Old 10-04-20, 04:13 PM
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Originally Posted by DeadGrandpa View Post
Um, ok. What about the choice of frame material? If you use more compliant material for the tubing, can you or do you need to adjust the HT or ST angles? And what about the tube thickness, which is another variable. Another thing I've found is that wider tires at lower pressure can make up for overly stiff geometry and frame material.

There you go.
Would you mind elaborating how a more compliant frame material would necessitate a change in angles? My wild guess would be, that more compliant frame designs may allow for slightly improved stability over imperfections, and therefore, respond well to more aggressive angles?

I know that materials such as aluminum and carbon often call for larger diameter tubing versus something like steel or titanium. Im assuming that, with modern technology, frame builders look for ways to make the aluminum frames very stiff and rigid, whereas with a steel frame, they can focus on offering some compliance because its already such a strong material.

Originally Posted by mstateglfr View Post
OP- a lot of current mountain bikes tend to have a more vertical seat tube angle. 74 and 75 degree seat tubes are common.
Others do have slacker seat tubes. Its a varied range since mountain bikes are so segmented in design.
I guess its about offering the right balance between a planted rear end without causing the front end to become too light. But with everyone being dofferent in size and stature, a standard size does not fit all...

Originally Posted by 70sSanO View Post
Too many variables to summarize with HA & SA. Stem length and bar width impact how responsive, or non-responsive, any bike feels. And that is just one more piece of the pie. Slant top tubes, BB to axle, etc. all figure in the equation.

John
What would be the benefits of a slanted top tube? A taller stack versus a relatively lower seat tube? What about some of those road bikes which have an arched top tube? What are the benefits in that?
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Old 10-04-20, 05:11 PM
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Slanted top tubes were first commonplace on mountain bikes. They'd already dropped the top tube for more stand-over and raised the bottom bracket for more ground clearance. That made the head tubes very short, but when the suspension fork was added they needed more room underneath, so the whole head tube moved up. Several things were tried, like Cannondale's Killer V. The slant was just the simplest. The road bikers adopted it because it gave them a stiffer triangle for pedaling at the same time as a long seat post for more comfort. It also goes along with threadless stems, which also came from MTB because quill stems would slam down into the steerer, but don't provide much added stack.

The steeper seat tubes are a new development and they are a compromise of designing the bike with a long front center and long reach and slack steering, so a standing rider is in the middle of the bike and not OTB. This blog post by Peter Verdone describes the basic idea. This is pretty much a done deal on mountain bikes. Some people have tried it on gravel bikes (Evil Chamois Hagar is an example) but it seems for now that gravel bikes will remain pretty much just armored-up endurance road bikes.
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Old 10-04-20, 05:34 PM
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Originally Posted by Darth Lefty View Post
Slanted top tubes were first commonplace on mountain bikes. They'd already dropped the top tube for more stand-over and raised the bottom bracket for more ground clearance. That made the head tubes very short, but when the suspension fork was added they needed more room underneath, so the whole head tube moved up. Several things were tried, like Cannondale's Killer V. The slant was just the simplest. The road bikers adopted it because it gave them a stiffer triangle for pedaling at the same time as a long seat post for more comfort. It also goes along with threadless stems, which also came from MTB because quill stems would slam down into the steerer, but don't provide much added stack.

The steeper seat tubes are a new development and they are a compromise of designing the bike with a long front center and long reach and slack steering, so a standing rider is in the middle of the bike and not OTB. This blog post by Peter Verdone describes the basic idea. This is pretty much a done deal on mountain bikes. Some people have tried it on gravel bikes (Evil Chamois Hagar is an example) but it seems for now that gravel bikes will remain pretty much just armored-up endurance road bikes.
how does the Chamois Hagar manage to get away with such an aggressive drop in chainstays without the pedals hitting the ground? Clearly, getting the geometry right has more to do with what is best suited to your personal needs. For me personally, I find that I lean forward slightly which puts enough weight over the front axle to cause a fair bit of understeer. I find that I have to be extremely gently during turn in, especially on gravel. My front wheel just plows. I'm planning to try using a wider and more aggressive front tire in the future.

I'm a taller and bigger rider, which may have something to do with the fact that I prefer a very upright riding position. But if I did manage to use some sort of swept back handlebars to achieve that stable rear biased handling im looking for, there just won't be enough weight over the front axle during steeper climbing. I think some sort of stem which you can adjust on the fly would be a nifty idea.

Would you mind explaining the difference in stability/ versus precision trade off when comparing the head tube angle to fork rake? Can you achieve handling which is both stable and precise, especially at speed by contrasting these two measurements?
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Old 10-04-20, 05:54 PM
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I had a 98 Zaskar LE. Full XTR, White or Nukeproof hubs (I had a lot of mountain bikes then, can't remember which had which wheelset). Sweet ride.

Compared to my middle of the road 2016 Trek 29er though, it's a piece of trash.

​​​​​​
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Old 10-04-20, 08:47 PM
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Originally Posted by rosefarts View Post
I had a 98 Zaskar LE. Full XTR, White or Nukeproof hubs (I had a lot of mountain bikes then, can't remember which had which wheelset). Sweet ride.

Compared to my middle of the road 2016 Trek 29er though, it's a piece of trash.

​​​​​​
Really? What don't you like about it?
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