# Frame design: sagged vs unsagged



## golden boy (Oct 29, 2008)

*Frame #4 is done and in the wild*

I'm back on this forum after a bit of a hiatus, preparing to build frame #4. I have a simple question: when you are designing a frame for front suspension, do you lay it out with all dimensions corresponding to the sagged or unsagged suspension state?

I have been drawing mine in the sagged state, since this reflects the actual ridden condition.


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## One Pivot (Nov 20, 2009)

There's no reliable (or even semi reliable!) way to measure or ride in a specific front sag percentage. It also makes comparisons between frames much less accurate.

Geo really should be based on AC height at full extension.


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## golden boy (Oct 29, 2008)

I get where you're coming from, but we also know that when riding, the suspension will rarely be fully extended. It just seems like the recommended sag percentage serves as a better baseline for frame geometry, IMO.


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## golden boy (Oct 29, 2008)

By the way, here's the driving dimensions for frame #4. Disregard the seat tube as modeled. And the untrimmed tubes. It is largely based on a Salsa Woodsmoke I demo'ed last summer that blew me away. Interestingly, most non-media reviews of this bike that I have read were not complimentary.


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## Walt (Jan 23, 2004)

I use sagged. There is no real consistency in the industry on this, though, so when you're looking at geometry charts online, you have to be careful how you interpret them.

-Walt


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## golden boy (Oct 29, 2008)

Walt said:


> I use sagged. There is no real consistency in the industry on this, though, so when you're looking at geometry charts online, you have to be careful how you interpret them.
> 
> -Walt


Exactly. Impossible to compare manufacturers' geo charts without that consistency. Drives me a little nuts.


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## scottzg (Sep 27, 2006)

I do unsagged because i know exactly how any non-**** hardtail with a 130mm fork behaves unsagged. When i design around a different fork i just take a glance at how it looks sagged and keep that stuff in mind. Also because climbing behavior is the most fussy to me, and then the fork is basically unsagged.

I don't think it's that important so long as you know what you're changing compared to the frames you know. No comments on FS; the rear suspension interacts with the geo and makes my head spin.


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## Eric Malcolm (Dec 18, 2011)

scottzg said:


> I do unsagged because i know exactly how any non-**** hardtail with a 130mm fork behaves unsagged. When i design around a different fork i just take a glance at how it looks sagged and keep that stuff in mind. Also because climbing behavior is the most fussy to me, and then the fork is basically unsagged.
> 
> I don't think it's that important so long as you know what you're changing compared to the frames you know. No comments on FS; the rear suspension interacts with the geo and makes my head spin.


My sentiments exactly. A baffling subject.

Eric


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## pvd (Jan 4, 2006)

Always sagged.


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## jsudar (Dec 9, 2011)

I drew mine sagged. Made more sense to me.

I also have a fair number of bike geometries in a spreadsheet. I have geo as the the manufacturer stated it, but then I spent some time doing math or drawings or both so I could compare a bunch of different hardtails all with the same fork. This gave me a better apples to apples comparison between bikes. 
This was a number of years ago, so its all old 26er bikes and no so relevant now. It was difficult, as some manufacturers are very vague about how they measure their geo.


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## pvd (Jan 4, 2006)

BTW, the design of that geometry is very steep and very short. You may want to look at more progressive designs to copy.


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## ru-tang (May 20, 2009)

This is a timely thread, I'm drawing up a new frame and thinking about the same things. After the frame is built, I was going to get a fork built for it. 

For you fork builders out there, I'm drawing up a 29er frame around a 130mm fork. I'm guessing that the fork would be built to the sag height, maybe subtract a bit to run 29+ tires?

Would I better off designing around 120 fork if I want to swap out for a rigid, or is 130 not going to be a problem down the road?


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## briderdt (Dec 14, 2012)

I design around a sagged fork, and when spec'ing a rigid fork as a replacement (or a swap for when some one wants to ride extensive pavement), I make it to the sagged A-C.


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## Erichimedes (Jul 30, 2010)

I design around sagged suspension. The only time a bike is in an unsagged state is when it's sitting on the ground not being ridden. I don't know why you'd design around that. Almost every number on the bike changes the second you get on it.


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## Walt (Jan 23, 2004)

ru-tang said:


> Would I better off designing around 120 fork if I want to swap out for a rigid, or is 130 not going to be a problem down the road?


That depends on the total axle-crown length minus expected sag. The longest commonly available rigid forks are in the 490mm axle-crown range. I occasionally build 500 or even 510mm ones for people but that's about the upper limit. So if you want to run a 180mm Fox 40 and swap to rigid... ain't gonna happen (but someone tried to get me to make him one once).

-Walt


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## scottzg (Sep 27, 2006)

Erichimedes said:


> The only time a bike is in an unsagged state is when it's sitting on the ground not being ridden. I don't know why you'd design around that. Almost every number on the bike changes the second you get on it.


When i'm climbing steeps seated the bike is completely unsagged regardless of fork travel, and that's when 3mm of saddle setback or chainstay length (edit- or BB height) can most alter the bike's behavior, at least for me. The most fussy place in the bike's travel to design around is also where the fork doesn't matter. It seems obvious to start my design here, then double check geo at sag afterwards. IMO.

Once you're sagged the fork's behavior interacts with the frame geo and it's not nearly so precise.

...And that's why i design around unsagged. For hardtails i don't really think it matters so long as you have a method that gets you the results you want. :thumbsup:


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## Erichimedes (Jul 30, 2010)

scottzg said:


> ...And that's why i design around unsagged. For hardtails i don't really think it matters so long as you have a method that gets you the results you want. :thumbsup:


I can see your reasoning.

I guess my reasoning is that I'm taking an average of the bike throughout an entire ride. But it's true, both methods produce great products, so it's just a matter of which side you like your toast buttered on.


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## Drew Diller (Jan 4, 2010)

This thread has made me noodle quietly on my own time, when I'm sick of noodling about everything else. I don't *think* this is a thread hijack, but forgive me if it momentarily becomes that.

Is there any point to, or prior art for, a means to deliberately collapse and lock the front suspension for the purpose of lowering and forwarding one's center mass relative to the rear contact patch when climbing an extremely steep grade?


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## scottzg (Sep 27, 2006)

Drew Diller said:


> This thread has made me noodle quietly on my own time, when I'm sick of noodling about everything else. I don't *think* this is a thread hijack, but forgive me if it momentarily becomes that.
> 
> Is there any point to, or prior art for, a means to deliberately collapse and lock the front suspension for the purpose of lowering and forwarding one's center mass relative to the rear contact patch when climbing an extremely steep grade?


Marzocchi had a system that cranked up the rebound for that purpose, and almost everyone had a system that shortened the fork's spring on the fly 10 years ago. None of those systems worked super great (rockshox's coil system was the best, imo), and now they've all been rendered obsolete by no longer using frame geometry that was essentially 'road bike + suspension fork.'

Some of those old systems are still available, like talas, although now that geometry is more sensible you'll end up with your feet on the ground and an 80* seat angle if you use them.

The fact that we can design around sag is a testament to how much better geometry has gotten, imo. It was tough being tall and a long travel hardtail enthusiast a decade ago.


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## golden boy (Oct 29, 2008)

*Frame #4 is done and in the wild*

I finished this frame recently so thought I would share some photos. Here it is, all built up. I call it the Lava Bomb.







Lots of firsts in this frame for me. First use of curved main tubes, first manipulation of chainstays and seatstays (for more tire clearance), first integrated seat clamp (not likely to do again), and first use of Wright-style dropouts with separate brake mount (also not likely to do again).

And a shot in the wild:







I'm pleased with the way it rides and handles. As far as I'm concerned, if you're gonna ride a hardtail, 27.5+ is the way to go for sure.


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