# 29er Geo. Check



## jgerhardt (Aug 31, 2009)

So, finally I am getting back to building a mtb and I have a few questions about geometry.
With Road bikes the geometry can be pretty straightforward, especially since you don't have to worry about sagged vs. unsagged. Since the last 6 bikes I have built have all been some version of your typical roadie I haven't had any real issues. Now that I am building my first MTB in almost a year I think I want to run some of my numbers buy all of you and make sure I am on the right track.








Using a 100mm fox f29 with an axle to crown measurement of 500mm I am using a sagged dim of 475mm with a rake of 44mm. With a 71 degree HT this gives me a 80mm trail.

The biggest difference with what I did with my first MTB was that it was designed using a non-sagged axle to crown measurement and a HT angle of 70.5 degrees. While this did end up with a decent sagged HT angle (almsot 72 degrees) the ST angle was pretty darn steep (74 degrees) and it dropped an already low BB even lower.


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## ade ward (Jun 23, 2009)

I can't say if you are right or wrong ,
My curent 29er which I think rides very well is 
HA 71 ( fork 480/45) SA 71.6 BB height 300 CS 417-427 effective TT 609

I have been told the bb is too low , but it feels great a real sit in bike i have had a couple of pedal
Strikes but not a regular thing 

I am just having another made with a few updates but keeping most of the geometry the same but shorter chain stays 407-427 not to actualy run any shorter but to try a belt drive at 417 mm


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## Campbell (Nov 13, 2011)

Congratulations on a build start. Working on a rebuild myself at the moment but I'm almost complete, just contemplating on whether or not I should add a direct mount derailleur option.

I'd slacken the HA out along with everyone else. Takes the edge off if you live anywhere it can get steeper. It will push your L out so shortening CS helps liven things up.


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## Walt (Jan 23, 2004)

*Yes*

Always design the bike to handle the way you want when there is a rider onboard - which means *with sag* if using suspension. Nobody cares what any of the angles are when the bike is just sitting on your roof rack!

-Walt


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## chequamagon (Oct 4, 2006)

Walt said:


> Nobody cares what any of the angles are when the bike is just sitting on your roof rack!


Have you been to the 29er forum lately?


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## customfab (Jun 8, 2008)

Walt said:


> Always design the bike to handle the way you want when there is a rider onboard - which means *with sag* if using suspension. Nobody cares what any of the angles are when the bike is just sitting on your roof rack!
> 
> -Walt


While that is true, static measurements are the base line for comparison.


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## Walt (Jan 23, 2004)

*What?*

I don't understand what you're trying to say - the trail number, or bb height, or whatever is not useful for any kind of suspension bike (and I include hardtails with suspension forks) unless you take sag into account. Period.

Maybe we are not talking about the same thing here. Most people have an amount of sag that they prefer, which of course can depend on lots of things (travel, type of suspension system, terrain, etc). So given that information, it's easiest to design the bike to handle how you want it to under those conditions to begin with, rather than working backward from an unsprung state and having to calculate out where the bike will end up when there's a rider onboard.

Can you do an unsprung design? Sure. But why? It's extra work, it's harder to tell how the bike will ride without doing lots of extra math, and it makes the entire design process much less transparent and intuitive for everyone involved.

-Walt



customfab said:


> While that is true, static measurements are the base line for comparison.


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## jgerhardt (Aug 31, 2009)

Campbell said:


> I'd slacken the HA out along with everyone else. Takes the edge off if you live anywhere it can get steeper. It will push your L out so shortening CS helps liven things up.


This frame is actually for my dad. When he last rode my 29er he loved the way it handled and that frame has a sagged HT angle of 72 Degrees. I would hate to give him anything slacker as he is not a very fast downhill rider and thrives on the uphills and tight singletrack. After this build I am going to build myself another frame and that one will probably have a slacker HT in the 69 - 70 degree range.



Walt said:


> Always design the bike to handle the way you want when there is a rider onboard - which means *with sag* if using suspension. Nobody cares what any of the angles are when the bike is just sitting on your roof rack!
> 
> -Walt


Walt, I agree completely. The only sticking point I am having is more with myself getting over having to design around something that changes and is not static.
To be honest the only place I ever really hear about sagged geometry is on these forums. 
Don't get me wrong I understand how it affects the geometry it is just different to how I was originally taught.
When I look at different bike manufacturers and they show their bike geometry on their sites are the angles they show based on sagged or unsagged measurements?


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## Walt (Jan 23, 2004)

*A pet peeve...*

Many geometry figures you see online from manufacturers aren't standardized in any way. Seat tubes are reported to TT/ST center, or top of TT, or top of ST... randomly. Standover can be at the middle of the toptube, or under the nose of the saddle, or anywhere in between. And of course some report unsagged numbers and others report sagged ones. Generally the better companies use sagged numbers (at least for hardtails) - sometimes you can check their drawings to be sure, if they post them.

It sucks that there's no consistency, but c'est la vie. We're still mixing SAE and metric units all over when talking about bikes, and how many BB standards are there now? The bike industry is what it is.

You can of course design any way you want, but you admit yourself that one of the problems with your initial 29er was that you didn't anticipate some of the changes that sag would cause. Think of it this way: the unsprung condition might be static, but it's also NEVER the state the bike will be in when it's being ridden. Why design for that?

-Walt



jgerhardt said:


> Walt, I agree completely. The only sticking point I am having is more with myself getting over having to design around something that changes and is not static.
> To be honest the only place I ever really hear about sagged geometry is on these forums.
> Don't get me wrong I understand how it affects the geometry it is just different to how I was originally taught.
> When I look at different bike manufacturers and they show their bike geometry on their sites are the angles they show based on sagged or unsagged measurements?


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## customfab (Jun 8, 2008)

Walt said:


> I don't understand what you're trying to say - the trail number, or bb height, or whatever is not useful for any kind of suspension bike (and I include hardtails with suspension forks) unless you take sag into account. Period.
> 
> Maybe we are not talking about the same thing here. Most people have an amount of sag that they prefer, which of course can depend on lots of things (travel, type of suspension system, terrain, etc). So given that information, it's easiest to design the bike to handle how you want it to under those conditions to begin with, rather than working backward from an unsprung state and having to calculate out where the bike will end up when there's a rider onboard.
> 
> ...


What I said is pretty simple. A static measurement is the base line for comparing the geometry from one bike to the next. What you are talking about while valid is making things more complicated than they need to be. How many riders know what there head angle is at sag, how many care? My point is that if you know what static head angle you like and run a consistent amount of sag for a given fork travel it comes out in the wash. If you are going to complicate things by trying to account for a sagged head angle are you going to complete the process and get tire size and the amount of deflection it has when loaded? On top of that what is your tolerance for HTA? Production bikes are often +/- 1/2 a degree, that's about the difference of an inch of sag. On top of all that we are talking about mountain bikes were everything is constantly in a state of flux.


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## Walt (Jan 23, 2004)

*We'll have to agree to disagree.*

In fact, yes, I do account for tire size when I design a frame (deflection is pretty similar at the PSI ranges that people actually ride, if we exclude fatbikes/road bikes/weird stuff and are just talking about XC). I think that's pretty crucial. A 2.1 vs a 2.3 (so, ~5mm of so of radius) can be the difference between a lot of annoying pedal strikes and very few, so if you're not using tire size as part of your design work, you're not doing your job.

Yes, everything is in a state of flux - but you can design around a default state where at least you've got a rider on the bike. It's more likely to be in that state than totally unsprung!

Think of it this way. You're cruising fast on a straight section of trail, you see a turn coming, and you get ready to lean/countersteer/whatever. The way the bike will react is going to be based on the geometry at that moment - rider onboard, wheels turning, suspension under load. Is that condition going to apply all the time? No, of course not. But it's actually pretty commonly where the bike will be. In any situation where the fork is at less than normal sag or topped out, the front wheel will either be off the ground or at the least significantly less loaded than normal. Good luck making any kind of course correction using the front tire there!

To me, deciding on a sag number, then doing the whole design is very simple. I see doing it the other way as much more complex (rotate the bike around the rear axle and re-calculate all the angles at any given fork length?), but to each their own.

Production frames are not +/- 1/2 degree, in my experience. Even if they are, that's not a standard I'd use to determine how I do a design! Most serious riders do indeed have a set amount of sag they prefer (and in fact folks who do suspension work keep careful track of PSI/spring rate, sag, and damping setting numbers) and designing around that is dead simple.

So the bottom line is this: I don't care much about how the bike handles when it's not loaded. So I don't design that way.

-Walt



customfab said:


> What I said is pretty simple. A static measurement is the base line for comparing the geometry from one bike to the next. What you are talking about while valid is making things more complicated than they need to be. How many riders know what there head angle is at sag, how many care? My point is that if you know what static head angle you like and run a consistent amount of sag for a given fork travel it comes out in the wash. If you are going to complicate things by trying to account for a sagged head angle are you going to complete the process and get tire size and the amount of deflection it has when loaded? On top of that what is your tolerance for HTA? Production bikes are often +/- 1/2 a degree, that's about the difference of an inch of sag. On top of all that we are talking about mountain bikes were everything is constantly in a state of flux.


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## customfab (Jun 8, 2008)

Walt said:


> In fact, yes, I do account for tire size when I design a frame (deflection is pretty similar at the PSI ranges that people actually ride, if we exclude fatbikes/road bikes/weird stuff and are just talking about XC). I think that's pretty crucial. A 2.1 vs a 2.3 (so, ~5mm of so of radius) can be the difference between a lot of annoying pedal strikes and very few, so if you're not using tire size as part of your design work, you're not doing your job.


I really think your splitting hairs that most certainly do not need to be split on this one.



Walt said:


> Think of it this way. You're cruising fast on a straight section of trail, you see a turn coming, and you get ready to lean/countersteer/whatever. The way the bike will react is going to be based on the geometry at that moment - rider onboard, wheels turning, suspension under load. Is that condition going to apply all the time? No, of course not. But it's actually pretty commonly where the bike will be. In any situation where the fork is at less than normal sag or topped out, the front wheel will either be off the ground or at the least significantly less loaded than normal. Good luck making any kind of course correction using the front tire there!
> 
> To me, deciding on a sag number, then doing the whole design is very simple. I see doing it the other way as much more complex (rotate the bike around the rear axle and re-calculate all the angles at any given fork length?), but to each their own.
> 
> ...


What this comes down to is your point of reference. If you have a history of HA with sag and you and your customers are comfortable designing that way then it's all good. If your like me and the rest of the industry and you compare numbers based on static measurements then that's fine as well. So long as you keep in mind what your point of reference is and make sure it's an apples to apples comparison. The only exception I would make to this and possibly divert to your method is when comparing bikes of different intended uses. IE a 5" travel FS bike to a 3.5" travel hardtail.


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## Walt (Jan 23, 2004)

*Yes, splitting hairs.*

As long as you consistently do things the same way, and you know what you/your customers like, I agree. It does not matter. But as the OP noted, he got himself into trouble with his first design because he didn't think about sag - so designing for sag in the first place might have been useful there.

There's not an industry standard, though. For example:
Karate Monkey | Bikes | Surly Bikes

Example: Surly's KM is well known to be "80mm suspension corrected", which for Surly means a 468mm fork length (listed on their geometry charts). Just one problem - an actual 80mm travel 29er fork is more like 480-485mm. So they design around sag, clearly (in fact, about 15-20mm worth).

Another example: Specialized lists their Hardrock 29er hardtails with 72 deg HTAs. With the stock 46mm rake fork (Rockshox Tora or Reba) and a normal 2.1" tire, that's going to be in the neighborhood of 72mm of trail - a bit on the low side for a mountain bike, but nothing totally crazy.

Remember that trail for a road bike tends to hover around 60mm, CX bikes tend to be in the high 60s/low 70s, and, say, a DH bike could be over 100mm. So our Specialized is already in the low range for mountain bikes, assuming they are incorporating sag into their published numbers.

If, on the other hand, we assume that Specialized is listing the *unsagged* head tube angle, we start getting a bit wacky. Assume a normal rider will run 15-20mm of sag on an 80mm fork. That's essentially a degree here. So with sag, we're at 73 degrees now - up in road bike territory. Trail drops to 67mm - probably nigh-unrideable on rough terrain for a lot of folks, and in fact with 23c tires on those 700c rims, we're right at about the same trail number as a road bike (a bit twitchier, actually, since road forks tend to use a bit less rake)!

29ers aren't road bikes with fat tires, and no big company is going to sell an entry-level mountain bike with a <70mm trail number. So Specialized and Surly, at least, use sag to do their design work and in their published geometry.

You can find counterexamples, of course - Canfield I know uses unsagged numbers, for example. I don't have the time to catalog all the big brands, and I'm sure there are plenty that do it without sag, but it's certainly not the standard.

In conclusion, I have no life and deserve your pity/contempt for wasting half an hour on this.

-Walt



customfab said:


> What this comes down to is your point of reference. If you have a history of HA with sag and you and your customers are comfortable designing that way then it's all good. If your like me and the rest of the industry and you compare numbers based on static measurements then that's fine as well. So long as you keep in mind what your point of reference is and make sure it's an apples to apples comparison. The only exception I would make to this and possibly divert to your method is when comparing bikes of different intended uses. IE a 5" travel FS bike to a 3.5" travel hardtail.


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## Mojo Man (Sep 1, 2007)

Hi Guys,
I'm having a custom bike built too.

May question is what length stem does everyone tryically use for a 29 XC hard tail race bike?
I've always felt good using a 80-90mm, but my builder says they desing their frames around a 100mm, seems so long to me.

what's everyone opinion?

Mojo


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## pvd (Jan 4, 2006)

That design looks like a way too high bb for that fork plus a super steep head angle. I'd put some more work into a detailed design to ensure good fit too. Take a look of a current print for a new bike of mine as reference.

A current print for an upcoming bike. | Peter Verdone Designs


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## Francis Buxton (Apr 2, 2004)

I've always followed the same methodology as Walt. The other method is really only good for comparing your bike to major manufacturers, and then you don't know exactly what they're giving you, b/c they don't disclose all of the gory details.

I have a 29er with a first-gen Reba at 100mm with a 72 HTA. 12" BB. It handles great in the twisties, and it's a ripper. My only complaint with it is that I don't like to do steep roll-offs that are taller than 12" (we have a number of spots on my local trails where we have 18-30" rollers). With 100mm travel, when I go to roll down these (in pretty tight areas), the suspension compresses too much, making the HTA get a little steep, and I get fearful of going over the bars.

I recently bought a Stumpjumper FSR 29 with 130mm travel and a 69.5 HTA, and it handles those sections like a rock star. It has a little bit higher BB, and about the same CS (17.6").

If your dad's not planning to do any steep technical rollers like mentioned above, and he's not riding California "singletrack", I'd say your design is pretty good. You might look into lowering the BB a bit, but BB height is pretty personal. 11.5", and you will likely get a lot of pedal strikes if you are a constant spinner. 12" is a pretty balanced BB height for me. I don't get many pedal strikes (especially when I ride fixed), but I'm not "up in the air". Again, personal preference. 

I can rip downhill on Arkansas singletrack like Syllamo and the Womble trail as fast as a lot of guys with 150mm suspension on that bike, so the HTA should be fine. For my next bike, I'll probably go a little slacker to see where my personal balance point is for being able to comfortably do those tech rollers while still being able to handle some tighter turns in midwest singletrack.


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## jgerhardt (Aug 31, 2009)

Walt said:


> In conclusion, I have no life and deserve your pity/contempt for wasting half an hour on this.
> 
> -Walt


Walt, I greatly appreciate the amount of time you have spent on this. I have been running in circles and finally just threw out the question so I can hopefully get a better understanding of all of this.



pvd said:


> That design looks like a way too high bb for that fork plus a super steep head angle. I'd put some more work into a detailed design to ensure good fit too. Take a look of a current print for a new bike of mine as reference.


That drawing I posted is definitely NOT my completed drawing. I just wanted to post what was pertinent to my question however in hindsight I probably should have posted the one with all the dims. As for the BB height the last MTB I built I placed the BB at 11.14" and with 175mm cranks I am getting way more pedal strikes than I should (that is measured with 25% sag). I think with this one I might have gotten a little gun-shy and went a little to far in the other direction.

As for stem length and saddle position, is there a good resource I can check out to help me figure that stuff out? I have always been of the mind that was all figured out once the bike was finished.


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## jgerhardt (Aug 31, 2009)

Updated drawing with a little lower BB height slacker seat tube angle. 








This is with 25mm of sag.


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## ade ward (Jun 23, 2009)

Not sure but to me 633 front centre looks a bit short there may be toe overlap


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## Mojo Man (Sep 1, 2007)

What is your body size for that TTL you are using? I'm doing a similar custom geo and was woundering what the perfect length would be.

Also, are you using the standard english threaded BB of the new PF30?

Mojo


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## jgerhardt (Aug 31, 2009)

Mojo Man said:


> What is your body size for that TTL you are using? I'm doing a similar custom geo and was woundering what the perfect length would be.
> 
> Also, are you using the standard english threaded BB of the new PF30?
> 
> Mojo


The frame above is for my dad. He is 6'-0" tall with an inseam of 33.5", a torso length of 25" and an arm length of 21.75".
Using the charts I got at UBI, I came up with the ETT above.

edit: oh and I did fiddle with the seat tube angle to get a little bit longer front/center than what is shown above.


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## Mojo Man (Sep 1, 2007)

jgerhardt said:


> The frame above is for my dad. He is 6'-0" tall with an inseam of 33.5", a torso length of 25" and an arm length of 21.75".
> Using the charts I got at UBI, I came up with the ETT above.
> 
> edit: oh and I did fiddle with the seat tube angle to get a little bit longer front/center than what is shown above.


I'm not a bike geo expert, but isn't 23.15" (588mm) ETTL too short for your fathers size? Typically that is more of a small medium, or 16.5" frame, no?

I use a 23-23.25" ETTL and I'm 5'8" with an inseam of 31.7"

Just woundering,

Mojo


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## jgerhardt (Aug 31, 2009)

For the ETTL, I combined the Torso length and Arm length:
25"(635mm) + 21.75"(552.45mm) = 1187.45mm
The chart then tells me to use a 588mm ETT.

edit: From the inseam I got a 553mm EST length.


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## pvd (Jan 4, 2006)

Folks,

A frames size is adjusted via Front Center not ETTL. TTL is a very very poor way of designing a frame. You can end up with a serious peice of garbage by going down the TTL route once you get outside of a very narrow window of design. The combination of FC, saddle (x,y) and handlebar (x,y) will very generally describe the scale of the frame and fit.


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## Blaster1200 (Feb 20, 2004)

pvd said:


> Folks,
> 
> A frames size is adjusted via Front Center not ETTL. TTL is a very very poor way of designing a frame. You can end up with a serious peice of garbage by going down the TTL route once you get outside of a very narrow window of design. The combination of FC, saddle (x,y) and handlebar (x,y) will very generally describe the scale of the frame and fit.


Additionally, TTL or ETTL doesn't take into consideration of seattube angle. A few degrees of seattube angle can be a half inch or so of ETTL. For example, some design frames to use s straight seat post so they use a slacker seattube angle, whereas those who design the frame for a setback seat post may use a steeper angle. The goal is to get the seat in a given position, but when you're measuring the TTL/ETTL, you're not actually taking the actual seat position into consideration.


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## jgerhardt (Aug 31, 2009)

pvd said:


> Folks,
> 
> A frames size is adjusted via Front Center not ETTL. TTL is a very very poor way of designing a frame. You can end up with a serious peice of garbage by going down the TTL route once you get outside of a very narrow window of design. The combination of FC, saddle (x,y) and handlebar (x,y) will very generally describe the scale of the frame and fit.


I have heard you mention this before, is there a resource somewhere that you would recommend to help me figure this out? To be honest the more I dive into this, the more conflicting info I get.


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## pvd (Jan 4, 2006)

Take a long look at the print that I posted. Note that the front is connected to the rear via FC. Look at the detail I put into the seat and handlebar position. You need to study op a little on fitting but you should do that anyway.


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## mhickey79 (Feb 22, 2007)

jgerhardt said:


> The frame above is for my dad. He is 6'-0" tall with an inseam of 33.5", a torso length of 25" and an arm length of 21.75".
> Using the charts I got at UBI, I came up with the ETT above.
> 
> edit: oh and I did fiddle with the seat tube angle to get a little bit longer front/center than what is shown above.


I used UBI's measurements when I was there a few years ago and ended up with a frame that was too small for me, unless I used a 110 stem, instead of the 90 that I wanted to use.
Their method suggested an ETT of 580mm for me (I'm 5'9"). Since then, I've found that I fit pretty well on bikes that have an ETT of 590-600. I'm now riding an 18" Surly Karate Monkey with an ETT of 600mm (90mm stem) and loving the bike and the fit. Yeah, I know that the story is anecdotal, but I agree with those who said your dad might want to stretch out a bit more.


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## jgerhardt (Aug 31, 2009)

I finished these two frames a little while ago and after some time in the saddle I decided to give a little update:









After some consideration I went ahead and got a copy of BikeCad and using their system, along with some of the advice from Walt and others completely redesigned the geometry of both frames.









I did my dad's frame first and stuck with a pretty standard frame design and geometry with a 71 degree HT and a 73 degree ST









With my frame I wanted to do something a little more fun with the frame design and also used a little slacker HT angle of 69.5 degrees and ST angle of 72.

The slacker HT angle is really noticeable over what I was used to and the new frames are a blast to ride!


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## C Dunlop (Sep 26, 2008)

> We're still mixing SAE and metric units all over when talking about bikes,


this.



> and how many BB standards are there now? The bike industry is what it is.
> -Walt


Not this. Completely off topic but this is one of my pet peeves. There are far less BB standards than there used to be.

now: BSA, PF30, BB30, BB90 and ITA (which is on the way out) and a few proprietary ones like BBright and BB386.

then: BSA, ITA, French, Swiss, Chater Lea, and Raleigh Super Corsa. Later joined by Pinarello's press fit system. All of those were available in JIS (regular and low profile) and Campag splines for square taper cranks. That was followed by the ISIS/powerdrive period and mega ISIS. You also needed to stock BBs in lengths from about 111m to around about 121mm. :madman:

BTW Walt, thanks for fixing my frame in Boulder last year. I made it another 3 months and 6,000 miles to San Jose del Cabo on that piece of plate chromo welded between my stays.


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