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Discussion Starter · #1 ·
Designing a frame with super short chainstays but with room for wide tyres and it seems moving to an 83mm bb shell will open up a lot of room without lengthening the stays.

I was considering the implication of this wrt chainline on a regular boost rear end. From what I gather, the chainline will be pushed out by at least 5mm best case (using the raceface q-factor specs).

My question is, does moving to an 83mm bb necessitate the need to use a wider rear hub? should I be looking at superboost for the rear?

Cheers,

Alex
 

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There isn't enough information here to answer well.
Generally, you're making a bike much worse for pedaling by using wider stance setups. Also, what do you mean by 'short' or 'wide'? What purpose does this server?

FWIW, I can do 410mm stays with 622-64 tires and Q156/52 cranks.
 

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Discussion Starter · #3 ·
Hi Peter thanks for chiming in.

I'm looking to use a sliding dropout giving me adjustability between 410-430mm chainstays. I'd like to fit 27.5x2.8 minimum in the shortest setting. I plan on trying 29x2.4-2.6 in the front (yeah I'm short and trying to avoid butt buzz).

Playing around with tubes, cutouts and yokes in bikecad I can't quite seen to make it a comfortable fit. Yokes need to be incredibly short and tubes seem to have issue with chainring clearance if the bend is sharp enough to clear the tyre. I haven't tried modelling steel plates into the design as I'm not entirely sure how.

In all this in not sure how this will play along with stiffness. I'd like to achieve some vertical compliance but maintain a laterally stiff rear end.

I saw some of your work Peter, I wish I had the fabrication talent to pull that off.
 

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Stating that a narrower Q-factor is always a benefit is a bit too generalized assumption in my opinion. Individual anatomy, hip width for starters, plays a big role.

Especially on mountain bikes, I find having a bit wider Q-factor only beneficial by providing more stability without hindering any other metric. Interestingly, Chris Kovarik designed his signature Dagga flat pedals with the same goal in mind, besides having a metric ton of grip and robust build.

It is somewhat different on road bikes in which the majority of the time is spent on the saddle spinning the pedals. In those cases, it might be warranted to aim for a narrow Q-factor.

In short, I am fan of 83mm BB shells and Super Boost spacing if it's an option.
 

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This is a first pass at what I'm trying to do.
It's looking like a decent start. You have some odd dimensioning in there that isn't helping you any. We don't use "top tube length" when describing bikes and your don't have front center listed. You also have nothing for handlebar placement. You would do well to go through and refine your drawing dimensions to understand better. My site has lots of examples.

That 27.5x2.8 tire doesn't make any sense. It's going to be an issue with your leg clearance. It's also not a very high performance choice. Going with a 29x2.3-2.5 would be much wiser. You can just do a segmented seat tube to make the room.

REVISED: Yes, 680mm saddle height is short. A 27.5" wheel would make sense but over 2.5" wide would be silly. Take the extra space and get your rear center down to 390-410mm range. 410mm will be very long for your size. I'm 5'10" and have use 410mm rear centers.
 

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Discussion Starter · #8 ·
It's looking like a decent start. You have some odd dimensioning in there that isn't helping you any. We don't use "top tube length" when describing bikes and your don't have front center listed. You also have nothing for handlebar placement. You would do well to go through and refine your drawing dimensions to understand better. My site has lots of examples.

That 27.5x2.8 tire doesn't make any sense. It's going to be an issue with your leg clearance. It's also not a very high performance choice. Going with a 29x2.3-2.5 would be much wiser. You can just do a segmented seat tube to make the room.

REVISED: Yes, 680mm saddle height is short. A 27.5" wheel would make sense but over 2.5" wide would be silly. Take the extra space and get your rear center down to 390-410mm range. 410mm will be very long for your size. I'm 5'10" and have use 410mm rear centers.
Thanks Peter,

I've been tweaking this back and forth for a few days now. I added some dimensions ... Noted re: the ETT, I still use this to gauge my seated pedaling position on the bike, helps me understand how stretched I'll be.

I'm trying to maintain the flexibility to use different wheel sizes at different chainstay lengths. I was trying to get 29x2.6 into a 410mm RC but I'm hampered by a lack of fabrication skills and working within the limits of the builder I'm using.

I'm getting comfortable with something like this. 275x2.8s is something I'm interested to try, just to help smooth some of the harshness out of trails, not convinced I'll like them though. I've tried 2.6 but can't find many reasons to use them over a 2.35-2.4 to be honest, especially in the front.

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You're starting to get somewhere.

1. ETTL is utterly useless. If you want a number to understand how stretched you are, use handgrip to center of saddle. That's the actual number that tells you that. ETTL tells you nothing.

2. Have you done a PVD/RAD calculation? That may open up some room for a nicer setup: PVD/RAD Refined | Peter Verdone Designs

3. You need to get that seat tube bend much lower or eliminate it entirely. Just slam it into the down tube. You can toss a gusset in there if you like.

4. Front wheel diameter should be 750mm for a 29x2.5 wheel. 622 + 63.5 x 2 = 749

5. The bike is short but I question the need to bend the down tube with a 64.5 degree head tube. You should check that and see if you really have to.

6. I still think that you are in outer space with the tire size stuff. sacrificing geometry and fit for stupid tires makes no sense. Just use 2.5" tires and harden the f&&& up. You'll have a much better bike in the end that will be way more fun to ride. You just don't have the room especially if your fab is challenged. Stop reading the internet about dumb tires.

7. You should be looking for narrow crank width options. A Q156 at the least. You can do narrower if you know some ninja tricks. This is super important for smaller stature folk.

8. Chainstays are still way too long.
 

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Discussion Starter · #10 ·
You're starting to get somewhere.

1. ETTL is utterly useless. If you want a number to understand how stretched you are, use handgrip to center of saddle. That's the actual number that tells you that. ETTL tells you nothing.

2. Have you done a PVD/RAD calculation? That may open up some room for a nicer setup: PVD/RAD Refined | Peter Verdone Designs

3. You need to get that seat tube bend much lower or eliminate it entirely. Just slam it into the down tube. You can toss a gusset in there if you like.

4. Front wheel diameter should be 750mm for a 29x2.5 wheel. 622 + 63.5 x 2 = 749

5. The bike is short but I question the need to bend the down tube with a 64.5 degree head tube. You should check that and see if you really have to.

6. I still think that you are in outer space with the tire size stuff. sacrificing geometry and fit for stupid tires makes no sense. Just use 2.5" tires and harden the f&&& up. You'll have a much better bike in the end that will be way more fun to ride. You just don't have the room especially if your fab is challenged. Stop reading the internet about dumb tires.

7. You should be looking for narrow crank width options. A Q156 at the least. You can do narrower if you know some ninja tricks. This is super important for smaller stature folk.

8. Chainstays are still way too long.
Appreciate the insights Peter.

I went through the RAD calcs back before you had updated the measuring technique with the sticks and strings. Compared it to LLB RAD and the two were pretty close. From memory Lee's came out at 780 and yours closer to 800. I should do it again with the new measuring technique you wrote up.

Yeah I've gone back and forth with the seat tube, prefer straight but don't want a stupid steep seat angle to clear the bigger tyres (point taken on the stupid tyres lol). I'll have a bit more of a play.

I was using 29x2.6 as a size for the front. I'll probably never run anything wider than 2.4 though.

I'd prefer a straight down tube ... I used a 'fox like' fork in bikecad for the model thus the bend to clear. I should double check the actual fork dimensions and maybe revise.

I have an abnormally wide stance, in fact I have to use spindle spacers if clipped in.

Tyre size is more fomo than anything. 2.8s seem like a good way to get slightly better rollover without going 29er. I'd rather have the clearance and not need it than the other way around. I run 275x2.35 on my full sus and would never go wider on that.
 

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1. You can totally ignore the LLB numbers. That system doesn't work and produces bad numbers. My revised system is a bit easier to measure.

2. The point of the offset seat tube is to clear tires and still have a proper seat position. 75 effective seat angle on a hardtail is pretty good unless you live in PNW.

3. Ok. Wide stance. You can use a lower bb if you are using 165mm cranks.

4. Why are you designing for terrible tires if you have no plans on using them. Seriously, rear end clearances are murder when using good parts. Why are you making it impossible by designing for crap you don't intend to use? Seriously, your fit and geometry are far far more important than running 'internet' tires.
 

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Discussion Starter · #12 ·
Yeah I just played with the seat tube. Straight will get me to around 75° and allow a longer dropper if I wanted to.

Yeah I'm on 165mm cranks. Just not sure how low before pedal strikes begin being an issue.

The tyre dilemma keeps bugging me. You're right, it is making things really tricky. Just trying to keep options open. I suppose with sliding dropouts I can always get clearance at the longer positions.
 

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Discussion Starter · #14 ·
If you try to make every option possible, you'll end up with a terrible bike or it cant be made.

Focus on what you really want and what you will honestly use. That makes the job hard enough.
Point taken. This started as a fun do it all trail bike. I don't want a wheels on the ground charge the chunk experience, I want to be thinking about lines and feel confident getting creative and popping off features. At the same time I don't want a bike that feels super sketchy on anything but a smooth flow trail.

This is the latest iteration (hooping the fab can get those chainstays that short).

The revised measurement process for PVDRAD makes things feel more consistent. Measured 3 times all within 2mm of each other. They came out at
Crank Axis to Grip Radius
794​
Crank Axis to Grip Angle
58​
Crank Axis to Grip X
423​
Crank Axis to Grip Y
672​

Stack height is challenged slightly here. Debating dropping to a 120mm fork or raising the bb 5mm.
1942842
 

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One thing I realized...
Doh, I missed that in your print. Your print for a hardtail should be shown in JRA, loaded, in equilibrium.. sagged. I use 30%. Others use 25%. It depends on the state that you tend to ride the bike. BikeCAD has a dimension so you can show the fork length and sagged length. Both are good to show.

Also, fine tuning of cockpit geometry can make a real difference when we are looking to squeeze in more from our geometry and still have a good fit. I've been moving to higher sweep angles than in the past (12-14 degrees) but the info in this post should get you into a better place. Bars, stems, and spacers | Peter Verdone Designs
 

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Discussion Starter · #17 · (Edited)
Doh, I missed that in your print. Your print for a hardtail should be shown in JRA, loaded, in equilibrium.. sagged. I use 30%. Others use 25%. It depends on the state that you tend to ride the bike. BikeCAD has a dimension so you can show the fork length and sagged length. Both are good to show.

Also, fine tuning of cockpit geometry can make a real difference when we are looking to squeeze in more from our geometry and still have a good fit. I've been moving to higher sweep angles than in the past (12-14 degrees) but the info in this post should get you into a better place. Bars, stems, and spacers | Peter Verdone Designs
Found the setting. So basically you're applying the rad results to a sagged geo, locking the frame, and removing sag from the model to get the frame dimensions to send to the builder right?

I know if I send through sagged dimensions it's just going to add an extra level of ambiguity too the process.

In this case my model is looking too low (~12mm) and too long (~5mm).

Also, when dealing with different wheel sizes, how do you approach what dimensions to send to the builder? The models I've seen them output always have front and rear axles aligned. Is this what you mean by in equilibrium?

Funny you sent that link, after applying the rad measurement to my model I've just spent half a day trying to best measure and model my bars and stem in bikecad. You're right, there's practically no useful information published by manufacturers.
 

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Discussion Starter · #20 ·
If you look at the prints on my site, you will see that I've designed in JRA, not in topped out. You're doing the opposite.

Here is a recent print of my Warbird and when I moved from a Lyrik to a ZEB with differing geometry. You can see the dimensioning that I would supply.


Understood, so I'd apply the rad dimensions to the jra model, get that right.

Dealing with a builder expecting to see stack, reach, BB drop, as a static geo though, I'd then work backwards to supply supply the relevant geo. I know a builder should be able to take the design and extrapolate for themselves but I want to minimise any chance of misinterpretation over email. The mixed wheel size is already causing a crap ton of miscommunication.
 
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