REVIEWS DEALS BIKESHOPS TRAILS

Frame design for newbies

4780 Views 54 Replies 11 Participants Last post by stuartm2, Feb 7, 2022

stuartm2

Discussion Starter · Jan 25, 2022

This started as a discussion on another thread which was quickly going to go off-topic so I figured it made sense to start afresh. I'm hoping PVD will join in here because it's his system I'm particularly interested in understanding - I've read a fair bit of his content and am genuinely intrigued by it, but have found it somewhat difficult to penetrate as a novice builder.

As a novice, let's take a step back from the discussion over driving vs. driven parameters until we've done the groundwork. I want to design a bike. I figure the two foundations in this process are 'me' and 'the riding I want to do'. These seem to be the things which will ultimately drive every other decision in the hierarchy of decisions, which will eventually arrive at a finished design. Does that sound correct?

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pvd

· #2 · Jan 25, 2022

2

stuartm2 said:
Ok, I misunderstood your use of tyre height as height off the ground rather than height over the rim. We're arriving at the same dimension (tyre outer diameter) but I'm measuring it directly as I already have a pair of wheels/tyres of the size I want to run.

Per the last discussion, The reason that rim and tire height are used is so that wheel size is easily changed and identified in the drawing or spreadsheet. You could use 'wheel diameter' in your drawing or calculation and be entirely correct but it leaves out a lot of information and flexibility that are vital in the understanding of the bicycle, the tires. Notice that in my print, I see what wheel and tires have been chosen.

All chassis design starts at the tires. The problem we have with bikes is that the tire positions need to be arrived at through rules of thumb, calculations, or testing as they relate to the CoM. The touchpoints of the bike relative to the ground will change that in slight ways and different riders in others. Then there's the use of the bike and how this alters our choices. Thus, things get fuzzy.

stuartm2 said:
I was talking about grip reach/stack, not frame reach/stack - you refer to it as grip position above. Grip and saddle positions seem like fairly immutable driving dimensions but I don't see them in either of your lists. Wondering why they're not considered 'driving' dimensions.

They are driving dimensions but is a separate component of the design. Handgrip position are decoupled from the frame via the stem and handlebar. So, you can have the CoM and wheel relationship you are looking for and then place the grips where they are needed. Since I'm looking for (at 5'10") front center distances of approximately 850mm, I'm doing a lot of fancy work to get the grips placed but it doesn't change how the bike will work.

I could start with the grip position if my only goal was to maximize front center with standard handlebars and stems but the design and planning work for this is not what I'm looking for and still (in this era) outside of what most folks building bikes are willing to look at.

Bars, stems, and spacers | Peter Verdone Designs

I'll repost the print I shared so that it is in this thread:

At the end of the day, we are trying to use values that communicate meaningful parameters of the bike that let us know how it will behave and fit. We want to eliminate ghost values like reach, top tube length, or ground trail and use only values that get us to where we want and describe the bike accurately.

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stuartm2

Discussion Starter · #3 · Jan 25, 2022

pvd said:
Per the last discussion, The reason that rim and tire height are used ...

Ok, so the rim diameter and tyre height combined allow us to derive the wheel diameter (or radius; either way, the actual dimension we need in our chassis design) but specifying them separately ensures we don't obscure the influence each component has on that value? They are the actual 'driving' dimensions.

pvd said:
All chassis design starts at the tires. The problem we have with bikes is that the tire positions need to be arrived at through rules of thumb, calculations, or testing as they relate to the CoM. The touchpoints of the bike relative to the ground will change that in slight ways and different riders in others. Then there's the use of the bike and how this alters our choices. Thus, things get fuzzy.

Good, the fuzziness is what I'm having trouble with

Let's dig into that...

pvd said:
[Grip and saddle positions] are driving dimensions but is a separate component of the design. Handgrip position are decoupled from the frame via the stem and handlebar. So, you can have the CoM and wheel relationship you are looking for and then place the grips where they are needed.

I assumed the starting point for a design would be the rider, but you're saying the starting point should be the wheels/tyres and CoM? Let's say I want to run 27.5x2.8" (584x71mm) Continental Trail King tyres on DT Swiss XM551 rims. How do I decide where to put the wheels, how do I figure out the CoM, and what should the relationship between them be?

pvd

· #4 · Jan 25, 2022

stuartm2 said:
I assumed the starting point for a design would be the rider, but you're saying the starting point should be the wheels/tyres and CoM? Let's say I want to run 27.5x2.8" (584x71mm) Continental Trail King tyres on DT Swiss XM551 rims. How do I decide where to put the wheels, how do I figure out the CoM, and what should the relationship between them be?

Now you get to the real issue, CoM is extremely difficult to determine for a bicycle and rider. The situation is quite dynamic and near impossible to pin down. Then there's the fact that everyone is different and the measurement is not easy.

The anti-squat problem. | Peter Verdone Designs

Also, why would you want a 584-71 tire. That's a pretty terrible tire for most uses?

stuartm2

Discussion Starter · #5 · Jan 25, 2022

pvd said:
Now you get to the real issue, CoM is extremely difficult to determine for a bicycle and rider. The situation is quite dynamic and near impossible to pin down. Then there's the fact that everyone is different and the measurement is not easy.

The anti-squat problem. | Peter Verdone Designs

Let's assume only the front end will be bobbing up and down on this build. I'll have a read of your linked article and come back later.

pvd said:
Also, why would you want a 584-71 tire. That's a pretty terrible tire for most uses?

They really suit the local terrain - mostly a mix of narrow and rocky technical singletrack, and wide chunky gravel tracks. Of the various tyres I've ridden, the Apex Protection Trail Kings in this size really suit where and how I ride in terms of handling, feel and tear resistance while offering essentially the same outer diameter as a regular 29er. Anyway, let's not get bogged down in each others' component choices. Can we just assume I want a pretty terrible tyre on this bike and stay focussed on the chassis design?

stuartm2

Discussion Starter · #6 · Jan 25, 2022

pvd said:
... CoM ...

Ok, that article was interesting but is mostly about full suspension bikes, not hardtails. It also suggests that you haven't actually started out on the designs for the bikes you've completed to date with actual CoM data. Is that true?

stuartm2

Discussion Starter · #7 · Jan 25, 2022

pvd said:
All chassis design starts at the tires. The problem we have with bikes is that the tire positions need to be arrived at through rules of thumb, calculations, or testing as they relate to the CoM.

pvd said:
So, you can have the CoM and wheel relationship you are looking for

I may have misunderstood what you were attempting to achieve with the discussion around wheels and CoM. Is your purpose to help me understand the motivations for "long" and "low" in modern bike geometry in abstract terms, rather than attempting to come up with an actual value from which to generate specific design dimensions? If it is, let's assume I'm on board with the whole "long, low and slack" thing, conceptually, and move on to looking at how (with currently available and affordable tools, and with tested methodologies) I might, as a novice designer, ascertain exactly what "long" and "low" mean in terms of generating dimensions to enter into BikeCAD.

pvd

· #8 · Jan 25, 2022

stuartm2 said:
Ok, that article was interesting but is mostly about full suspension bikes, not hardtails. It also suggests that you haven't actually started out on the designs for the bikes you've completed to date with actual CoM data. Is that true?

The CoM discussion is the same with hardtails as FS bikes.

I haven't used CoM to design the placement of my front wheel....yet. As stated, trying to locate very precisely the CoM of a rider on a bike is a fools errand. It's just not really where we are with this art. Still, it is what is happening in the background. Could we have a fixed CoM, we OF COURSE would use it. But we don't so we can't. Instead we move to the next best thing, common front centers for differently sized riders in a variety of use cases. This is the fuzziness. Still, Front center describes one of the most important dimensions of the bike, the front wheel from the BB which is a shadow of the CoM triangle.

pvd

· #9 · Jan 25, 2022

stuartm2 said:
I may have misunderstood what you were attempting to achieve with the discussion around wheels and CoM.

The hardest thing for a novice designer to get their heads around is that two things are happening when we design the bike. We are designing a handling system where the wheels are placed relative to the CoM with a castor created in the angle of the head and the offset of the front axle from that. On the other side, we are placing a rider on this system in a way that is comfortable, channels their energy production, is aerodynamic (for road), and gives them the leverage they need to control the bike. It's a stew of ideas all happening at once and the result is either excellent or garbage.

So, what parameters do we use to describe the bike so that when we change one, it has meaningful and understandable results for this stew. Single parameter adjustment is the goal but we don't always get that. Still, the print that I show and the spreadsheets that I show are the best I've developed in the last 35 years.

I encourage you to make a drawing and spreadsheet that does as well. Having gone through the exercise of developing my own MATH for this system I have an understanding of what makes sense and what doesn't that others lack. Maybe you should do this and see where it takes you.

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pvd

· #10 · Jan 26, 2022

I will say that I should probably be using the lateral component of the front and rear centers as a driving parameter rather than the direct method. The problem with that is that it's far more difficult to measure in an approachable way. This would eliminate the effects on that component with changes in wheel diameters, giving more predictable results. This is something for another development round if I can get my head around doing it that way.

eMcK

· #11 · Jan 26, 2022

Since this is a bike for you, it might be best to start with you. What you currently ride, where you ride, and what you want to improve about your riding experience. While you can certainly do a lot of math to help get you closer to your "perfect" bike, there isn't some kind of magic bullet to know what front center to use or head angle, or fork travel, or saddle to grip distance, etc.

How about start with a model of your current bike (or your bike you like the most) in bike cad and go from there? Would be a good idea to measure it as well, not just go off the published numbers.

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stuartm2

Discussion Starter · #12 · Jan 26, 2022

Thanks. I do have various measurements from bikes I own and have made an initial design based on those and on general 'progressive' bike geometries. I've also built a first frame from mild steel to an earlier design as a practice run. I'm really keen to now explore the theory behind 'modern' MTB design - the hows and whys which experienced builders like PVD use to arrive at a particular geometry for an individual, especially in circumstances where you can't really get away with either winging it through a few prototypes until you get it right, or just copying someone else's work wholesale.

I was expecting to start with rider fit - I know PVD has a system of fitting he uses (PVD-RAD) and I have my numbers from that system ready when we get there - so I was surprised that he wanted to start with wheel placement and CoM. But he's the one who knows his system and how it works, so I'm going to let him lead us down the rabbit hole and see where we end up in the hope that it helps other who're interested in the cutting edge of such designs to penetrate such a complex topic.

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pvd

· #13 · Jan 26, 2022

stuartm2 said:
I do have various measurements from bikes I own and have made an initial design based on those and on general 'progressive' bike geometries. I've also built a first frame from mild steel to an earlier design as a practice run. I'm really keen to now explore the theory behind 'modern' MTB design - the hows and whys which experienced builders like PVD use to arrive at a particular geometry for an individual, especially in circumstances where you can't really get away with either winging it through a few prototypes until you get it right, or just copying someone else's work wholesale.

Critique is a great place to start. If you post the print of your bike, we can tear it apart and have you defend your decisions. I find that many people dont understand 'why' they have made the decisions that they do but feel that they are justified. A defence makes that all clear.

Also, just like watching YouTube videos, it's passive to just talk. If you are generating a real print in this, you are forced to get your hands dirty.

stuartm2

Discussion Starter · #14 · Jan 26, 2022

pvd said:
Critique is a great place to start. If you post the print of your bike, we can tear it apart and have you defend your decisions. I find that many people dont understand 'why' they have made the decisions that they do but feel that they are justified. A defence makes that all clear.

Also, just like watching YouTube videos, it's passive to just talk. If you are generating a real print in this, you are forced to get your hands dirty.

Have at it!

Slope Font Triangle Parallel Auto part

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pvd

· #15 · Jan 26, 2022

stuartm2 said:
Have at it!

Please fix your fork measures.

stuartm2

Discussion Starter · #16 · Jan 26, 2022

pvd said:
Please fix your fork measures.

Tell me what's wrong with them and I will

pvd

· #17 · Jan 26, 2022

stuartm2 said:
Tell me what's wrong with them and I will

Turn that statement around. This is how learning works.

stuartm2

Discussion Starter · #18 · Jan 26, 2022

Rockshox Revelation 29": 150mm travel, 42mm offset, 561mm A2C (516mm @ 30% sag). I'm here to learn but the numbers on the drawing match those provided by the manufacturer.

Ben.land101

· #19 · Jan 26, 2022

stuartm2 said:
Tell me what's wrong with them and I will

Check out the print PVD posted above, note the measurements used.
One of the big things is axle-to-crown, vs. fork length. Often used interchangeably, but they are two different measures.

uncajohn

· #20 · Jan 26, 2022

pvd said:
Per the last discussion, The reason that rim and tire height are used is so that wheel size is easily changed and identified in the drawing or spreadsheet. You could use 'wheel diameter' in your drawing or calculation and be entirely correct but it leaves out a lot of information and flexibility that are vital in the understanding of the bicycle, the tires. Notice that in my print, I see what wheel and tires have been chosen.

All chassis design starts at the tires. The problem we have with bikes is that the tire positions need to be arrived at through rules of thumb, calculations, or testing as they relate to the CoM. The touchpoints of the bike relative to the ground will change that in slight ways and different riders in others. Then there's the use of the bike and how this alters our choices. Thus, things get fuzzy.

They are driving dimensions but is a separate component of the design. Handgrip position are decoupled from the frame via the stem and handlebar. So, you can have the CoM and wheel relationship you are looking for and then place the grips where they are needed. Since I'm looking for (at 5'10") front center distances of approximately 850mm, I'm doing a lot of fancy work to get the grips placed but it doesn't change how the bike will work.

I could start with the grip position if my only goal was to maximize front center with standard handlebars and stems but the design and planning work for this is not what I'm looking for and still (in this era) outside of what most folks building bikes are willing to look at.

Bars, stems, and spacers | Peter Verdone Designs

I'll repost the print I shared so that it is in this thread:

At the end of the day, we are trying to use values that communicate meaningful parameters of the bike that let us know how it will behave and fit. We want to eliminate ghost values like reach, top tube length, or ground trail and use only values that get us to where we want and describe the bike accurately.

Just a question.

Don’t we want to have a relatively flexible band of choices over our tire sizes?

Given the fact that some change their tires according the ridding situations, don’t we have to take this into account too?

Do we want to “lock” our precisely calculated custom frame, over a very specific tire / brand?

Or.

Maybe when swapping tires (a narrower one for muddy trails..?) the difference will be minimal.

On the other hand, when designing such a machine, we know what we want (how & were we ride!)...

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