[crembz]

Hi there,

I'm trying to design a custom frame to get made up as an all purpose fun hardtail. If not found that many ;options locally so I'm looking into getting the frame built.

Requirements:

Comfortable for 30-50km rides around town at moderate pace (sealed & gravel surfaces)
Must be playful and handle a few light jumps, bunny hops etc
Can venture out onto some singletrack occasionally (blue trails no black)

From looking around I've sort of thinking of something between the Ibis DV9 and the Chameleon.

I'm 168cm, 84kg with a 78cm inseam.

This is what I've come up with so far, any thoughts about how this thing might actually end up riding?

 

[pvd]

Please put more work into your dimentioning.

 

[scottzg]

Some thoughts and opinions--

What travel fork? Looks like it might be an exciting ride if you bottom it out. Check what the bike looks like at 100% sag; front-center and head angle.

I don't see any benefit to such a long head tube. It's poor use of prime real estate. Either shorten it, run a longer fork... If that's your actual saddle height you're really limiting your ability to fit yourself. IMO if your head tube isn't ~120mm you should have thought about why that's the case.

Looks like you're trying to keep the front-center real short? That's fine on urban stuff, but you don't give much (anything, imo) by pushing the front end out a bit more. I don't see any value in a short FC on a bike that will be jumped and ridden off road.

That BB is HIGH, especially for your use-case. Consider moving to the ~315mm range, which is still fairly conservative. I use bikeCAD's lean angle measure for BB height; i like ~36* for mtbs.

IMO when you're designing for yourself, start with front-center and rider compartment, then move on to describe the seat position and handlebar drop, then steering behavior (head angle, offset, reach). Chainstays last. You might design a production frame. Put up drawings that have accurate saddle height, rider compartment, front-center, stem length, fork length, and wheelbase. It makes it easier to evaluate.

I'd be surprised if a pro builder would be willing to sign his name to this, as pictured. Half the value in having a custom frame is the builder's intimate knowledge of frame design. Going through this process is still valuable for good communication with the builder, though.


I'm not an expert. Hope this is helpful

Please put more work into your dimentioning.

You posted that while i was replying... knew it was coming. hahaha

 

[crembz]

Thanks for the advice. I spent a little more time on this ... I'm not expert but working through the app little at a time. I don't seem to be able to select wheelbase in the view(greyed out).

I'm modelling this around a 120mm fork with an A2C of ~530mm (Im thinking a 120mm revelation). I'm also using 165mm cranks. Assuming 29x2.2 tyres in this also.

0% sag


20%sag


100%sag

 

[crembz]

Yeah I'd definitely let the builder add their own expertise and knowledge to the design. I'd just like to get to a reasonable ballpark as a starting point rather than starting the discussion from a blank piece of paper.

 

[Forest Rider]

I think the chainstay is too long and seat tube is too steep.

LOL -totally kidding. And didn't mean to derail the thread which is probably going to happen now. Sorry.
Just poking fun at other threads using this as a portal.

Please put more work into your dimentioning.

....or is it dimensioning?

 

[crembz]

I think the chainstay is too long and seat tube is too steep.

LOL -totally kidding. And didn't mean to derail the thread which is probably going to happen now. Sorry.
Just poking fun at other threads using this as a portal.



....or is it dimensioning?

lol fair point though re numbers ... I'm trying to put some though behind these. I'm focused on an outcome of ... "can cover distance" + "mixed terrain urban riding" + "playful" without really caring about what the numbers need to be to get there.

In my limited knowledge:

Admittedly I'm not too familiar with the importance of front center other than stability for speed downhill

HTA I understand affects stability downhill, rollover and steering precision/speed/twitchiness = higher for my purpose

Chainstays affect playfulness/poppiness = short for my purpose

STA affects pedalability and comfort = conservative-steep for my purpose

Seat tube affects clearance for dropper posts = short because I'm short

Stack affects bar height/comfort in relation to seat and hands - feet dimension

Reach affects hip angles and stretch to bars relative to STA, seat height and Stack.

I'm trying to model stack and reach based on RAD using the leelikesbikes ridelogic sizing calculator. The issue I'm having is that unless I run 100mm travel forks on this steepish HTA I'm going to be quite far from the ideal RAD:

The other option being to invert the handlebars. It's a shame bikecad cant model the RAD i.e. bb-bar grip.

The other decisions I need to make but don't quite understand the impacts of are bb offset and 44mm vs tapered headtube.

 

[scottzg]

Don't use RAD, it's worse than nothing.

 

[crembz]

Don't use RAD, it's worse than nothing.

Lol oh well that changes the way I've been thinking. Are there any other systems worth considering in your experience? Just want to nail the fit on this frame.

 

[crembz]

Spent a little more time on this. Slackened out the HTA a little to bring the stack down slightly. Shortened the HT and added in the headset dimensions. Also penciled in 2.6" wide tyres and offset the seat tube slightly to accommodate.

I'm concerned by the stack still being somewhat high compared to my current bikes and the reach being longer than my current bikes.

0% sag


25mm sag


120mm sag


If someone with a better understanding of geometry than I could take a look and comment on the expected riding characteristics of the frame I'd really appreciate it.

Long distance rides (30-50kms)?
Urban/Gravel/Singletrack handling?
Playfulness/skills (manuals, bunny hops, jumps)?

Thanks heaps.

 

[pvd]

Much better work on the print.

You only need to focus on 25-30% sag dimensions.

1. Change the head angle to 66-67 degrees. It's way too steep.
2. Your chainstay are way too long for your height. You want to get down to 410mm with 29" a 29" wheel but you are probably into 27.5" territory so maybe 405mm.

3. Bend the seat tube. Period.

4. Your effective seat tube angle is way too steep for long rides. Get it down to 74-73.5 degrees.

 

[pvd]

Also, your handlebars look quite high. Consider going to a 90mm head tube. You can go even shorter than 90mm (85mm) but few builders can do that kind of work.

You can try to approximate your fit by using a trick that I recently discussed. The key to fit is to focus on your hand grips, saddle, and pedals. Other placements are part of handling.

I'm soooo RAD! | Peter Verdone Designs

Also, be warned to use good cockpit part dimensions....

Bars, stems, and spacers | Peter Verdone Designs

 

[scottzg]

You only need to focus on 25-30% sag dimensions.

Designing around sag is a terrible way to go about it. Clearly it works for you, but it's not a good suggestion for newbies.

-0% sag is hugely important; it determines how steep you can climb seated, and it's easily repeatable. Heck 100% is more useful than some number in the middle.
-25% is an arbitrary amount, and not a useful one. It's like, standing in the attack position on level ground? You're closer to the 50% when descending, and 10% when you're seated on flat ground.
-it conflates spring ramp, suspension behavior, and geometry when you're actually riding on the trail. It won't necessarily produce consistent results from bike to bike or fork to fork.
-production geo is almost always listed unsagged, so designing sagged makes it difficult to extrapolate from what's out there. (no problem for you, obviously!)

This is just IMO, but i think it's pretty clear-cut.

Lol oh well that changes the way I've been thinking. Are there any other systems worth considering in your experience? Just want to nail the fit on this frame.

Do you have a 26 or 650b (because stack) bike where the distance from the handlebars to the seat feels just right? Maybe something close, where you can swap a stem in (or mount a seatpost backward) to see how that cockpit length feels? You can use any bike to figure out how you want the rider compartment and handlebar drop, and then export that number to your custom frame and design around it.

That's how i do it, anyway. I like being able to ride around a bunch with the actual fit, rather than taking a guess with a calculator.

It's a bummer you're stuck with a straight seat tube and 430mm reach/chainstays.

 

[pvd]

.
-production geo is almost always listed unsagged...

Product managers and marketing people don't have any clue about bike geometry or design and that is make very clear by how they list the specs. Anyone would do well to ignore all marketing material.

I've studied this subject. I've written about it at length. I use 30% sag when designing hardtails and this has been very useful.

 

[EatsDirt]

Product managers and marketing people don't have any clue about bike geometry or design and that is make very clear by how they list the specs. Anyone would do well to ignore all marketing material.

I've studied this subject. I've written about it at length. I use 30% sag when designing hardtails and this has been very useful.

If everything you do and think is so far superior, why is it that you are just a guy shaking a stick online?

Serious question.

 

[crembz]

Thanks very much for contributing, truly appreciate the differing perspectives on this.

Much better work on the print.

You only need to focus on 25-30% sag dimensions.

1. Change the head angle to 66-67 degrees. It's way too steep.
2. Your chainstay are way too long for your height. You want to get down to 410mm with 29" a 29" wheel but you are probably into 27.5" territory so maybe 405mm.

3. Bend the seat tube. Period.

4. Your effective seat tube angle is way too steep for long rides. Get it down to 74-73.5 degrees.

Thanks for the insights. I assume your recommended changes are at 25% sag? i.e. 67* @ 25% sag?

I'm working within the limits of the frame builder at the moment, thus the 430mm chainstays and straight seat tube. I'm not comfortable with the stack and reach as it is quite far away from why I currently ride and find comfortable.

I have posed the questions to the frame builder as I think it would open up the design quite a bit.

Do you have a 26 or 650b (because stack) bike where the distance from the handlebars to the seat feels just right? Maybe something close, where you can swap a stem in (or mount a seatpost backward) to see how that cockpit length feels? You can use any bike to figure out how you want the rider compartment and handlebar drop, and then export that number to your custom frame and design around it.

That's how i do it, anyway. I like being able to ride around a bunch with the actual fit, rather than taking a guess with a calculator.

It's a bummer you're stuck with a straight seat tube and 430mm reach/chainstays.

I do have a nukeproof scout 275 and a spec enduro 29 both in a small. Interestingly the spec sheets show them as having a similar stack of 606 & 608mm respectively. Reach is 400 & 415 respectively.

I have however put a 29er front on the nukeproof and a 650b rear on the enduro which in both cases would have increased stack, I estimate stack on nukeproof is roughly 617mm now with a reach of 386mm.

Agreed, I'm thinking if I'm stuck with 430 reach/chainstays it may not get to me to where I need to be. Might as well buy something off the shelf, question there is what.

 

[pvd]

You're really going to have to go with 29"F/27.5"R. That will open up a bit of room and make this a much better bike for you. You'll be able to shorten the stays 25mm and slack out the seat tube without bending.

Also, 31.6mm seatpost or it didn't happen. 170mm is probably a good stroke.

 

[crembz]

You're really going to have to go with 29"F/27.5"R. That will open up a bit of room and make this a much better bike for you. You'll be able to shorten the stays 25mm and slack out the seat tube without bending.

Also, 31.6mm seatpost or it didn't happen. 170mm is probably a good stroke.

Interesting, the reason I thought 29er f/r for this bike is to cover distance. Serious question ... In my understanding, putting a 650b back on the rear will negate this objective would it not? In which case a 650b may actually be a better choice than a mullet? I hear what you're saying though.

31.6 and 170mm seat post is exactly what I run on the Scout.

In fact this build is looking more and more like a nukeproof scout mullet ... Oh wait I have one of those lol 🤣.

 

[pvd]

If the bike doesn't fit you, it doesn't matter how rad your kit is. The speed will have to come from optimising your saddle and bar placement. Let the wheels do what they need to do without ruining that.

 

[crembz]

If the bike doesn't fit you, it doesn't matter how rad your kit is. The speed will have to come from optimising your saddle and bar placement. Let the wheels do what they need to do without ruining that.

Agreed, what I'm getting at is whether there is any benefit in a 29er front for this application if going with a 650b rear?

It's looking like I'll need to use a bent seat tube or a very offset bb to get a 29er to work for me. Or as you say use a smaller wheels.

20mm bb offset assuming 2.4" 29er: