MTB input from the pros

[false_aesthetic]

Folks,

I just can't get the idea of a MTB out of my head. Maybe part of it is listening to the FatCyclist Leadville podcast. Maybe it's because I've almost been plowed over a few too many times in Los Angeles lately. Whatever it is I wanna make one (and I feel like my TIG skills are at the point where I won't be seriously risking my limbs).

I haven't ridden a MTB since the very early 2000s (back then, that's all I did). I'd love to get a bit of feedback from the folks that are riding/building since I'm pretty unfamiliar with this side of the bike world and modern geometry. The Santa Monica + San Gabriel Mtns are my back yard.

My bud has already gifted me a brand new RockShox Revelation RC 120mm -- the A-C + tire size reflect that.


Me:
5'8"; 31" inseam (787mm); 155lbs

Road Contact Points:
Saddle height: 702mm
Setback: 47mm
Saddle-Bar Drop: 70mm
SaddleTip - H-Bar Center: 517mm

Thanks all.
-T-

[steve garro]

You have it at 70* un-sagged so sag that 25-30% and it's going to be crazy steep and your BB will be really low



- Garro.

[false_aesthetic]

Hey Steve,

As drawn I have 24mm (20%) of sag entered into BikeCAD. Would you suggest doing 36mm?

[steve garro]

Hey Steve,

As drawn I have 24mm (20%) of sag entered into BikeCAD. Would you suggest doing 36mm?

Your drawing says 512mm ATC?


That's full length


Manufacturer: RockShox

Model: RockShox Revelation RC Suspension Fork
Size: 15x110mm Thru-axle
Type: Tapered
Style: 27Five

Offset: 46
Axle to Crown Length: 512
Crown Race: 40
Steerer Tube Length: 265
Brake Type : Post Mount 180
Wheel Size: 27.5"
Travel: 140
Defined Color: Black
Steerer Type: Tapered
External Adjustments: Lockout Threshold
External Adjustments: Low Speed Compression Damping
External Adjustments: Rebound Damping
Positive Spring: Air
Front Axle: 15mm Thru x 110mm
Negative Spring: Air
Stanchion Diameter: 35
Lockout Switch: Crown
Maximum Tire Width: 82

512 seems short for a 140 fork?


- Garro.

[false_aesthetic]

I see what you're saying. I added A-C minus sag.

The fork that I have is 120mm. The specs on the leg confirm that A-C is 512mm.

TomMTB2.jpg

[Blacklabbikes]

I'm not a pro but I'd have:

a longer HT ~ 110mm
more bb drop ~ 65mm

I'd also loose the short taper chainstays or the polydrops. I don't think the aesthetic works.

All IMO.

[steve garro]

IMG_4590.JPG

I see what you're saying. I added A-C minus sag.

The fork that I have is 120mm. The specs on the leg confirm that A-C is 512mm.

TomMTB2.jpg

First......ditch the 28.6 TT = go 31.8

480 sagged ATC = that's proper.

I would open up the head tube to 100mm just to eliminate a DT/TT compound miter

Lengthen the CS to 425 (IMO)

Don't drop the BB more - this one shown is at 40 sagged

Other then that looks pretty sound

You'll end up with something very close to this

- Garro.

[false_aesthetic]

Thanks Steve! I appreciate the input.

For the 40mm BB drop are you doing that because the client is using a longer crankset or do you just prefer even more clearance?

[Boedie]

If I'm not mistaken, Bike Cad calculates the correct axle to crown when the sag is entered into the fork details. It will show the a-c measurement or a-c minus sag, depending on what you tell it to display. Hope that helps.
Chris

[Boedie]

If I'm not mistaken, Bike Cad calculates the correct axle to crown when the sag is entered into the fork details. It will show the a-c measurement or a-c minus sag, depending on what you tell it to display. Hope that helps.
Chris

Oops, sorry, just saw this discussed above. Can't figure out how to delete my message. Using a tablet.
Chris

[MDEnvEngr]

See here for some thoughts on modern mtb geometry. I have been heading this way for a couple of years with the kids' bikes and they report good things. I also see Kris at 44 is trending this way with a couple prototypes:

Naked's framebuilder notes on a prototype

[false_aesthetic]

Naked's framebuilder notes on a prototype

That's an interesting read for me. I'm probably going to have to make a few iterations before I have a 5th grader's understanding of this stuff. At least I have some time to do it and a job that supports it.

[steve garro]

Thanks Steve! I appreciate the input.

For the 40mm BB drop are you doing that because the client is using a longer crankset or do you just prefer even more clearance?

To achieve the target BB height while riding

[steve garro]

To achieve the target BB height while riding

That's an interesting read for me. I'm probably going to have to make a few iterations before I have a 5th grader's understanding of this stuff. At least I have some time to do it and a job that supports it.

Best thing you can do is build one and ride it and use that as a base point for you own flavor.

Everyone wants to point towards what others are doing as what you or they or everyone else are doing as what you should do.

WWDD

What Would Dario Do?



- Garro.

[MDEnvEngr]

That's an interesting read for me. I'm probably going to have to make a few iterations before I have a 5th grader's understanding of this stuff. At least I have some time to do it and a job that supports it.

With a nod to Garro's comments below, you'll have to try a few versions to get what you like probably. Perhaps you don't need to go with the curved seat tube wicked short chainstays right off the start (if you do - Waltworks sells them). But 2 things you really should do to catch up: design for and use a dropper post - I was a skeptic for a long time but this component absolutely transforms the ride; an:, a wicked short stem - keep your bars where you like them and stretch your front center to use a tiny (<50mm) stem. Those 2 things will get you mostly caught up to "modern geometry."

[steve garro]

Those 2 things will get you mostly caught up to "modern geometry."

But, is that something a person wants?

You know, one of the most important questions you can ask your client for a MTB is "Can you wheelie?" and if yes, "Can you Manual?"


If you can or cannot determines ALLOT about the needs of the client, geometry, frame design, and tubing and component choice.


Think about it and after a few posts I'll tell you why (IMO)


- Garro.

[false_aesthetic]

I want to hug all of you.

Seriously.

[steve garro]

I want to hug all of you.

Seriously.

Let's get growing!


- Garro.

[Donald]

Some good stuff here. If you haven't ridden since the early 2000s, this will be something pretty similar to what you (were) used to and, as others have said, a good starting point to experiment.
Down the road, if you want to get a dropper (get a dropper!), then you can really open up the world of bikes that go up AND down really well. All the advice about longer front centers and steeper seattubes will suddenly be applicable.

BB height is something that's completely dependent on the rider and their ability to tolerate/avoid pedal strikes. I, personally, sit in the camp of get it as low as you can, slap a high-engagement hub and some short cranks (I ride 165mm) on and get rowdy.

*Disclaimer* I ride in the Southeast. Lots of punchy ups and downs, exposed (usually slick) roots and rocks. I have no knowledge of the trails near you, so I've tried to steer away from giving specific 'this bike' advice.

[steve garro]

I want to hug all of you.

Seriously.

Here is the kind of thing I work with, this is my latest actual request:




"I would very much like for you to build me a bike. Here's what it would be:

29 running 2.2-2.4 rear, 2.3-2.5 front
I'll put a 120 fork on it, maybe someday I'll be tempted to go up to 130
It will be set up geared. Most likely a triple.
I'd like to be able to run an internal routed dropper. May or may not.
Bottle bosses: one seat tube, one on top of down tube, and one on bottom of down tube.
Probably 148 rear spacing. But I'd like your opinion about that.
Straight tubes in the main triangle, whatever you want to do in the rear. One top tube.
Ability to run a tapered steerer, which I assume is standard these days. 44mm strait head tube and tapered headset probably?
Brakes will be XTR v-brake levers and Avid BB7s. Probably a 180 or 200 front rotor. Don't know if that matters.

Not obsessed by weight. It would be nice if it was a little lighter than a Surly frame, but it will be used by a fairly big man. It won't be a museum piece, I will ride it into whatever danger I am willing to face. I'm basically a 200 lb 6 footer. I used to be 6'1" but who's to say now.

I need a pretty slack seat tube angle to get KOPS. All my bikes have to have setback posts. I fit a typical Large frame's cockpit, but always with lots of post showing. Standover is never an issue on a stock Large frame. I run a fairly high bar. Roughly an inch + higher than saddle. Since the saddle will be relatively high, a relatively tall head tube is best.

I like bikes that are responsive. I enjoy riding fast on a twisty bit of singletrack, but I've never been the kind of fast like some of the people I know. I like the bike to be nimble at speed. Slack angle bikes feel sluggish to me at low speed. I spend a lot of time at low speed these days. I don't like pedal strikes.

Now the important stuff. Those details up there are all I really need to tell you. I want the bike to fit, I want it to be ready to be spec'd the way I am going to spec it, and I want it to be prepared for my weight to be durable. But mainly I want it to be a Coconino. I don't care what head tube angle you choose. I don't care how long the chainstays are. I don't care what brand(s) of tubing you use. I just want it to ride like a Coconino. I've ridden *****'s. I took his blue one that's spec'd with STI XTR 9 speed to ride the Antelope Peak Challenge down near Oracle several years ago. I've run into riders on Coconinos all over during my travels. Once during a night lap at the 24 in the Old Pueblo I stood around on course with a Coconino rider and we split an orange. I've never met anyone on a Coconino who wasn't thrilled with how their bike rode.

I don't care what you charge me for the frame. I read Steve Friedman's article about Dario Pegoretti in Bicycling after he died. It really made me feel. I have been thinking about getting a Coconino for quite a while. The way that article made me feel gave me a vision for how this could happen. The bike will be far more valuable to me than whatever it will cost.



Let me know if this is a project you'd like to take on. I really hope you will."






- Garro.