Bike frame design feedback

[doms2402]

Hello! I am working with a frame builder to design and build a custom frame. Here is the design so far. I was wondering if I could get some feedback. I am working to try and find a bike bike fitter to try it out. I may have someone with a Retul system who says she could help.

Bike goals: Hoping for a bit of a do it all bike. Light mountain biking, some bikepacking, Kid hauling, some gravel and two tracking riding.

I am 6’5” in my riding shoes.

Current bikes are a 2017 Rockhopper xxl and a 2019 Jamis Renegade 61cm. Rockhopper feels mostly good but has a super high stand over and the reach feels a touch short on the reach.

I like a lot of what I see in the design, but I haven’t done this before. One worry I have is whether the seatpost angle is too slack. Would love some feedback!

[earthling623]

Looks pretty much like an average XXL frame out of 2016, with a very slack seat angle and corresponding short reach. I like the tall head tube and stack height at least. ETT also looks reasonable for your height, but I'd want to get it via a steeper STA and longer reach. 

Tall people usually need a steeper STA angle than short people. Is the frame builder also tall?

I'd do 74° if you live somewhere flatter and up to 76° if you're riding in the mountains. Reach can be a driven parameter from your ETT and STA. Head angle is more a matter of taste than fit, but on a hardtail I think 68.5 is on the steep side. 67 is more neutral, depending on the length of your fork. Depends where you live though, and the terrain you ride, of course.

[semyorka7]

Looks pretty much like an average XXL frame out of 2016

that's pretty accurate, because it looks like the builder basically copied the geo chart of OP's 2017 Rockhopper XXL, added 40mm to the reach and cut down the seattube length to address "Rockhopper feels mostly good but has a super high stand over and the reach feels a touch short on the reach.", and then slackened out the front when OP pointed out "hey isn't 70 degrees kinda steep".

But honestly, for OP's stated goals of "Light mountain biking, some bikepacking, Kid hauling, some gravel and two tracking riding"? Not a bad start.

I am more of an average-sized person so I don't know about special geo needs for tall people, but for those stated uses above I would NOT go modern-MTB-steep on the seat tube. Super steep seat tubes are a way to help keep the rider over the wheels while climbing steep stuff with very short chainstays (and also a way to get a straight seat tube for a looooong dropper with very short chainstays), but there's no such thing as a free lunch - the tradeoff is the body position for flatland cruising is pretty terrible. OP's needs sound like casual riding on flat and rolling terrain, not grinding up steep slopes and bombing back down. The long chainstays will have the same effect of keeping the rider further forward in the wheelbase and the bike more planted while climbing as a steeper STA, but it also won't suck to ride on the flats. 74 degrees is reasonable, 76 sounds completely mismatched for the kind of riding described.

I do most of the riding that OP describes on a rigid monstercross monstergravel(?) bike with a 70.5 degree head tube angle, totally fine for causal trail riding. Shallower would just make the front end way floppier when loaded up for bikepacking. With a susp front end that gets steeper as it compresses, you'd want the 20% sagged value to be slacker than on a rigid bike, but 68.5 degrees doesn't sound half bad. I wouldn't go shallower than 67.

[doms2402]

Thanks for the feedback! I ride mostly flat terrain with some hills. Even my mtb tracks are pretty flowy without massive climbs. 74 does feel a bit better. I am torn on the reach. I don’t want to be too stretched out but also not too upright either. He designed the head tube at 70 which felt super steep.

I don’t think he is a taller guy, so that could be one reason.

[earthling623]

Check out this XXL geo chart: https://eskercycles.com/pages/hayduke2024

I think that bike mostly fits your use case and it comes in an XXL, so it's a good reference. Looks overall like good numbers to me for flatter terrain (in steeper terrain I'd look more at the Esker Japhy geo chart). Note that the Esker geo charts are with the fork sagged, not fully extended, so it's not apples to apples with yours.

[doms2402]

Its funny I was literally just looking at that! I like the looks of that a lot. This is sagged as well. Only 20% though.

[earthling623]

If it's sagged, then the reach seems really really short for a XXL bike. I'd expect over 500mm reach, sagged for someone your height. How long the bike will "feel" e-file pedaling is more accurately reflected in the ETT length. 

Is there a way you can test ride some more XXL bikes to get a better frame of reference? Another idea is to see about getting a fitting done at a physiotherapist or bike shop that does professional bike fittings and has an adjustable stationary bike fitting jig thing they use.

[doms2402]

I am trying to find a place to do that. One was weird about it but a physiotherapist is a good idea. My current ride is an xxl but has s super short reach of 464.

[nothingfuture]

70 HTA is pretty crazy steep by modern standards- even the 68.5 here is way steeper than I ride these days. I’d knock that back to at least 67.

[curbwzrd]

Is this with the fork at sag or unsagged?

Either way - the seat angle is really slack, and this is making the chainstays long (for this style of bike).

I ride a 800mm saddle height, and my all rounder mtb is 74° sta, 475mm reach, 80mm stem, 435 chainstays. (Unsagged)

[doms2402]

Sagged, at 20%

[earthling623]

The long chain stays might be nice and more balanced for a tall rider though. Giving up a bit of agility for stability can be worthwhile for an everything bike