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Saul KK 06-25-19 09:53 PM

Gravel, touring, commuter bike design questions
I'm in the process of designing another bike for myself, and thought it would be a good idea to get some feedback on a few things from some more experienced frame builders.

This bike is going to be my daily commuter, and I also want to be able to do some weekend gravel adventures on it. Also possibly some over nighters with some gear, so will be putting bosses on the forks and seat stays. I will probably be running a light handlebar/front rack bag most of the time also.

I have proportionally longer legs than upper body, and am not particularly flexible, which is why I'm going for a slightly more upright riding position. I'm achieving this by steepening the ST angle, slackening the HT angle, and shortening the stem. The other reason for these front end angles/dimensions is that I want to have absolutely zero toe overlap with up to 2.4" X 650b tires.

I have been playing around with riding positions on the last bike I made, and this looks to be about right for me as far as I can tell.

The part I am not sure about, is how much trail I should be aiming for with a 71 head angle?

I plan on mostly running 700c X 32-42mm tires for commuting, but I want the option to run 650b X 2-2.4". Are there any special considerations when making a bike that can run both 700c and 650b? Aside from obviously frame tire clearance.

I am still deciding on lugged vs fillet brazed, which will also slightly depend on whether I can find lugs with the correct anlges. I will also be doing a yoke/plate style BB chainstay area for max tire clearance.

I will be running SRAM Force 1x11 hydraulic disc groupset with post mount disc brakes.

I will be doing paragon syntace 12x142 rear and 12x100 front thru axles.

I am thinking of using oversize columbus zona tubing in 8/5/8 or 7/5/7 along with zona 29er chainstays and seatstays. Does this sound about right?

I weigh about 70kg/155lbs

I plan on using the pacenti style lugged MTB crown from ceeway, and bending my own fork blades. I am a little concerned about fork vibration while heavy braking with this style of fork and disc brakes, is that likely to be a problem? And which blades would you recommend for stiffness and being able to bend for rake?

Is there anything you would suggest I change about this design? Or any suggestions before I start ordering parts?

Thanks a lot in advance!

unterhausen 06-26-19 06:51 AM

my gravel bike has a 71.5 degree head angle and 45 mm rake for a massive 1" flop. Okay, it's not that massive, but I don't like it. I think it would ride a lot better with 55mm rake and still have decent trail.

Andrew R Stewart 06-26-19 10:58 AM

I would not suggest a .7/.5/.7 tube set for a commuter or for any tourer. Too easy to dent. Is the seat set back what you have tried already with the seat/bar reach and heights? Andy

HTupolev 06-26-19 01:49 PM

Originally Posted by Saul KK (Post 20997370)
I'm going for a slightly more upright riding position. I'm achieving this by steepening the ST angle​​​​​​

Could you explain this? Usually upright positions use a more rearward saddle position.

unterhausen 06-26-19 03:12 PM

that's true to maintain a performance position, you should use a slacker seat tube angle for a more upright position. He may just want to shorten the top tube though.

dsaul 06-26-19 03:33 PM

That looks very similar to my 650b gravel bike. I have a 71 degree HTA and 50mm of rake for about 65mm of trail with 47mm tires. I went with a longer front center and a 50mm stem with a bit less saddle to bar drop for a more MTB like feel on singletrack trails. I really like it, but my background is more in the MTB realm, so the flop is nothing compared to my full rigid MTB with a 68 degree HTA.

Saul KK 06-27-19 07:04 AM

Unterhausen, I haven't looked into wheel flop at all as a consideration. I better do a bit of research on what it all means.

Andy, what tube thicknesses would you suggest? Yes the seat post has I think a 20-25mm set back on the bike I am trialing the fit on. I know I could use a 0 setback post instead of a 74 ST angle, but visually I like the ST to bisect the centre of the saddle (if it continued).

HTupolev, I figured since I'm using an 80mm stem and short reach handle bars, which is essentially moving my centre of mass back, that by bringing the seat forward somewhat I can keep my body mass more central. This is all just my best guess.

Dsaul, that's good to hear that you've had positive experience with a somewhat similar steering geometry.

Really appreciate all the input!

unterhausen 06-27-19 08:35 AM

I think 8/6/8 in anything oversize is really heavy. I'm willing to live with an occasional dent. But if you are using a high strength steel, dents are a lot less likely. Is the bike going to spend much time in a bike rack at work? If no, I wouldn't worry about it. I commuted on my Spirit for Lugs bike, but it didn't sit in the rack at all. People ride aluminum bikes with thin tubes and put them in bike racks all the time and I don't see much obvious damage.

Flop is annoying when you stand up, especially at very low speeds. Otherwise, it's not really noticeable. You can learn to live with just about anything.

Andrew R Stewart 06-27-19 08:42 AM

Seat set back is the horizontal dimension from the tip of the saddle to the vertical line intersecting the center of the BB. Not referencing the seat post but is partially determined by post set back. I mentioned it because of what Eric said in his post #5 .

I prefer a .6mm wall center section on the bikes that will see a lot of load or a lot of leaning against stuff (like when locking the bike). For practical bikes a tad of frame weight is lost in the total riding weight more often then not and years later more likely to be still round and ridden. Andy

unterhausen 06-27-19 11:25 AM

We builders tend to think of the vertical line through the bb as a reference. But really, it's not important except as something you can measure. What matters is the minimum torso-femur angle, or hip angle. On a TT bike, the seat tube can be pushed forward to get a low profile. Similarly, for an upright position, the seat tube can be pushed back. Look at pedal-forward semi-recumbents. I always imagine taking a rider fixed at the minimum hip angle and rotating them around the bb. Any resulting position is a legitimate design.

If you are willing to sacrifice minimum hip angle, and take the performance hit, then making the seat tube more upright is a legitimate choice. Particularly if it's needed to help with other things such as toe overlap and the like. If it isn't needed for that, I wouldn't do it.

Of course, for an existing frame and desired handlebar position, then different setback seatposts make sense. But for a builder, we have complete freedom. The trick is to predict where those things will fall for a given rider.

TiHabanero 06-27-19 04:25 PM

I just finished brazing the main tubes on a touring bike. I designed it for a more upright position and found it was best achieved with a 71.5 seat angle and a BB drop of 9.0cm. Add to this a sloping top tube which allowed me to effectively extend or raise the heat tube. I drew out my fit on paper and drew the frame tubes to the fit. This worked well. The BB drop is a bit experimental as I have never gone lower than 8.0 before, but figured with 38-40mm tires it will not be a problem.

You might consider dropping the BB a tad to see how that affects the fit.

Saul KK 06-30-19 09:45 PM

No my bike wont spend much time in a rack, and I'm keen to go for something a little thinner this time, but also aware that it will be somewhat less durable.

My last bike was 9/6/9 cromor tubing, so if I'm wanting to do a heavily loaded trip I can just use that bike.

I'm now also leaning more towards a carbon fork with eyelets, partially for weight, partially for looks, and partially due to cost of a disc brake brazing jig (also Incepi hasn't responded to my emails in about 3 weeks now).

Interesting points about relative positions of BB and seat. I imagine these ideal measurements, including ideal hip angle, must vary slightly from one person to the next, given that we are all structurally slightly different.

9cm of BB drop sounds like quite a bit. I was going back and forth between 7-8cm, maybe I should just go with 7.5cm. I do like the idea of a bit of extra clearance though when compared to a standard road bike.

duanedr 07-01-19 01:53 AM

Originally Posted by Saul KK (Post 21005143)
9cm of BB drop sounds like quite a bit. I was going back and forth between 7-8cm, maybe I should just go with 7.5cm. I do like the idea of a bit of extra clearance though when compared to a standard road bike.

BB Drop is how you arrive at what you care about - BB height. What folks are reacting to is your height of 281mm is pretty high for anything that isn't designed for single-track. Also, 74* STA is pretty steep for a use that would benefit from more compliance and a more rearward weight bias.

In general, i would evaluate lowering the BB and rotating the rider rearward around the new BB location. Shorter cranks help with hip/lower back flexibility issues as well. As others have said, interviews and watching you ride are needed for more specifics.

Doug Fattic 07-01-19 06:58 AM

I think it is unlikely that your very high bottom bracket height and steep seat angle are anywhere near ideal. If you were somehow able to build 2 different frames and tried both out I bet you would like the one this is swallower and lower better.

The way I like to find seat tube angle is by using an adjustable fitting bicycle to help establish the seat, pedal and handlebar relationship and let that dictate the frame design. If the purpose of the custom frame is not racing or performance riding but rather to be comfortable than it makes sense to have a higher handlebar position (like you have done). As the bars come up the seat needs to go back for reasons already mentioned. This is why Dutch bikes (or old Schwinns) with upright handlebars have very swallow seat angles. You would find that moving the saddle back takes weight and pressure off of your hands on the brake hoods. Riders that have only been fit on production frames discover that more saddle setback (more than their bike allowed) is a wonderful revelation in riding comfort.

Of course as your seat angle gets swallower the front of your bike comes back getting the front tire in the way of your toes. Now you have to decide what you want to compromise. If overlap is a real problem you can build around MTB 559 wheels instead. Or you can live with the overlap.

Like duanedr said, BB drop is determined by BB height. I like it to be as low as possible. If I was building a frame like yours for myself, my seat tube angle would be around 72 and my BB height using 170mm cranks would be around 260mm.

unterhausen 07-01-19 07:48 AM

I really don't care much about bb height. Maybe bitd when we rode platform pedals and pedal strike was a real issue. I like to have more than 70mm of bb drop though. I couldn't tell you what the bb height is on any of my bikes. Some people are really obsessed with stand over clearance. I am riding a frame that's too big for me right now and have never noticed an issue.

fietsbob 07-01-19 08:53 AM

I had 2 pair on top of the down tube, 1 beneath , + on the seat tube so 4 sets ..

Now seeing bottle bosses on the underside of the top tube for pump clips

Seeing bosses in sets of 3 on the latest Trek 520 for big bottle/anything cages.

duanedr 07-01-19 02:36 PM

Originally Posted by Doug Fattic (Post 21005440)
my BB height using 170mm cranks would be around 260mm.

That's exactly where I start for road bikes-90mm longer than the crank. From there I factor in terrain, expected use, pedals, crank width to drive adjustments upward.

Saul KK 07-05-19 06:37 AM

Apologies for the slow response, I don't always have the brain power to respond properly after work. I am very grateful for all the input and time people take to help me and others out, and the willingness to share knowledge is amazing!

I've ordered a 50mm stem to put on my current bike, and along with moving the seat back as far as it will go, I should be able to somewhat mimic a 72 ST, and see how that feels.

Bellow are some photos of the revised frame plan. I've adjusted the ST to 72 as suggested, as well as shortening the head stem to 60mm.

I'll be using a 175mm crankset, and platform pedals, so will require a little more clearance than click-in style. I understand BB height is the important number, but given it varies with tire/rim size and I plan to use both 700c and 650b with different size tires, I find it easier to talk about BB drop. I am also checking BB height with various tire/rim combinations. I've added 5mm of BB drop, but not sure I want to go much lower. With 650b X 2.2" I have a BB height of 272mm.

I've also slightly adjusted the fork rake to represent the carbon fork I will buy.


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