Flat-bar touring frame geometry discussion
I'm planning to take the Paul Brodie frame building course next summer, with plans to build a flat-bar, IGH, Belt-drive touring bike.
I've been comparing geometry, and want some feedback. I think I want to build something a little bit "long". Not crazy, but maybe an extra cm in the chainstays for pannier clearance, and a slightly longer than normal top tube, mated to a shorter stem, to give front wheel fender-to-toe clearance.
This is also influenced by the trend in DH to run a longer top tube, short stem combo, for stability.
I understand that roadies like the shorter wheelbase for racing and such, but would this be a bear to ride? On a long hill climb, for example, would it be that much harder to ride? I like long skateboards and snowboards, so I was kind of relating it to that.
Any ideas about how this would factor in to head angle measurement? Again, like in DH, there is a trend towards slacker head angles. I would likely take a flat-bar touring average, and subtract a degree. I'm not looking for "nimble". More like "stable".
The bike would also see use as a commuter, 25kms each way, maybe 2 or 3 days a week.
Any feedback would be appreciated. Difference in touring geo for flat bar vs. drop? Negative effects of a slightly long wheelbase? Suggestions or comments? Seems like not too many companies building flat-bar touring bikes. The surly Ogre is one that I'm using as a geo starting point.
Thanks a lot.
From your description of what you're looking for, I don't think a longer WB would hurt you. Certainly not a cm on the CS. You might even consider more depending on what you're considering for a starting point.
Going from a drop bar to a flat bar, you need a longer stem or TT just to maintain reach, unless much more upright is what you're looking for. Also, a slacker HTA will kick the front wheel out more, so combine those things and I don't think it'd be necessary to specifically design around a long TT/short stem to get the front center long enough so that you don't get overlap when using fenders. Unless of course you already have issues.
IMO, when you're talking about a "casual" road bike (tourer, commuter, etc),as long as you don't go too crazy, there's quite a bit of variation you can do and it will still ride more or less (to quote Surly) "like a bike."
There are 3 major influences on stability in your design that you should focus on rather than trends.
Front Centre, Head Tube Angle, and Trail.
Feldy has touched on the drop-bar to flat bar variation. Say you set your HT at 71 degrees, and you want a slow steer response, fork offset needs to be in the 38-42mm range to give a higher Trail. If the off-set is 42mm plus, the bike will be more steery, and hunting for corners to turn into. If you want the bike to rail along, use the shorter offset. Head tube angle has less influence in this case as a rigid frame/fork is then focused on Front Centre (BB to front axle). Having a longer chainstay will not have a great influence, as long as it needs to be to fit your ancillaries on and have sufficient stiffness so that the long wheelbase is not too flexy. You can stiffen things up a bit with choosing a little bit larger Down Tube diameter. There is a lot to consider, not just 'trends', good design should be around sound principles.
If I don't make an attempt, how will I know if it will work?
If you'll mostly ride on paved/well maintained surfaces, you can just worry about making sure you have heel/toe clearance for your bags and fenders and let the chips fall where they may in terms of wheel positioning. Since you're not going to bunnyhop any logs or ride down 45 degree gullies (ie, like a DH bike would) you don't need to worry as much about fine tuning weight distribution. Much more important to make sure you can pedal without clipping your heels on your panniers.
So I guess what I'm saying is: go for it, long TT and chainstays won't hurt anything here and will be helpful for letting the bike perform the functions you want it to.
Heel clearance for the rear pannier is absolute deal killer if you get it wrong.
DH bikes are getting really slack but they are also using really wide bars. I doubt you're going to use bars that wide, so I wouldn't try to go slacker. If you're running front panniers you're going to get a lot of stability from their mass anyways. When you take them off a slack front end will feel floppy without the panniers slowing things down. Having a longer front center might mitigate this a little bit, but then the question becomes why are you trying to design a touring bike like a mountain bike, when there are plenty of touring bikes to work from?
Yeah, let me be more clear: You should not think about DH bikes, or mountain bikes at all, when doing a design for a touring bike. A touring bike will never see airtime, rough singletrack, or any of the other stuff a mountain bike is designed for.
What you want is a road bike with flat bars, maybe some extra tire/fender clearance, and room for bags. Start there, not with mountain bike geometry.
Proper chainstay length for a true touring bike is the whole chainstay.
Longer the better.
Yes, I make touring bikes, and have ridden them all over the world.
- Steve Garro, Coconino Cycles.
I'm not getting what type of bike you're looking for making - a road touring/commuter bike or a MTB touring bike that could be commuted on? There are so many variations on "touring" bike and bikepacking is the latest.
Are you primarily riding roads (dirt and paved) or are you riding trails while touring? The Ogre is meant for it all which means it does it all pretty OK (IMO). Their geometry is 72/73 with 17" chainstays with the axle pushed all the way forward on in the horizontal drops (so let's say 17.3" when you tension the chain). Those aren't terribly long chainstays for pannier use and if you have big feet they're pretty close to hitting I'd think. The angles are pretty steep and the BB somewhat low for serious MTB use (at least these days) but I think would be nice for an all-around touring bike. They based the geometry on the Karate Monkey to put it in perspective which was one of the first 29" wheeled MTB bikes.
If you're looking for a more dirt road and road touring bike I'd glance at the Rivendell Atlantis geometry, and Co-Motion's website. Rivendell likes pretty long chainstays for heel/pannier clearance as well as for stability with loaded bikes at speed. Check out the Joe Appaloosa from Rivendell, it's extremely long in both wheelbase and chainstays. I'd personally not go under 17.5" if I were making this frame and that is really the shortest I'd go for a true loaded touring bike in general.
If you weren't limited by pannier use I'd be saying just make a bike with the geometry you prefer to ride most of the time and put frame bags on (or Old Man Mountain racks) when touring. But being your first frame, there may likely be more...? Don't try to do it all in one frame is my advice, especially if you'll be doing loaded touring. Talk about 'testing' your first handbuilt frame...
As mentioned above, look at something like the Surly Long Haul Trucker's geo, not the Ogre. Long chainstays, lots of BB drop. Stretch out your front center for flat bars, and there you go.
I don't build frames, but if you're going to load it up make it stiff as ****. That's the way I look at it. Otherwise you'll be wobbling about trying to stay upright. 26" wheels are good. Try to keep the rear panniers as low and forward as possible while avoiding heel strike. You can't really make the stays too long. Look at the big dummy.
Just use big diameter thick walled tubes. No puny stuff.
I know nothing about steering really except that you want it to be easy to go straight. It's harder to start off when heavily loaded, it's harder to hold a wobbly bike straight for long rides while heavily loaded, and it's harder to deal with a squirrel-y bike through pot holes while heavily loaded. So slack is probably where it's at.
Go look at a cross check frame. That thing is wobbly as hell when you got more than 40lbs on the rear of it. 70lbs or more and you'll be lucky to stay upright at all (I've tried that before for over 20 miles). Now take a look at something like the Thorn Nomad or Tout Terrain Silk Road. What differences do you see in tubing, rake, stay length, and pannier mounting position? Those differences are what will help you figure this out.
Nut strike is the only downside. A sloping top tube is wise.
Originally Posted by Smudgemo
What's slack? Look at the classic touring bikes like the Bruce Gordon's - they have 72 head angles. I could see <em>slacker</em> than a road bike, but I don't see any point in <b>slack</b>, especially when compared to todays mountain bikes (which may be unsagged numbers anyway!)
Originally Posted by aBicycle
I shouldn't have even used the word slack. The kind of steering that makes it easy to stay going straight no matter what happens is what I should have said.
Awesome disclaimer - thanks for the honesty.
Originally Posted by aBicycle
You know what makes a great touring bike?
For reals - lots to learn from say......a 198something Rockhopper.
One with rack mounts.
Those cats knew what was up & just getting out was a big part of MTB riding.
- Steve Garro, Coconino Cycles
Thanks for the feedback everyone. Here's where I'm at so far, and I have a few more questions.
Took the geometry of the LHT and Vaya as a starting point for bb drop and chainstay length. Added quite a bit of length to the top tube and used a 220mm x 44 head tube, as I'd like to use flat bars (currently have 1.5" risers), and want to stretch out a bit.
Front center works out to 666, which is why the working title for this bike is currently "The Beast".
Rocker dropouts are from Paragon.
@Eric, you had a suggestion to use a fork with 38 to 42mm offset. Does such a fork exist? I need to choose a fork to go with this. Salsa and surly have 45mm, and anything else seems to have even more.
Anyone have a suggestion for a target mechanical trail? I'd like stable handling.
Any other feedback or suggestions? Cheers and thanks.
Hi, sorry for the delay in replying.
I have re-read the whole thread to get the bigger picture. Looking at your drawing, I can see you are a tall person. So, with a low 74 mm BB drop and possible pannier use you are looking good.
The fork you are looking for some-one else may have to chime in to help you. You're looking for a wide crown 700x35 or so tire clearance fork with bag mounting off the shelf. Brake type is Disc, Off-set options other than 45mm.
May need to get one made. Sorry, my initial comments were of the design drawing type of thing, and I make my own forks, but live in New Zealand. Some-one closer to home would be able to assist here.
Observations from personal experience close to where you are at. My own ride is also 71.5 degree head with 668mm front centre. My BB drop is only 40mm. We differ in end use. I like to pedal/drive out of corners and have 180mm cranks.
Re: Fork off-set. I originally started with 45mm, but I found it to be a bit too willing to turn. My ride position is lean forward so my COG remains low for my height. Yours is more upright so I am guessing we would would end up similar. I prefer to use the higher COG to flow through turns as I lean my shoulder and a light hand on the bar. If you like to turn by bar movement only, stay at 45mm. I made another fork that is 39mm. I like this very much. So solid at in the straight ahead at any speed, and suits the shoulder lean method. I gravel grind alot so I find if I hit a soft patch of gravel, though the bike slows, direction remains forward and I don't lose as much momentum, the 45mm had the inclination to flop a bit and therefore braked the bike and change my direction.
To counter the higher trail, I have a handle bar setup that allows me opt for a reach 90mm which keeps low speed riding steerable, but I can change to a narrower and longer 250mm reach that is total control on gravel roads.
As one of the comments above says, experiment. A generic 45mm fork is a good starting point, and is easier to mass produce, but some of the fine tuning comes from assessing where you want to go.
Don't be frightened by making a fork for yourself. A unicrown requires good Tig Skills, but if you step back and go old school, there are some nice wide crowns that are easy to braze and make a good strong fork. Forks are really no harder to do than setting up your chainstays. Could be a back-up project to do after you have the frame built...
Anyway, above all, have fun, its a great challenge.
If I don't make an attempt, how will I know if it will work?
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