Hi everyone, I’m designing a steel frame and fork for off-road touring with my dog, who will be riding in a basket up front or running alongside. After the helpful advice offered in this thread I have some more questions about the general frame design. I won’t be assembling the frame myself but would like to go to the builder with the geometry and a good idea of what tubing I want. I’m interested in any feedback or comments on my design, particularly any glaring issues that I haven’t noticed.
Overall the plan is for a 27.5 drop bar off-road touring bike, I’ve been looking to bikes like the Tanglefoot Hardtack/ Stooge Rambler/ Surly Grappler for inspiration, mostly the long front centre and high stack. I’m 5’11 but most of that is in my legs so medium frames often seem too long for me but I need the stack found on a large if I am to get the bars up near the saddle.
This is where I am with geo so far;
My plan at the moment is for a curved top-tube with second TT below a-la klunker/cruiser, this is almost entirely for aesthetic reasons but will serve to separate the large frame bag into two. My plan was to size the TT and DT as I would if there were no second TT and make the second TT from small diameter tubing (25.4/22.2mm?) to keep unnecessary weight down. Should I consider the second TT more structural and size down the TT tubing diameter?
Regarding fork length, aside from ensuring it can clear the tyre what else is considered when designing a fork? This won’t be suspension corrected and I’m hoping to maximise frame bag space, so I’m thinking of a short fork (420mm a-c) and long head tube. The high fork offset (63mm) is to reduce trail and help with handling when my 10kg/22lbs dog is in the basket. I assume a shorter fork is generally stiffer too?
For the head tube length, I want it to be long for frame bag space and to set my drop bars high without tonnes of spacers. I see some bikes using spacers rather than a long head tube, I assume because it saves weight and maybe maximises standover without a steep top tube. Is there a reason I haven’t considered for shortening the headtube?
To keep weight down I plan on butted tubing for the TT and DT, I understand externally butted tubing is available for STs but is there a weight saving using a 27.2mm ST over 30.8/31.6? Are there any other places that weight can be saved? For example using a segmented fork rather than a lugged or plate crown?
I know I have asked a lot of questions so thanks a bunch in advance for anyone that can help me out!
Always a good idea to come into it educated! It will result in a better bike.
I would pull in the reach even more, unless it was intentional. It is a very common mistake to increase the stack while keeping the reach constant, you end up with a much bigger bike. The reach needs to come closer as the stack gets taller.
Sounds like a good plan! The second TT is just for style. For bonus points, you can try to find a butted 25.4 that is short enough to be used as a second TT.
You would need to make sure there are fork blades that can achieve the axle to crown you want, and have the correct wall thickness to handle the extra load. I suggest finding a builder who is a fork expert. You don’t want to mess around, especially when you have two lives at stake! Meriwether Cycles is pretty good at making steel forks, but judging by your spelling of “tyre” you are not in North America
Correct, spacers are mostly for standover and style reasons. Also, the smaller frames are stiffer and lighter. If you were designing around existing fork options, you must ensure the steerer is long enough for the headtube and spacers you want to run.
27.2 seat posts are a lot more comfortable than the bigger sizes. 31.6 is preferred for dropper posts. However, with a gravel-style frame (long seat tube), a 34.9x31.6 seat tube is going to be HEAVY. I would stick with the 29mm eternally butted seat tubes.
Thanks, that makes sense. In my mind the higher the bars the closer they get to the saddle but that’ an effect of the spacers above the headtube. Now that I think about it I can see how it pushes the headtube further away. I’m unsure how best to judge how much shorter I should make it to counter this though. Are there any fit measurements that are less effected so I can compare a lower stack bike I’m happy with?
Thanks for the recommendation, I’m not in the US but will bear that in mind. Are there any designs of forks that are going to be stronger? I’m thinking a curved leg fork would suit the frame better and be more comfortable but perhaps a straight leg fork will be more solid.
I’ll most likely be using a 27.2 seatpost on the frame but I considrred a shim and a 30.8 ST to allow me to use a decent dropper. I’m not sure I will ever use one though because I get fed up of the play they all seem to develop. It might be best to just go with the 29mm seat tube, they look better to me as well.
For the curved top tube, is there any issue or difficulty bending a butted tube over a straight guage? Also I assume its best to weld the lower TT to the headtube or the downtube above the thin wall section?
To compare a taller stack bike, as I am riding on a more aggressive bike, I sit up and visualize my hands coming back. IMO, with that stack, you probably need to pull in the reach by 20mm.
Another way of doing it is to start with the minimum front center so that you can get away with an acceptable amount of toe overlap, then extend the headtube as high as possible. This works pretty well for everyone under 5’8 (173cm).
I’m not a fork expert, so be sure to use a disc brake fork blade. I have seen too many people chasing flexy forks, only for the fork to crack at the brake mount. I think the problem is that curved steel fork blades and disc brakes never shared the same timeline.
From a geometry perspective, there is no difference in strength between curved vs straight fork blades. However, straight fork blades are more modern. Therefore, they are stiffer and stronger (larger diameter, thicker wall thicknesses).
I don’t think 30.9 seat tubes exist? If so, the options are really limited. If you want the strength and stiffness for loading your bike, I would go with 31.6 at the cost of an extra 100g of weight.
You would roll a curved top tube. This all depends on what tools and skills your builder has access to.
Headtube vs DT would probably be an aesthetic decision. It will depend on where you want the additional TT to land on the ST.
In ya drawing it seems like your dimensioning Reach and Stack. These are ok and handy for comparing off the shelf frames but for a custom bike I would be dimensioning fit to the grip position (or at least the centre of the bar clamp for simplicity). That will show up the changes that a taller head tube gives more clearly.
You can also run the math quick as a right angle triangle. Assuming the same frame reach and stem length then a 50mm taller headtube at your 69° HTA will position the handlebars 20mm further away than a lower stack bike that uses spacers to gain the height.
I’ve just measured up the seat to handlebar dimensions on a bike that fits though and it seems I’m actually pretty close, I think mostly because I’m also planning on a -20mm shorter stem which has made up the difference. I’m going to measure some other bikes that fit me and see how they stack up.
makes a lot of sense, with the weight I’ll have up front a sturdy fork will be best.
Sorry, I meant the internal diameter, to fit a 30.9 seat post. The bike will be loaded for touring but I generally use a bikepacking style set-up, so just a framebag and a seatbag on the back, with the rest on the front of the bike. I’m hoping to get away with a slightly less burly frame as I won’t have any heavy panniers. I’ll see what the builder thinks.
Thanks, just been doing some reading up on this. I know the builder I have in mind has done similar frames so I’m confident they know what they’re doing. I’ll discuss with them what they can do for me!
Thanks, again it’s probably something the builder should know I guess!
Makes sense! You should defo add the saddle to handlebar dimention to your drawings. That’s the important number. Stem length is a driven parameter not a driving one.
Tom Sturdy’s advice to me was that larger front load requires stiffer top tube. I have several weird bikes with front load on the fork, on the headtube and on the front deck, frames i have both bought and built and the advice seems to be valid. Especially with the longer headtube, I would expect that your frame could actually benefit from a second top tube. With the front load, a noodly ride can quickly escalate to speed wobble/shimmy even at low speeds and that’s no fun. D. G. Wilson has like 5 pages on shimmy theory in his book Bicycling Science and the short version is that the reasons for speed wobble are not clearly understood.
That said, I don’t mean that to say that it’s double top tube or nothing. Even if the frame turns out to actually wobble, I had great luck fixing speed wobble on several cargo bikes with the Cane Creek Viscoset, which does not seem to be manufactured any more, but is sometimes still available on Ebay.
My GUESS would also be that an ovalised top tube might be stiffer in some directions than a round tube of the same diameter? Someone more experienced than me would have to confirm that.
I think an ovalized top tube makes sense to reduce shimmy because it increases lateral stiffness.
To reduce shimmy pack as much of the luggage as centrally on the bike as possible, i.e. not behind/ in front of the rear / front axle. Overall frame stiffness and damping also helps for example by running lower tire pressure.
I’ve moved things around a little on my drawing, including matching the saddle to handlebar dimension of my current bike.
You’re right, I think I’ve landed on 50mm stem because I’m trying to make the front-centre long while staying within my fit dimensions (and not using a 0mm stem or something).
I had also considered if a 44mm or tapered headtube would help with stiffening things up (and allowing larger diameter tubing) but at the length I want it would be enormous. I thought a second top tube should help enough with stiffness but if the consensus is that a larger or ovalised top tube is better still then I’ll discuss it with the builder.
Hopefully with a big frame bag I can get everything heavy central and low down, with lightweight bulky things in a seat bag. Just the dog that messes it up but hopefully I can sort a rack that is as low and close to the headtube as possible.
I have heard of the Viscoset before, good to know it might help if I do get some shimmy. At the moment I’m riding a Surly Straggler, the front rack is set quite high because of the 700c wheel and quite far forward because the low handlebars stop it from being mounted nearer the headtube. Even with all this and 2 panniers at the back I haven’t noticed issues with shimmy. The high trail and dog weight does make steering heavy though, which I hope to fix with this frame/fork.
