dose

Hi,
I intend to get a custom frame built, based on my own geometry. I don't have any experience designing frames so I am interested in feedback from the experts here (I hope this is an appropriate sub-forum, as I suppose you guys know the best what to look out for). It's meant to be an allrounder (for I guess my definition of it :-)); I intend to use it for street trials, dirt jumping, single tracks, and a bit of touring. I modeled it after a Spank Alfonso (medium size) frame, which I currently have and love. It basically fits the bill and everything I want to do on it I can (with varying degrees of success, probably mostly limited by my own inabilities, though). But it's an old frame, it's difficult to find proper dropper posts (weird seat tube diameter), it's non-boost, no thru-axle, front-center is a bit too short (my toes touch the front tire occasionally), I'd rather it had a slightly lower top tube (i.e., lower stand over height), it has no tapered steerer, I'd *think* it's probably pretty heavy (as it's pretty beefy). Anyway, the build is going to be titanium. I've attached a model I've created with relevant measurements (in millimeters). Also happy to upload the model of the Spank frame I used as a baseline if that would be of interest.



As I said, I am curious to hear general comments & concerns. On top of that, I'd love recommendations on tube diameters and thickness to use (I have not yet inquired about certain details with the builder). Basically, I want to make sure that this thing can take *anything* I may or may not throw at it (I don't intend to ever buy another frame, if you will): falls, heavy cases, anything. Are double/triple butted titanium tubes a thing? I'd guess that the most common fracture zone on a jump frame is probably the head-tube down-tube connection (if you case heavily). On the Spank Alfonso this piece is beefed up like crazy (I've never been worried about it there). How could that area be strengthened on a titanium frame? What should I ask for? (the builder is a professional) Any other areas to watch out for? Any feedback is appreciated.

The model already includes a dropper post (I used it for measurements) and has a sketch of stem and handlebar. Feel free to ignore those details. Some numbers may change slightly (e.g., it's not quite clear at this point if the chainstay can accommodate my tires and that obviously depends on tube diameter and other factors, but if anything changes should be minor and with chainstay specifically not affect the majority of the geometry).

guy153

First thought is that 700c wheels with only a 15mm BB drop may put the BB unnecessarily high. You may be basing that drop off the Alfonso but if that's a trials type of bike it may have much smaller wheels. Better to shoot for around the same ground clearance instead.

dose

Thanks for your response. Fair point. It's just 26" wheels (with arguably rather large tires). I have no intention to ever go to smaller wheels. I went 5mm higher than what I measured the Alfonso to have (20mm drop), to nudge it into a direction of having more ground clearance. I almost certainly do not want to put it lower than my current frame (as that height has never posed a problem in terms of ride comfort and if anything ground clearance could be larger), but could leave it untouched to be on the safe side. I understand that there are even straight dirt jump frames out there with only 14mm drop (I found that to be the case for the NS Bike Decade). I believe street trials frames (also 26") are in a different ballpark still, with -20mm or so (i.e., BB above axle line; and with that much higher).

guy153

Interesting! Trials frames are seriously weird from the point of view of basically every other kind of frame. If you've ridden frames with a similar drop and wheel size then it should be all right. In fact the ones with smaller wheels that I'm thinking of probably have negative BB drop.

Andrew R Stewart 

"intend to use it for street trials, dirt jumping, single tracks, and a bit of touring." dose

Well, that's quite a spread of use. Like the person who wants to get a car that they will drive to work, do some plowing with and sometimes take in a track day. This from someone who has little clue as to frame design I have less surprise at this request, and then asking us to help provide what will only be a comprise in every type of use.

Who is doing the actual fabrication? have they experience in designing frames (I SURE hope they do...)? Have they offered any thoughts? Perhaps we are not reading into what is really wanted well, trials bikes don't go places and have no high gears (and much of the movements are hopping in place and lunging with a single pedal stroke). Is this what you meant by "street trials" (admittedly a term I don't see used by others)? Andy

guy153

Although specialized trials bikes are very bizarre (hydraulic rim brakes, rims roughened with an angle grinder, the freewheel at the bottom bracket) people with enough skill can do a lot of tricks on much more normal bikes. So I think a compromise is possible. The main requirement is just that it be extremely strong.

dsaul

I'm betting on Waltly Titanium. They offer "design it yourself" Titanium frames for relatively low prices.

unterhausen

I was even tempted to send a design to waltly

dose

May I ask that we please stay on topic and not side track the conversation with questions that have no relevance to what I am asking? Thanks! Also, friendly reminder that you may keep any unhelpful elitist sentiments to yourself. If you have nothing to add, please stay quiet. Thanks again.

To reiterate, here is what I was hoping to get input for:
> I am curious to hear general comments & concerns. I'd love
> recommendations on tube diameters and thickness to use (I have not yet
> inquired about certain details with the builder). Basically, I want to
> make sure that this thing can take *anything* I may or may not throw at
> it (I don't intend to ever buy another frame, if you will): falls, heavy
> cases, anything. Are double/triple butted titanium tubes a thing? I'd
> guess that the most common fracture zone on a jump frame is probably the
> head-tube down-tube connection (if you case heavily). On the Spank
> Alfonso this piece is beefed up like crazy (I've never been worried
> about it there). How could that area be strengthened on a titanium
> frame? What should I ask for? (the builder is a professional) Any other
> areas to watch out for? Any feedback is appreciated.

In case it's unclear: when I say "general comments" I am referring to general comments on this very topic of this specific frame design. I am not interested in your opinion of whether Titanium frames are good/bad or to digress into providing an introduction to street trials. I hope that clarifies my questions some more.

guy153

Reynolds make some double-butted Ti, but it's very expensive and high-zoot stuff:

https://www.reynoldstechnology.biz/m...t-3-2point5ti/

I think most people use plain gauge. I saw a video about the Moots factory and they were buying tubes in 20 foot lengths, so those obviously can't be butted at all.

If I was going to make your frame I would use Reynolds 853 DZB (which is steel, triple-butted, and strong AF). I'm not convinced by gussets; I think it's better to use triple butted tubes. Yes agree HT/DT is probably the weak spot. Also HT/TT.

Street trials are on topic because how you're going to use the frame affects the design!

Andrew R Stewart 

When one posts in a public forum one has to understand that replies will be subject to those posters' manor and thoughts. I see this as part of the cost of asking for help. Please get past your dislike for potential tangents, just disregard them and take in the good replies.

My advice is to use as thick walled and as large in diameter tubing that your other parts will allow. As anything can be bent or damaged under an aggressive rider, the heaviest and strongest stuff will last longer. Sure the bike will be heavy and less responsive but it will tend to survive the best when crashed. By writing
"Basically, I want to
> make sure that this thing can take *anything* I may or may not throw at
> it (I don't intend to ever buy another frame, if you will): falls, heavy
> cases, anything."
you are saying that only the toughest frame will suit you, anything less won't. I generally take heed when absolutes are stated as the claim/need. Often I later find out that the speaker/OP actually doesn't want a bike that serves only the most extreme type of use they foresee, as that design takes away from the rest (90%?) of their riding works better with a lighter and easier handling rig. But this is why the more one rides and expands to different natures of on bike enjoyment the more likely that rider will have more than one bike. Andy (who doesn't agree with the "one bike to rule all" thought)

unterhausen

I would definitely consult with the builder for things like diameters and wall thickness. If you are going to Waltly, I'm sure they have some limits on what they want to deal with as far as materials. Other contract builders will be the same.

A number of U.S. builders externally butt their tubes because you can buy or make tube grinders that will do it. I suspect the improvement in ride is mostly a matter of how much you think it's going to improve the ride. I don't know if any of the Asian builders do that, but in the U.S. it's a high price option.

As Andy says, you don't get to control the contents of your thread and I'm sorry if anything sounded elitist to you, but I'm at a loss as to what it might be.

dose

Thanks for the responses. And for the pointer to Waltly. I do see that they have a double butted options for most tubes, actually, and their form gives me at least some ideas on tube diameters. Yes, I will consult with the builder, but wanted to get other people's opinions first. On the topic of wall diameter & thickness, are there general suggestions with respect to the relationship between the two (specifically for Ti)? I understand that bigger is generally better, but are there sweet spots if I, say, pin diameter or tube thickness to a certain (personally chosen) upper bound? E.g., browsing the forum I stumbled over the idea that wall thickness should not go below 1/50th the tube's diameter. Is there a point of diminishing return with respect to increasing wall thickness for any particular diameter? Any analyses you could point to?

> you are saying that only the toughest frame will suit you, anything less won't.

It's not an ideal formulation and perhaps I should have left it out. I kind of know what the frame will be subjected to (which realistically is unlikely to be that much in absolute terms; certainly not on a regular basis), but it is hard to convey in words. I am sure that picking a specific super strong Titanium is unnecessary for my use case, especially if it comes at a steep increase in cost. But perhaps a good way of getting to a useful answer is by way of comparison: geometry aside (which obviously has an impact; but for the sake of argument we shall assume equality) let's say I want to match the strength of a given 7005 aluminum alloy tube. Are there general guidelines? If I match diameter and thickness of an aluminum tube in Ti the result should be stronger (and heavier). How small could I go to come out roughly equal (I do understand that different materials react differently to different kinds of strengths, I am mostly looking for rules of thumb).

unterhausen

There is a lower limit on wall thickness, so bigger diameter tubes tend to be heavier and stiffer. And not be as thin as they could be in a world without buckling and dents.
I imagine Waltly has tubes that are bigger diameter than you really need. But it's somewhat difficult for me to even come up with a criteria by which you would decide.