Best style of suspension?
- Thread starter titch__16
- Start date
There is no real performance difference between 68 & 73mm BB shells
The advantage of a 73mm BB shell is that you can move the chainstays further apart giving greater tyre clearance than with a 68mm BB shell
I'd contact a frame builder and see if they will sell you what you want
The advantage of a 73mm BB shell is that you can move the chainstays further apart giving greater tyre clearance than with a 68mm BB shell
I'd contact a frame builder and see if they will sell you what you want
titch, I'm not sure which modelling software you're using (did someone say 3Dmax?) but I'd strongly suggest looking carefully at where the compressive and shearing forces are going through your frame.
I am not an engineer, but I am a professional bike mechanic (Cytech 3, for those to whom it matters!) and I've seen a lot of broken frames and analysed how they've broken: the point where your rear shock attaches to the seat tube looks incredibly weak, to me. If you're planning on a bolt-on collar, it will need to be exceptionally strongly clamped, or any compressive force will simply twist it around the seat tube. Your seat tube will need to be strengthened in this area to withstand both the clamping and twisting forces. Don't make the mistake of thinking that all the force from the rear end compressing will be absorbed by the rear shock; the shock needs a solid mount point at both ends, if one of them is weak then it will not compress, just transfer all the force into the mount point and twist it round the frame.
I don't want to put a dampener on your design, I'm interested by your process and I hope you come up with something innovative and functional. But I also hope we can all give you the benefit of our experience, as well as some encouragement.
'one-legged' fork designs can work because there's a strong stable mount point around the head tube and front hubs can be built with big bearings, so the compression forces that would twist the wheel sideways can be minimised. I used to have a USE S.U.B. fork and I was surprised how strong it was and how well it tracked - very stiff.
The problems I foresee with your 'one-legged' back end design stem from the twisting forces that will be created by suspension compression and by pedalling force. Your frame simply does not look strong enough to withstand these forces: the pivot by the bottom bracket seems very small, although the 'beefed-up' chainstays may help withstand the twisting force, and the top mount design on the seat tube seems far too weak, as I mentioned earlier.
Again, sorry for putting such a downer on it, I mean no offence, I hope this is constructive criticism and gives you some points to work on! All the best,
Pete
I am not an engineer, but I am a professional bike mechanic (Cytech 3, for those to whom it matters!) and I've seen a lot of broken frames and analysed how they've broken: the point where your rear shock attaches to the seat tube looks incredibly weak, to me. If you're planning on a bolt-on collar, it will need to be exceptionally strongly clamped, or any compressive force will simply twist it around the seat tube. Your seat tube will need to be strengthened in this area to withstand both the clamping and twisting forces. Don't make the mistake of thinking that all the force from the rear end compressing will be absorbed by the rear shock; the shock needs a solid mount point at both ends, if one of them is weak then it will not compress, just transfer all the force into the mount point and twist it round the frame.
I don't want to put a dampener on your design, I'm interested by your process and I hope you come up with something innovative and functional. But I also hope we can all give you the benefit of our experience, as well as some encouragement.
'one-legged' fork designs can work because there's a strong stable mount point around the head tube and front hubs can be built with big bearings, so the compression forces that would twist the wheel sideways can be minimised. I used to have a USE S.U.B. fork and I was surprised how strong it was and how well it tracked - very stiff.
The problems I foresee with your 'one-legged' back end design stem from the twisting forces that will be created by suspension compression and by pedalling force. Your frame simply does not look strong enough to withstand these forces: the pivot by the bottom bracket seems very small, although the 'beefed-up' chainstays may help withstand the twisting force, and the top mount design on the seat tube seems far too weak, as I mentioned earlier.
Again, sorry for putting such a downer on it, I mean no offence, I hope this is constructive criticism and gives you some points to work on! All the best,
Pete
titch__16
Retro Guru
Thanks for the advice, I've just redesigned a couple of mounts. One is a clamp with a higher surface area, the other will basically be a tube to fit directly over the seat tube and the welded into place.
The BB pivot will have a 10mm bolt and both ends of the rear fork will have 6mm. I think they should both be sufficient, but still I'm open to suggestions
The BB pivot will have a 10mm bolt and both ends of the rear fork will have 6mm. I think they should both be sufficient, but still I'm open to suggestions
Titch, I'm with Pierre... I don't know if the seat tube will be able to withstand the twisting and shear forces of the rear shock. Perhaps moving the mount to a gusset between the seat tube and the top tube, so the top tube takes some of the load?
Of course, your design may be fine. A lot of existing bikes have their shocks mounted to the seat tube, but a one-sided suspension changes the dynamics a bit. Unfortunately, without sophisticated software (or engineering knowledge and a slide rule ) that can analyze the stresses, you're going to have to rely on trial-and-error. And since you may not have the resources to do destructive testing and build multiple prototypes of the frame, you'll probably have to be conservative in your design in order to eliminate the "error" part of the equation. Which means, in the end, the weight-saving benefit of the one-sided design may be negated or even reversed.
But still, go for it. You'll certainly learn something, and you may hit the sweet spot and end up with a great bike.
Of course, your design may be fine. A lot of existing bikes have their shocks mounted to the seat tube, but a one-sided suspension changes the dynamics a bit. Unfortunately, without sophisticated software (or engineering knowledge and a slide rule
) that can analyze the stresses, you're going to have to rely on trial-and-error. And since you may not have the resources to do destructive testing and build multiple prototypes of the frame, you'll probably have to be conservative in your design in order to eliminate the "error" part of the equation. Which means, in the end, the weight-saving benefit of the one-sided design may be negated or even reversed. But still, go for it. You'll certainly learn something, and you may hit the sweet spot and end up with a great bike.
Nice! That's exactly what I was thinking of, and your image has saved me the bother of having to try and describe it!
I'd certainly make the mount point much chunkier too - at the moment it looks a bit like a canti / V-brake post, and you'll need something more substantial to withstand the force of you and the bike bouncing on it without shearing off. If it doesn't need to rotate 360 degrees, it doesn't need to be round... just an idea, but you could reinforce the side of the post that points up toward the top tube while still maintaining a perfectly functional (but non-circular) pivot point bearing. If the bearing's not a complete circle, maybe a cap over the entire pivot (as part of the shock arm) will both keep the mud out and prevent the arm 'pulling' off the frame.
Sorry that's a bit of a sketchy description - as you've made clear, a picture can tell a thousand words! I'll attempt some scribbling if you want me to try and clarify...
I'd certainly make the mount point much chunkier too - at the moment it looks a bit like a canti / V-brake post, and you'll need something more substantial to withstand the force of you and the bike bouncing on it without shearing off. If it doesn't need to rotate 360 degrees, it doesn't need to be round... just an idea, but you could reinforce the side of the post that points up toward the top tube while still maintaining a perfectly functional (but non-circular) pivot point bearing. If the bearing's not a complete circle, maybe a cap over the entire pivot (as part of the shock arm) will both keep the mud out and prevent the arm 'pulling' off the frame.
Sorry that's a bit of a sketchy description - as you've made clear, a picture can tell a thousand words! I'll attempt some scribbling if you want me to try and clarify...
Just a thought, but could you have either a dual shock either side of the frame in your last image and have your single leg mounting to a linkage joining the shocks together? I'm certainly no engineer and have no experience in this area at all, but all the mention of shearing/twisting worries me, and by having something on both sides would, at least in my head, equal out the forces? I'm thinking of triple crown fork style, but with only one leg.
I initially thought of a dummy side that worked with the main shock, providing some rigidity and support to the main shock, very much like early judy forks, but maybe this isn't such a great idea, but I thought I'd share it just in. Ase it prompted debate and further ideas.
Another thought struck me as I was typing, does the one leg have the shock in it? Following on from my initial idea above, you could have the suspension unit piercing the seat tube, much like the old rts, but higher up of course so you can still follow through with the one sided design. Again, in my head at least, the forces will be being applied to something centred, so would that reduce the stress put through the area you mount it to?
Loving your work so far, it's great to see the idea evolve.
I initially thought of a dummy side that worked with the main shock, providing some rigidity and support to the main shock, very much like early judy forks, but maybe this isn't such a great idea, but I thought I'd share it just in. Ase it prompted debate and further ideas.
Another thought struck me as I was typing, does the one leg have the shock in it? Following on from my initial idea above, you could have the suspension unit piercing the seat tube, much like the old rts, but higher up of course so you can still follow through with the one sided design. Again, in my head at least, the forces will be being applied to something centred, so would that reduce the stress put through the area you mount it to?
Loving your work so far, it's great to see the idea evolve.
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