tonyharker

Hello i've started this thread as i'm interested on getting some feedback on an idea for bike design i've got.

I'm an engineering graduate, currently taking an MA in design, and at the moment i'm designing a bike which uses a front linkage desige to reduce the effects of fork compression on the steering geometry of a rigid framed bike, and i'm looking for advice, comments, or any other feedback. The design as i have it on paper for a 90mm travel fork reduces the change in headtube angle from about 5 degrees to about .25of a degree.

This sounds like a good thing as it will mean an almost constant trail (difference between effective headtube line contact point, and where the tyre actually touches the ground) give or take about 1mm, so the design should have far more predictable handling characteristics, as currently frames designed for 100mm forks have 70 ish head angles so that they reach the 'sweet spot in handling (typically seen as a 73 ish head angle) at around 60mm of the way through their travel, and of course the trail is varying all the time through the angle change from 70-75 ish degree headtube angle. Essentially the geometry is in almost constant compromise. Through adjusting it throughout the travel you could get a constant 73 degrees, therefore to my mind better handling.

Do people agree with this? i don't know if someone's made this and people just couldn't get on with the handling or simillar?

One knock on effect of the change in headtube angle is wheelbase elongation. Over a typical fork compression (without correcting headtube angle) the effective wheelbase lengthens by approx 20mm (for 90mm travel fork) due to the frame rocking forward. With a constant headtube angle there is the rocking forward and the effect of correcting the headtuvbe angle, therefore resulting in a +35mm wheelbase change over the full 90mm. does this sound like a problem? in theory longer wheelbase = more stability, but as the wheelbase is only extending forward the centry of mass of the bike will effectively shift backward slightly, but then this may well be compensated for if the rider rocks forward slightly as their handlebars drop.

There are a fair number of complicated factors at work, all of which i would welcome comments on, but the other important question is do mountain bikers need/want this?

cyclops

Just cheat and use BMW's telelever design. Or just copy the earles fork, or the saxon fork. or the hossack fork, (BMW currenrtly uses a modified hossack fork on its four cylider bikes), or you could go all avant-garde and copy Bimota's work on hub centre steering. The one thing all these designs have in common is excess weight compared to conventional telescopics and increased manufacturing costs. In a road motorcycle an extra kilo or three is forgivable, especially if its sprung weight, but not on a bicycle. Dont be discouraged, the forces involved in mtb front suspesion are rather less than those on a motorcyle, the scope exists for lightweight versions of the above systems. Some of the above sytems work very well, others less so. A renowned expert on the subject that publishes on the web is Brit Tony Foale, google him.

Do we need this? Do we want this? Meh... 10 years ago, or even 5, I would have said, (and did), that hydro discs were technology for its own sake and a ludicrous gimmick, now I wouldn't be without them. As tech gizmos are introduced we try them and they live or die on their own merits. Discs live, I-drive is dying, Bio-pace is dead. Cyclops sticks his head out and predicts that the 29" single speed really is a silly idea with a finite lifespan.

ed 

I don't see how this can be accomplished with a rigid frame.

Bumps move up toward the bike, thus you need suspension that moves up and down...not fore and aft.

I can understand an "anti-dive" fork but no matter what design you use, when the fork is compressed the head tube will be lower to the ground. The rear end is the fulcrum. Thus the HA will change. With a 90mm setup...you're wasting your time b/c you won't feel the effects of it with such a short travel fork. You should be working with 150mm or so.

tonyharker

hi cyclops thanks for the reply! i'm currently using a simillar underlever thing to BMW, but i've managed to design it without using the spherical bearings, and also using linkages instead of sliders and a shock like the bmw, so should be lighter, and have much less friction in the system, as well as being stiffer as i have contact points between the frame and fork with fewer degrees of freedom.

I'll just look up tony foale, thanks for the info! I know what you mean about how unpredictable the bike industry is for things like this discs, i even remember how scheptical people were of narrower chains on 9 speed, however at least mountain bikers tend to be pretty keen to find out about and try new ideas, even if they won't actually part with money to do so, so i figure as an idea the constange geometry suspension could have a fighting chance.... I do feel it is a bit of a shame how much bikes are judged on weight, but it is about the only read-out of performance on which you can easily compare bikes, so i can't see it changing any time soon. Thanks for the comments and hope to hear from you again soon. t

tonyharker

hi Chelboed, I would be using a pivoted headtube, with linkages to adjust angle and fork length through compression, therefore can have up and down movement like a normal fork, while keeping the fork a constant angle relative to the ground.

I can see what you mean about the effects being small at 90mm, this is true, but basically as i'm mocking this bike up the only shock i have will give me 90mm of travel once in place with the set up as i have it, and also i feel that all this 120-150mm travel malarky is a little ott. i think a nice handling bike with 90mm of supple and actve suspension with good tracking and stiffness could be fun to ride, and bring rider skill/enjoyment to the fore a little more. However this is just my opinion, and i know i am very much in the minority in thinking that any mountain bike can survive with less than 100mm of travel.

ed 

Pivoted head tube...very interesting. Pic's / sketches?

As for the 120-150 malarky...I have a Fox Vanilla that I can take apart and move spacers around to achieve either 100mm or 130mm. I ran it for almost a year at 100mm on a Fisher hardtail. I ran it for about 2yrs at the 130mm travel setting on a Jamis 5" dually. Now I have it on a Jamis hardtail. I have tested it at 100mm and 130mm and there is no question that the 130mm setting is far superior. I live in Kansas where XC is the mainstay. Set at 100mm, the bike is a littler snappier feeling but that benefit easily gets overshadowed by the 130mm setting in plushness, comfortable position, and aggressive ride characteristics. It's only another inch, but feels like adding two. The taller ride height makes the bike feel like it "fits better". The slacker HA is a no-brainer for aggressive sections and DH areas. It's much better for technical riding believe-it-or-not. (prob. a riding style fit) I like to skip over sections of trail that are rock-strewn or covered in roots by using another rock or root to propel me up and over the rough sections. The 130mm setting gives me a comfortable feel in that respect.

I have another bike with a 60mm fork which is really no comparison. It's like trying to compare a Pinto to a Viper...but I can say that the lower position really sucks. I feel somewhat "out of my element" all hunkered over the front end of the bike. The stem on that bike is about 1/2" longer and the bars are narrower, but even if I had the exact same cockpit...I still wouldn't like the lower front end in comparison.


So sure, it's OTT for some people but right on target for others. It's all speculation, right?

scrublover

Anti-dive stuff has been tried in the mtb world before, and has never worked out as well as for stuff with a motor. They end up with more moving parts, more complicated linkages, and more hassle.

Good luck with your project, but it sounds as if you'll come up with all of that.

mx_599

tonyharker

hi Scrublover, thanks for your input. The design is not anti-dive (although the lengthening of the wheelbase will reduce dive slightly due to the more forward wheel path, it's more about keeping goemetry constsant, aiming for the precision and cinfidence you might have riding a rigid bike while still offering comfort and grip of suspension, essentially removing all the geometry compromises which frames have been built around from the advent of suspension.

Re more linkages, more complication i totally agree, however where the linkages jenuinely achieve something i think people will put up with it, look at the new marins, or whyte frames, or VPP designs, all pretty complicated, but result in a nice axle-path, and stiff frame due to multiple connections.

Have you heard of many geometry correction systems in your experience? i've seen the whyte PRST designs which have potential to do something simillar, but are mounted on a suspension frame, so can't really work to correct geometry, as the rear-end moves too.

Thanks for your thoughts and keep them coming!

tonyharker

Hi Chelboed,

I have drawings, mathmatica calculations, and well as computer models (solidworks), so have pretty thoroughly tested my linkages, geometry set up, clearances etc., I'm intending to get feedback on the concept at the moment, as my system has several crucial differences to current systems which i would like to keep fairly close to my chest until the a little further down the line. I am very happy to describe and discuss what my design will do, but for the moment i'm afraid i can't release pictures, however those who have seen the design have so far seen no problems/drawbacks besides complication, as with any linkage, the possibility for extra weight, but due to the way the forces are distributed in the design weight is unlikely to be as large a problem as in many linkage heavy designs, AND that the system is only a computer model and the benefeits are only theoretical, this is why i'm looking for feedback, and opinions as to the usefulness of the system to real riders (as well as currently working on building a prototype to test the ideas in the real world.

Also i'm very interested to find out more about your fork preferences. Much of what you say about ride position etc. could be as easily achieved with stem etc. adjustments. I know what you mean about the stability you feel by having a slacker headtube angle. From what i've read this has more to do with the fact that you're increasing the trail on the front end which increases the stability, but makes the bars harder to turn, making the bike feel harder to knock off line, but in return slightly harder to steer when you do want to change direction. I was considering building my fork with an adjustable drop-out so that handling could be tuned by the owner through adjusting the rake without having to do more drastic things like putting on longer or shorter forks, and getting away from that 72/73 deg headangle sweetspot.

xcracer13

I'm sorry but doesn't Bionicon already have a geometry adjusting hardtail system? Kona has the Magic Link.

ed 

1 - I'm sure there are OneMillion (pinky-to-bottom-lip) CAD freaks here on the forums just itching to steal your design and bring it to fruition to make billions of dollars.

2 - Actually the feel that I get from my 130mm bike cannot be duplicated with the 60mm bike by changing stems/bars because theres:

a. Extra Plush ride
b. Slightly longer wheelbase (marginal)
c. Taller front end
d. Taller BB height
3. slacker head angle

All of these things equal a bike that's more stable, easier to manual and generally get the front end off the ground, more ground clearance, and smoother. The only thing bars and stem would do is make it a more "upright" ride which wouldn't change the HA, BB height, or smoothness of ride.

3 - I don't have much problem here in KS with getting knocked off line, nor do I feel my bike steers slowly. It's nimble and stable. Not twitchy like a steep XC racer, but not Freeride slack.

4 - How is a 72/73 deg HA a sweet spot? Are you reading "road bike" geometry books or something? I'd say 69-71 at the most. I really like mine at 70'

never

I don't know about that being a sweet spot. My trail bike has a 67-68.5 degree head angle (depending on the travel) and it's MUCH more fun to ride than any of my XC hardtails with 71/72 head angles.

Plus, the MTB world is gradually moving away from steeper head angles. Riding and terrain are progressing, and the bikes are changing. I think what you're designing/describing would appeal to a rather limited audience.

tonyharker

hi xcracer, i've just checked the bionicon website they do a full sus where the angles are adjustable, however what i am proposing is that as the fork compresses a series of linkages will alter its angle relative to the frame so that the headtube angle never changes by more that .25 of a degree throughout full compression of the fork. Many systems seem to exist which allow you to change the angles of the bike, but these are all still affected as the bike rocks forward (on a hardtail) through suspension compression. Whereas my design from 0mm compression to 90mm compression will have near enough a 73 deg head angle and around say50mm rake, whereas a traditional system would have 70 deg head angle and 50mm rake at 0mm compression, but at 90mm compression it would have a 75 deg head angle and over 60mm rake. I feel that removing this inconsistancy from the geometry would be beneficial to control on the bike. However that is just what i think and i would welcome any thoughts, if you feel it is unnecessary, or if you like that variation in handling under compression, like the steering getting a little quicker?

thanks for your comment.

cryptid01

It's the URT that seals the deal I think.

ed 

I'd still like to see how you're going to keep the head tube in relation to the suspension at a constant degree w/o changing as the front of the bike comes down.The only way to get away with it IMO would be to change the free-spinning headset to a static or fixed coupling and put the steering in front of the linkage.

No matter how you look at it...as the suspension compresses, the front of the bike will get closer to the ground and the rear of the bike will stay back where it is b/c it's a rigid frame. The fork steers "at the head tube", thus the HA will still change in real life.

junkyard

I'm not sure I understand the purpose. So, the geometry changes somewhat when the fork compresses. I've never found that any slight change in geometry has caused any issues for my riding. I mean, when you come down to it, body position is ever changing when negotiating a trail. In essense, that is changing the geometry of your body relative to the frame as well. I don't see it as a necessary innovation.

xcracer13

Sorry, I don't think it's a necessary innovation.

dminor 

But think of the radical change in geometry of an 8"-travel fork. Your effective axle-to-crown length changes so drastically on a hard compression. And that usually happens at moments when you could most benefit from the advantages of your frame's normally-slack angles.

tonyharker

hi thanks for the feedback never and chelboed,

I see what you mean about headtube angles getting steeper, however as far as i understand this is primarily to offset the longer forks, in otherwords when you stick your 6 inch travel fork on a rigid frame the angle might be 68 degrees with the fork fully extended, but then when the fork is fully compressed your geometry will have steepened up by 7/8degrees leaving you with a 75 or 76 degree headangle. This slackening of angles seems to be primarily to counter the greater change in angles with the longer travel forks, as beyond about 75 degrees headangle bikes become pretty unsettling to ride.

i'm sorry i cannot share my images at the moment, i mean no offence by this, however anything crucial to my design that i share then enters into the public domain, and i no longer have any rights to it that's it pure and simple. I'm sorry chelboed i did not mean to offend or anything else with any of my comments, and was merely commenting on what you were saying you did and didn't like about your bike set up, i'm merely trying to get a cencus on the design, and establish peoples preferences. Thank you for clearing things up.

ed 

I was of the assumption that about 1"= about 1degree change. So a 68 degree HA with a 6" fork fully compressed would be 74, right?

tonyharker

thanks for posting the pic gastro, seeing that bmw design initially made me think what that type of thing could be capable if better designed, chelboed, the bmw shows how a simillar idea can be achieved, the linkage under the frame can push out the fork where it attatches to it, theefore as the suspension compresses the arc of that arm will push the lower pivot on the fork away from the frame, thereby correcting the headtube angle as the fork compresses, and keeping it fairly close to a constant angle.

Cheers for the comment junkyard, that is one of the main problems i have with the design, acceptance, people often think what they are used to is fine, and are reluctant to try new things, if you were to imagine using a bike which kept a constant headtube angle therefore constant trail and steering characteristic, then went to something where these changed as you went over bumps, or as the forks compressed going quickly round corners, do you then think you would feel the old system was lacking. I'm not saying what i'm proposing will change the world i'm merely wondering what your opinion of it would be if considered from the other side of the looking glass?

tonyharker

hi chelboed, yep, 1 deg per inch of compression is roughly correct, seems to work out around 1.3, but will vary according to wheelbase length.

dminor 

We appreciate your need to keep the details under your hat at this point. It's always good to see 'out of the box' thinking and people working to expand what we may eventually define as 'normal.' I can definitely see the value of what you are trying to achieve; I hope that it can pan out in a practical, manufacturable way.

When you can let the cat out of the bag, let us be among the first to see it .

Captain Crash

What is going to keep the *geometry correction* device from compressing if a heavy rider gets onto the bike, and the fork was a pretty stiff one?

EDIT: After looking at the picture that gastro posted, I realized the geometry correction mechanism IS the suspension.

Duh...