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?

ed 

Why did you start this thread with 2 different titles?

fosmith

i've always tried to ride "Platform" forks that resist compression unless you hit a bump. Manitou R7 with SPV (now Absolute damping) Fox X series forks with inertia valve, or Rock Shox with Floodgate. If you have a fork with lockout and blowoff threshold adjustment, it will have the same results.

I do hate it when you are diving into a turn, the fork compresses under load and steepens the head angle.

tonyharker

sorry it came up with an error when i initially tried to post, then i just read through and again, before retrying to post, and thought i might try and use a more interesting sounding title.. did try to remove one, but couldn't see how, fortunately the admins have taken care of it. thanks for pointing it out. t

ed 

Still though...I'm interested to hear your remark to my post in the other thread.

tonyharker

thanks for the reply fosmith, do you think there might be advantages , eg. bumpy corner? to the system, or do you feel everyhting is adequately taken care of by platform systems? i've tended to ride rigid untill recently, and the feeling of steering geometry changing (in fast cornering, or bumpy line changes) i still find pretty unnerving (although going from 0-5 inches of travel is likely to feel a little weird), and i just wondered if this might be a good solution.

The other question that popped into my mind was whether being do used to the geometry change with suspension might make a constant head angle feel weird, it could have advantages yet just be too different for people to feel it was worth getting used to.

any thoughts greatly appreciated.

tonyharker

oops, sorry turns out other thread is still there, just a little lower down the list... embarassing.

rankin116

Ask to have them merged.

mx_599

DirtPedalerB

I'm trying to think how this would work , maybe some kind of shock loaded headtube that compresses backward as the fork compresses preserving the bar height and angle.. I may be way off, how does it work?