Hi Richard, Sorry for the delay in response - way busy preparing for a conference next week...
OK keep in mind its been a few years since I have even had to contemplate mech design, and I have never worked in mechanical design of safety critical components. Below should not by any means be considered authoritative, and others on the list may have more reliable comments. Below is just my "thinking out loud"... * Firstly, the areas over which the forces are distributed. 50x75mm plates may be under 10 times less stress than say 4x10mm dia. (it isnt as clearcut as this though, seeing as this assumes the bolts are the only components over which the shear is transferred, and that the bolts have the same material properties as the plate/floor interface, which isnt the case) * Bolts are designed for axial tensile loadings, whereas in the proposed application the loading is presumably mainly shear. High tensile bolts in this application may actually be weaker? (strength comes at a price - high strength materials can be more brittle) * A common yet simple design criteria often used in initial calcs is that failure of a component will occur when the max shear stress reaches half the yield stress measured in a simple axial load test. The same theory therefore implies that a material's strength in shear is half its strength in axial loading. * No automotive safety critical components rely on bolts to support shear loads eg the spigot joint between strut and ball joint instead of relying on the bolts. * Although the floor is thin sheet, roll cage legs are located in the "corners" where presumably strength is added by proximity to the right angle section. Also in this loading, the cage mount would be trying to "puncture" the floor. I think the energy required to puncture material like this is far greater than that required to deform components in more "normal" loadings eg tension?? As mentioned earlier, this stuff is not "bagging" the idea, but just possible reasons that I can think of which may be drawbacks. I figure, given the analysis technology available today, there must be some pretty conclusive reason why the traditional mounting method is still used. Perhaps a reputable roll cage fabricator may have better answers. Anyone else have any ideas? Andrew > Andrew > > Thanks for your considered thoughts on the vertical roll cage mounting > points. I understand your points. > > The guts of the issue is whether the compressive forces acting on a > relatively small area of the floor pan (50 by 75mm) which is only thin > metal, would be more likely to rip through the floor before the same forces > acting through 4 high tensile bolts under shear pressure, snapped them. > > As a mechanical engineer do you have any info on the likely stronger > scenario? I have always felt that bolting to the thinnish flat floor plate > isn't that strong a mounting point. Bolting to a curved section, like the > wheel well, always seems to have much more strength. > > ----- Original Message ----- > From: "GREENBURY, Andrew Robert" <[EMAIL PROTECTED]> > To: <[EMAIL PROTECTED]> > Sent: Sunday, 7 July 2002 16:49 PM > Subject: Re: roll cage in 1600, mounting locations > > > > hi Richard, > > > > Interesting idea. To my mind, in a roll over with a cage with floor > > mounts, the loads would be transferred in a direction generally along the > > cage legs, and distributed over the mount area normal to the legs in a > > compressive manner. Because the majority of the load transfer to the rest > > of the car would be in this manner, the affixing bolts arent put into > > tension or anything, and so would do little more than locate the mounts, > > and maybe some shear transfer due to force transfer not being directly > > along the cage legs etc. > > > > However, in the same rollover scenario with vertical mounts, the force > > transfer from the cage to the car will be in shear as you pointed out, but > > limited by the capabilities of the bolts. Remember stress=force/area; a > > fat man lying on a bodyboard probably has more chance getting across a > > frozen lake than a small lady wearing high heels, because her small weight > > is distributed over tiny areas. > > > > In the worst case the transfer would only be through the combined small > > cross sectional area of the bolts (high stress concentration), best case > > would be this area plus some of the mount area (again limited by the > > bolts putting the mounts in tension). > > > > Just my thinking out loud - anyone got any views? > > > > Andrew > > > > On Fri, 5 Jul 2002, Richard Clough wrote: > > > > > Can anyone think of a reason why roll cage mounting brackets could not > be > > > installed vertically along the inside of the door sills rather than on > the > > > floor? > > > > > > My idea is to make a long mounting plate and weld it to the inside of > the > > > sill, or inside the sill cavity when the outer sills are replaced. This > > > would allow for the forward bars in the footwell and the centre hoop to > be > > > mounted off the bottom of the wall rather than to the floor. > > > > > > My feeling is that these vertical roll cage mounts would have some > > > advantages. They would be stronger than floor mounts in a roll over, as > the > > > forces would be shear forces rather than flat fronted forces on a thin > metal > > > floor pan. The full length side bars would stiffen the body. There are > no > > > bolts protruding under the floor getting damaged, and no problems > fitting > > > carpets or concealing holes in the floor if the cage is removed . > > > > > > Comments on this idea? > > > > > > ----- Original Message ----- > > > From: "James Morrison" <[EMAIL PROTECTED]> > > > To: <[EMAIL PROTECTED]> > > > Sent: Friday, 5 July 2002 12:30 PM > > > Subject: Re: roll cage in 1600, pedal locations > > > > > > > > > > my mates 1600 rally car has a full cage and stock pedal and yeh its > pretty > > > damn close to the cage, could get annoying. You may wanna think about a > > > bluebird (i think) pedal and cable accellator. > > > > > > > > James > > > > > > > > --- JUSTIN FRIEDRICHS <[EMAIL PROTECTED]> wrote: > > > > >Guys, > > > > >Do you need to modify the position of the acc pedal > > > > >and brake pedal when a roll cage is fitted to a 1600. > > > > >Cos I think the cage legs need to be very close to the > > > > >acc pedal. > > > > > > > > > >Cheers > > > > >Justin > > > > > > > > > >http://www.sold.com.au - SOLD.com.au > > > > >- Find yourself a bargain! > > > > > > > > > > > > > _____________________________________________________________ > > > > Get Your free Ozdat Email Account > > > > ---> http://www.ozdat.com > > > > > > > > _____________________________________________________________ > > > > Promote your group and strengthen ties to your members with > > > [EMAIL PROTECTED] by Everyone.net http://www.everyone.net/?btn=tag > > > > > > > > > > > > > > > > > > > --membersozdat------------------------------------------------------- OZDAT Mailing List Please Note:- Send (un)subscribe requests to [EMAIL PROTECTED] Send submissions to [EMAIL PROTECTED] No unauthorised redistribution of this email http://www.ozdat.com/ozdatonline/index.htm http://www.ozdat.com/ozdatonline/listindex.html http://www.mail-archive.com/[email protected]/ ---------------------------------------------------------------------
