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[Rods2]

Thinking about MotoGP bikes where they have found lack of frame flex decreases tyre adhesion over slight bumps, does the same apply under some conditions (like bumps with a loaded suspension) if you have too stiff a chassis on a car?

[chrisgixer]

Thinking about MotoGP bikes where they have found lack of frame flex decreases tyre adhesion over slight bumps, does the same apply under some conditions (like bumps with a loaded suspension) if you have too stiff a chassis on a car?

Not sure that applies to cars in the same way, because, as I understand it, the frame flex required in bikes is related to lean angle. And they are reaching some massive lean angles these days. 64/5 degrees in cases.
 So a bikes suspension works in a vertical plane, same as cars, but, as the bike leans over there's a sliding scale of decreasing travel verses an increasing scale on bump height. By that I mean, the more the bike leans over the higher the bump becomes along the new ANGLED plane of bikes suspension travel as its leant over.

Effectively. Lean the bike over far enough, approaching 90degrees say (hopefully not or the rider has binned it ) and there will be zero suspension travel in relation to bump height of the roads in perfections, and your then looking at frame flex to absorb the bumps.
 At the rear they look at swing arm flex to achieve this. Massively braced up and down, but thinner side to side when viewed with the bike vertical.  The idea is that its incredibly stiff in the direction of suspension travel, with slight flex in the side view to help with lean angle. Up front the frame sections forward of the engine mounts do the job there.

....afaik anyway.

Apart from anything else, cars lean the wrong way in a corner. Where as bikes lean in obviously. Much better. Forcing the tyre and suspension into the floor, increasing grip level as it goes. (Ignoring the all important direction of gravity for a minute, which is most inconvenient) It's not really something road riders will encounter, until they get into competition and slicks that generate serious grip levels, and the lean angles go up. Or down, depending on your point of view.
 The way round it, to a point, is for the rider to,pick the bike up in the corner while maintains body position. It's often described as hanging off. Hence the knee and elbow down styles we see these days. If they kept bike, head and body in line with lean angle, they'd ride off the edge of the tyre first off, and second the suspension would become less effective over bigger bumps, and frame flex plays more of a part. It's also another reason why a higher tyre profile/smaller wheel sixes are used, so it can deflect and absorb bumps a bit more. Tyre being the first point of suspension

That's my understanding of it anyway. Not saying its correct mind. Just how I understand it.
 

[TheBoy]

Is there an assumption, WRT racks, that the subframe is massively more rigid that what its bolted to? I thought they worked together, as a chassis, to give the rigidity?

[chrisgixer]

Is there an assumption, WRT racks, that the subframe is massively more rigid that what its bolted to? I thought they worked together, as a chassis, to give the rigidity?

Er, not sure I understand.

So far, my thoughts relate to the comparison of a steering box and idler bolted to a body, that's controlling wheels that are bolted to subframe(, via wishbones.)

Versus a rack that bolted to the same subframe that controls the wheels. This eliminates any possible flex in the body. Unless flex in the body is transfers to the subframe as well.

The reason for comparison is eliminating the "possible" flex in the body. As in when jacked on one corner....etc.


Or have I misunderstood?

[chrisgixer]

More ramblings. Along the same lines...

Put another way.

The box and idler are bolted at a height x amount above the level of the steering linkage, on the side of the frame rails/inner wing, for want of a better description. Call it body for now.

If the body flexes then the flex might mean movement on a plane equivalent to the suspension strut towers moving in and out. Slightly. If accepting that as possible, which is arguable obviously, then a chavy item such as a strut brace might help. Just to give another way of explaining it.

But, if the box and idler bracket are removed, then the only possibility for that body flex to affect handling is for tower flex to affect camber. A minimal or, less off a consideration given the flexible top mount and and lower ball joint, and the give in those anyway.


In sort, a rack would remove that possibility of body flex affecting steering, of you accept it as possible in the first place of course. Failing that, a strut brace. Ignoring the nasty design floor of drilling holes in the strut tower and causing fatigue cracks and failure of course.

[Nick W]

You really need to measure the flex that you're concerned about. That will be a fun job, and I bet you'll be surprised by any figures that you come up with.

Actually, I think you're massively over-thinking this and should get out more!

[chrisgixer]

You really need to measure the flex that you're concerned about. That will be a fun job, and I bet you'll be surprised by any figures that you come up with.

Actually, I think you're massively over-thinking this and should get out more!

Its a good job I don't give a flying shit what you think then. Isn't it.

[05omegav6]

The subframe is no more rigid than the body it's attached to, (don't forget it's C shaped...).

The box, bolted securely will only move very slightly as it's only attached to a small area on one chassis leg...

Whereas the subframe will flex across the car, and any rack attached to it will flex at both ends during cornering, by virtue of being attached to opposite sides of the car.

A strut brace will stop the turrets from flexing inboard and outboard under suspension load, (this is minimal anyway because the turrets are mounted against the bulkhead), but won't prevent chassis twist during cornering. Better off fitting diagonal bracing across the radiator aperture.

[05omegav6]

As a thought, assuming the V8 is roughly in postion, is there any room across the car to mount the rack directly to the car, approximately where the tie rod currently sits

[chrisgixer]

The subframe is no more rigid than the body it's attached to, (don't forget it's C shaped...).

The box, bolted securely will only move very slightly as it's only attached to a small area on one chassis leg...

Whereas the subframe will flex across the car, and any rack attached to it will flex at both ends during cornering, by virtue of being attached to opposite sides of the car.

A strut brace will stop the turrets from flexing inboard and outboard under suspension load, (this is minimal anyway because the turrets are mounted against the bulkhead), but won't prevent chassis twist during cornering. Better off fitting diagonal bracing across the radiator aperture.

Viewed that way, Mmmmyeah sort of agree. But viewed the other way, as presented?

[chrisgixer]

As a thought, assuming the V8 is roughly in postion, is there any room across the car to mount the rack directly to the car, approximately where the tie rod currently sits

Position of the tie rod is deceiving. As its u shaped. To clear the sump. The ball joints are higher that the lowest point of the centre tie rod.

So to fit a rack, the lowest part of the tie rid will mean the rack will need to be 50mm higher. The jury is out as to weather the bonnet will shut throttle body is the highest point and is a common problem on the monaro and r8's as the throttle body often rubs the bonnet lining.
 A bonnet bulge will work, with a bit of Sammy. But the more serious concern is the height if the transmission tunnel.

There is a bridge shape to the sump from the original design of the donor car. But even so, some of the webbing may need a nibble.

The rack Rats used, from an Almera, appears to be a fraction long though. It seems they may not have allowed for the fact that the wishbone pivot points toe out. They are not parallel.

....Or I have the wrong Almera rack which sounds more likely.

[Marks DTM Calib]

I would not use expanding foam anywhere......having experienced its use on Mk2 coaching stock it is BRILLIANT at holding water and NEVER dries out again.