large wheels\low COG

[5lab]

the toyobaru thing has got me thinking..

if you had a car with a low enough cog, and high enough axle stubs (driven by a large rolling diameter of the wheels) - could you set it up so the suspension was really soft, but it had no body roll? the forces would go completely laterally into the wheels, rather than across and down, so you could have a very compliant ride and no body roll?

the gt 86 has a 47cm cog. that means wheels with a rolling diameter of 36" to fit my plan (I acknowledge that just fitting big wheels wouldn't work, the suspension etc would need a bit of redesign). if we allow for 4" tyre sidewall, that's 28" wheels. Still pretty big by todays standards, but not a different league



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win?

[davepoth]

I'm not sure it would be possible. The springs would need to be above the stub axle, as well as some structure to support those. The added unsprung weight of 28"s would make the ride terrible anyway.

[_g_]

Presumably the springs could still be mounted fairly central to the wheel?

For instance motorbike rear shocks often have the shock (with spring) going through the main swingarm then anchored below it, to give it a bit of extra space.

I'm a bit fuzzy this morning; but does the size of wheels actually make THAT much difference to body roll or are there other factors?
I can see the obvious logic from having CoG central to the wheel, but not sure it works that way in practice?

[5lab]

_g_ said:

I'm a bit fuzzy this morning; but does the size of wheels actually make THAT much difference to body roll or are there other factors?
I can see the obvious logic from having CoG central to the wheel, but not sure it works that way in practice?

i don't know. in my mind, the force on a regular car pivots around the cog, so on cornering, the load is diagonally downwards on the outside wheel

if you had the cog at the same height as the wheel center, the load would be directly sideways??

if you had the cog below the wheel center, the car would lean 'into' corners



do heavier wheels ruin ride quality? a heavier wheel meants its more 'reluctant' to move, and certainly accelleration may be blighted, as well as grip levels, but if a wheel is less likely to move up or down, surely that'd improve ride quality if anything? also larger wheels would smooth the impact of any bump by having a more gradual slope from ground upwards (in teh same way that 29er mountain bikes offer a smoother ride than their 26er counterparts?)

[Kozy]

5lab said:

i don't know. in my mind, the force on a regular car pivots around the cog, so on cornering, the load is diagonally downwards on the outside wheel

if you had the cog at the same height as the wheel center, the load would be directly sideways??

if you had the cog below the wheel center, the car would lean 'into' corners

No it wont work, the diameter of the wheel has no bearing on this matter, what you are looking at is the roll centre. The car does not pivot around the CoG and despite common misconception, nor does it roll around the roll centre. If you located the RC at the same height as the CoG then yes, there would be no roll but you would still have the load transfer and it would be instantaneous, travelling directly through the suspension linkages, meaning pretty much an instant loss of grip at the contact patch.

In addition to this you also lose the ability to tune the handling with the springs, bars and dampers.

Roll is not bad so long as the wheels orientation to the road is OK, and you want MORE load transfer if you want to go faster.

Edited by Kozy on Monday 5th December 12:18

[5lab]

Kozy said:

5lab said:

i don't know. in my mind, the force on a regular car pivots around the cog, so on cornering, the load is diagonally downwards on the outside wheel

if you had the cog at the same height as the wheel center, the load would be directly sideways??

if you had the cog below the wheel center, the car would lean 'into' corners

No it wont work, the diameter of the wheel has no bearing on this matter, what you are looking at is the roll centre. The car does not pivot around the CoG and despite common misconception, nor does it roll around the roll centre. If you located the RC at the same height as the CoG then yes, there would be no roll but you would still have the load transfer and it would be instantaneous, travelling directly through the suspension linkages, meaning pretty much an instant loss of grip at the contact patch.

In addition to this you also lose the ability to tune the handling with the springs, bars and dampers.

Roll is not bad so long as the wheels orientation to the road is OK, and you want MORE load transfer if you want to go faster.

Edited by Kozy on Monday 5th December 12:18

you might have to help me with some of this. I thought roll was detremental to both reaction speed of steering (as the first bit of time is just the car moving over the wheels) and overall grip levels - hense why tarmac race cars have very little in the way of suspension (just enough to keep all 4 wheels on the tarmac). If you ran a car with no suspension on completely smooth roads, surely it'd have more grip than anything else?

[Kozy]

Roll is detrimental to suspension geometry, which is in turn detrimental to grip. You'll generally gain positive camber on the outside wheels in a corner, and at some point the positive camber combined with the load increase from load transfer will overcome the grip available and the tyre will slide. You can negate this by figuring out the maths/geometry and setting the static camber to a value that at maximum roll angle, orientates the tyre optimally to the road, at say -0.5°. I did this with my car and with relatively supple suspension I must say it worked exceptionally well. The downsides of running excessive camber are increased and un-even tyre wear (though it is normally toe that is the biggest player here) and a slight decrease in longitudinal traction.

You can control the geometry by stiffening the whole thing up sure, but harder suspension means less grip. Aside from alignment, grip is all about keeping the load on the tyre as constant as possible, which soft suspension is great at as it can easily massage the contact patch into all the undulations in the road surface. Stiffen it all up and the tyre starts to bounce, the load variations become large and fast and as a result, you lose grip. Yes, if you drive on a glass smooth surface then it can work, but that's not a real life scenario.

You are right about roll being detrimental to reaction times, but it is a balancing act between feel, feedback and response.

So you can gain grip by stiffening by virtue of geometry control, but lose it at the same time through contact patch load variation. Finding the optimum point is the tricky part, and is what makes good suspension so expensive.

Edited by Kozy on Monday 5th December 12:59

[Mr2Mike]

You can raise the roll center far enough to get it level with or above the COG without huge wheels, but you tend to gain some pretty horrible handling traits such as jacking.

[Kozy]

Mr2Mike said:

You can raise the roll center far enough to get it level with or above the COG without huge wheels, but you tend to gain some pretty horrible handling traits such as jacking.

Jacking is exactly what the OP is trying to achieve, it is simply the lateral version of anti-dive/squat, so 100% geometric anti-roll means massive jacking loads which keep the sprung mass fully supported.

As you say, it really is quite undesirable.

[littleredrooster]

5lab said:

...if you had the cog at the same height as the wheel center, the load would be directly sideways??

No it wouldn't. It goes through the point of reaction - i.e. the contact patch of the tyre, not the centre of the wheel. The wheel merely transmits the force.

5lab said:

if you had the cog below the wheel center, the car would lean 'into' corners

For the same reason quoted above, you would need the CoG to be lower than ground level to achieve this. It's the same principle as a high-wire walker who has an extended pole with weights below the wire; almost impossible to fall off.