the distance between the wishbones and knuckle height ? 1

[Ikac]

How the distance between upper and bottom wishbone is important. I design 3D model of tilt light car and I have problem with 3d model because I can't place the top front wishbone high. In technical view and dynamic aspect, what's happen if I place the top front wishbone lower, decrease its width and position knuckle asymmetric in the wheel, how much knuckle could be position low inside in the wheel.

 

[WolfHR]

I seem to recall that Honda and other manufacturers have been putting upper control arm ball joint outside the wheel, so it can be done. I also seem to recall that Len Terry in his book may have advised spacing them as widely as possible (a number of ~250mm/10" springs to mind*).

I think it could be argued that more closely balljoints are positioned, the bigger forces acting on wishbones will be. And I would think that all the camber/KPI/Caster/&c changes get more exaggerated with closely spaced ball joints.

* I don't know where I remember it from, but I also have a recollection of figure of something like 2/3 of wheel dia being mentioned in literaure as ballpark/target figur for distance between outer ball joints

HTH (as an appetiser before knowledgeable members join in)

 

[Ikac]

Thank you very much, it really help me. And I want to ask one more thing, could be the top front ball joint closer to the center of the wheel and the bottom ball joint respectably lower from the center of the wheel(spindle)

 

[BrianPetersen]

It's technically possible, but you may have geometry and/or ground clearance problems. Recall that the rear suspensions of traditional Jaguar and 1963 - 1982 Corvette used the rear drive shaft as the upper arm and had a separate lower arm. I've never seen a front suspension done that way.

The loading on the ball joints will be higher than with the arms higher up. What's stopping you from having the upper arm higher - to the point of having the upper arm outside the wheel as Honda and several others do?

 

[patprimmer]

The 1988 EA Australian Ford Falcon, many Hondas and at least some Mercedes Benz have the upper ball joint higher than and slightly over the tyre so as to reduce king pin inclination while minimising scrub radius I think. It also reduces camber change with lock and a need for excessive castor to counter camber change on the outside wheel.

Regards
Pat
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[GregLocock]

There are millions of cars with the upper ball joint inside the wheel. We call those short spindle SLAs. Kinematically they are fine, but I would stress that you do want as much separation as is feasible, at least in production.

There undoubtedly are advantages in placing the upper ball joint above the tire, but there are disadvantages as well (practical, not kinematic). We call those long spindle SLAs.

There is also a bit of a trend towards mid length SLAs, where the upper ball joint is alongside the tire. Obviously you tend to run a lot of KPI in that case, but other than that it seems to work fine.

Finally if you are really in trouble you could use a virtual upper ball joint as Audi do. That introduces complexity into your kinematics, increases cost and weight, and steering friction, but gives you a lot more freedom for KPI and scrub radius in particular.







Cheers

Greg Locock


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[patprimmer]

I guess a McPhearson strut has a really high top ball joint and also relatively steep king pin inclination or bad scrub radius.

The trend to bigger dia wheels with more negative offset must open up the space to get the ball joints further apart with a low king pin inclination while inside the wheel

Regards
Pat
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[Ikac]

Hey, I found the old Mercedes silver arrow have front suspension, like I want for my design. Because it is also open wheeler, I want the top front wishbone to be as low as possible, to get slick, aerodynamic shape. I think the this Mercedes has double wishbone, is there somebody know how this front suspension work and response?

 

[GregLocock]

From that photo I'd guess it is a short spindle SLA with torsion springs or possibly inboard coils and inboard or lever shocks or possibly even rotary friction dampers.

Cheers

Greg Locock


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[WolfHR]

Great guess, Greg- open coil springs (torsion bars with degressive, by some 30% at full bump, linkage were used at the rear, de Dion suspension) and hydaulic dampers, presumably rotary. Upper wishbone length was 10" and lower 11.8"- I think it was aimed at keeping minimal camber change*. Suspension travel was 5.75" (I think 3" bump and 2.75" rebound).

* that coupled with de Dion was supposed to make it understeery, which seems reasonable enough considering RWD with something like 600 BHP/ton, even though ZF cam and pawl LSD was used.