Interesting photo of rally suspension

I love rally cars and this shot shows some interesting stuff, at least for me. Note the amount of droop and the angles the front and rear struts are at. Easy to see how they can jump and stick the landing with all that travel......

If you find a picture of the knuckles the strut placement makes sense. The bottom of the strut goes down past the driveshafts to the bottom of the arm. That makes the strut longer and gives it more travel. It's a neat trick. 

TRAVEL GOOD :)

That's my favorite part of rally, watching that suspension just suck up the big hits. The rebound control is amazing.

Negative caster?

In reply to Rocambolesque :

The struts are not in line with the steering axis.

This is really interesting to me, in a "why dey do dat?" kind of way, because traditionally the strut was ahead of the ball joint, because this introduces brake antidive.  The one way that WRCars have anything in common with 3rd gen F-bodies and Fox body anythings...

Rocambolesque said:

Negative caster?

No. The top pivot and the lower ball joint are fairly conventionally located, it's just as Seth indicated, the shock body passes behind the axle center line to increase its length. I've seen other rally cars where the shock passes in front of the axle.

You can see the ball joint just peaking out below the brake disc.

Pete. (l33t FS) said:

In reply to Rocambolesque :

The struts are not in line with the steering axis.

 

This is really interesting to me, in a "why dey do dat?" kind of way, because traditionally the strut was ahead of the ball joint, because this introduces brake antidive.  The one way that WRCars have anything in common with 3rd gen F-bodies and Fox body anythings...

The strut is pretty close to vertical there. Put it in front and it's going to have a pretty serious angle to it unless you move the strut tower forward as well which may or may not be possible considering drivetrain, hood line, etc (maybe rules too?) and would result in weird caster. Being vertical it's going to be much happier about taking bog forces like landing from a big jump like that than if it was laid back a bunch.

dps214 said:

Pete. (l33t FS) said:

In reply to Rocambolesque :

The struts are not in line with the steering axis.

 

This is really interesting to me, in a "why dey do dat?" kind of way, because traditionally the strut was ahead of the ball joint, because this introduces brake antidive.  The one way that WRCars have anything in common with 3rd gen F-bodies and Fox body anythings...

The strut is pretty close to vertical there. Put it in front and it's going to have a pretty serious angle to it unless you move the strut tower forward as well which may or may not be possible considering drivetrain, hood line, etc (maybe rules too?) and would result in weird caster. Being vertical it's going to be much happier about taking bog forces like landing from a big jump like that than if it was laid back a bunch.

The part that has me curious is the angle of the control arm axis, and if they changed that to deal with the prodive-geometry in the strut.

I don't think suspension forces from road forces is going to have any signfiicant effect on the struts.  They are HUGE diameter upside-down units, and IIRC use linear bearings, not bushings.

I guess I wasn't clear, I didn't mean the struts weren't strong enough (though minimizing bending forces on something that gets used that hard can only help) I mostly meant the better motion ratio meant the springs and dampers don't need to be as beefy to handle the big forces like landing from ten feet in the air. Basically if you have to misalign the damper from the steering axis, it seems like it would be more efficient to move it closer to vertical than further away.

In reply to dps214 :

Well.... due to caster, and the angle of the lower control arm, the axle centerline doesn't move straight up and down anyway.

Look at practically any four-bar or SLA front suspension, the upper control arm generally will pivot on an axis pointing down to the rear.   This is for brake antidive: as the suspension cycles, the knuckle will rotate fore and aft.  The upshot is, braking forces from the rotor on the caliper will try to rotate the knuckle forwards, and this torques the suspension downwards.

In a strut suspension, you gain this effect by laying the strut axis back, either with increased caster, or moving the bottom of the strut forwards in relation to the ball joint.

Now, as for the lower control arm...  If you lower the rear pivot/raise the front pivot, forward thrust will push the suspension down into the ground.  This is why "anti lift kits" work so well on Subarus: they lower the rear pivot, so acceleration forces increase front grip relative to a neutral setting.  Downside is that longitudinal braking forces tend to go the opposite way.

Older WRCars had a lot of control arm angle, for increased grip under acceleration, and then they counteracted the braking force downside by increasing the brake antidive by moving the bottom of the strut forward.  They had to do that anyway, to allow for the super long strut bodies, so they may as well do it in a beneficial direction.  (This is one reason I really like R50/R53 Minis: they have this sort of geometry, albeit not as extreme)

The softer your suspension, the more closely you have to pay attention to your anti's, because the chassis IS going to want to move around a lot under acceleration and braking.  And those modern long travel suspensions have some remarkably soft springs (I have heard in the 100-150lb range in some cases!) and they handle bumps and jumps with some interesting damping characteristics, like inertia sensitive valving to tell the difference between "ride control" and "bumps" and "landing", and hydraulic bump stops, and who knows what else.  Tuning all that has to be an interesting process!

This all is why I am deeply curious.  The geometry looks "backwards".  It's the sort of geometry that old Audis had, and those cars were horrible for excessive chassis motion.  Audi Sport ran springs as high as 800lb in the GRAVEL cars...

In reply to Pete. (l33t FS) :

This photo is the front of an R5 Fiesta. To your point about how soft the springs might be, look at the number of coils in the spring. It's got to be pretty soft.

So much halfshaft plunge I'd imagine.

Look at the diameter also.  You can tolerate a lot of bending load and surface pressure with big bushings and shafts.  

Very cool stuff would love to learn more about it.  

I'd love to see some underside shots of that engine bay.  I think I now see how they can swap transmissions in ~14 mins - there looks to be nothing in the way.

Very cool.

In reply to ProDarwin :

If it's anything like the Focus WRC, the struts, knuckles, and axle are serviced as an assembly.  Axle cup stays in the transmission, too.  Big shouldered studs holding the ball joint and tie rod, instead of tapered studs you have to fight to separate.

If the front end is built like the front drive Escort rally car, all drivetrain mountings attach to the engine.  Engine may not be replaced during a rally, so the engine gets all the mounts.  Transmission gets nothing.

There's a lot of intereresting service park videos on YouTube.  Prodrive could have a service crew swap a Subaru transmission in five minutes.

The ad where I pulled the Fiesta photo from says the wheel travel , front and rear , is 280mm/300mm....so 11-12 inches. I'd like to try that down a dirt road.

Suspension shot from below.... doesn't the back tension rod on the lower arm look a little flimsy? These are cars that hang the inside from wheel 2 feet over the shoulder and catch rocks and stumps.