Roll centres + Rubbish saloons

[phoenix]

Originally Posted by Denis Bassom

I thought the roll centre was dictated by the point that the panhard rod crossed the centre of the diff housing and hence also reduced when the car was lowered?

That is only true when a panhard rod is added to a simple cart-sprung rear axle. As soon as you add other links (in your case four trailing arms) then the rollcentre loaction is dictated by other factors. Page 34 of Fred Puhn's book gives the details, which are to complex to descibe on here with just words. Suffice to say, that lowering the rear of a five linked axle can actually raise the roll centre considerably, thus increasing roll resistance and giving a previously neutral or under steering car oversteer.

[greenamex2]

Hmmm.

My panhard rod is so close to the axle that the links won't make massive amount of difference.

According to all the googling the fact that the rod slopes upwards from the body to the axle is not a good thing.

I suppose the question is whether the extra weight of a watts linkage is going to offset the geometry improvements.

I think I'll have a measure tomorrow and work out the roll centres.

May also drill some new panhard mounting holes and have a play in the last couple of races (don't read this bit Colin/Rob!).

[phoenix]

Originally Posted by Denis Bassom

My panhard rod is so close to the axle that the links won't make massive amount of difference.

I'm afraid that is not true. Having lowered the car the four trailing links are also now running upwards towards the axle, I imagine. This will give a raised roll centre.

What is also true is that the Panhard rod body mounting should definitely be above the axle mounting!

Remember that if you add the weight of a Watts linkage you will also remove the weight of the existing Panhard rod, so it's not all bad. I also think that the increased cornering speed potential of getting the suspension right will far outway the slight weight penalty.

[Notso Swift]

Originally Posted by phoenix

I'm afraid that is not true. Having lowered the car the four trailing links are also now running upwards towards the axle, I imagine. This will give a raised roll centre.

What is also true is that the Panhard rod body mounting should definitely be above the axle mounting!

Remember that if you add the weight of a Watts linkage you will also remove the weight of the existing Panhard rod, so it's not all bad. I also think that the increased cornering speed potential of getting the suspension right will far outway the slight weight penalty.

Agree the difference between a Watts link and a Panhard rod is about the equivilent to a good dump before you head out!

[greenamex2]

Originally Posted by phoenix

I'm afraid that is not true. Having lowered the car the four trailing links are also now running upwards towards the axle, I imagine. This will give a raised roll centre.

What is also true is that the Panhard rod body mounting should definitely be above the axle mounting!

Remember that if you add the weight of a Watts linkage you will also remove the weight of the existing Panhard rod, so it's not all bad. I also think that the increased cornering speed potential of getting the suspension right will far outway the slight weight penalty.

The lower links are pretty much level (err, now, more constructive interpretation of the regs this year), the upper links (which arent covered by either the Puhns or Staniforth books) are largely off with the fairies. They are VERY softly mounted because they are about one third the length of the lowers and rip out the mountings otherwise.

From the puhn book the virtual meeting pount of the lower arms is probably raising it a bit but the magic numbers are -

Front roll centre is 12cm above ground.
Rear roll centre is 23cm above ground (plus any effect of the lower arms).

How does this sound?

For info I am going to raise the body end of the panhard rod for the next race by 1-1.5 inches so it is just sloping downwards body to axle. Should be interesting to see what, if any, difference I feel. Will report back.

And the watts linkage is designed in case I do detect a difference.

[Al Weyman]

Lower the axle mounting points of the lower trailing arms Dennis so that the trailing arms are level, they definitely do not want to be pointing up in the sky as a result of lowering.

Disregard this Dennis, we cross posted!

[Al Weyman]

It sounds like a similiar set up to the old Talbot which definitely went bound especially if you used poly bushes, why not just adabt an Escort 5 link kit and have done with it.

[greenamex2]

Originally Posted by Al Weyman

It sounds like a similiar set up to the old Talbot which definitely went bound especially if you used poly bushes, why not just adabt an Escort 5 link kit and have done with it.

The draft tipex lines are already in drawn to correct the upper links as per the Escort kit.

It would be nice for the top of the axle not to nod backward and forwards as it moves up and down, or you accelerate/brake!

[glenn22481]

Dennis,

just out of interest and perhaps to further fuel the current thread...

What is your rear toe-in and camber set at?

Does it lift a rear while cornering?

[Notso Swift]

Originally Posted by Denis Bassom

From the puhn book the virtual meeting pount of the lower arms is probably raising it a bit but the magic numbers are -

Front roll centre is 12cm above ground.
Rear roll centre is 23cm above ground (plus any effect of the lower arms).

How does this sound?

I have usually worked on the front being 2/3 of the rear RC and it seems to give a nice feel.
That said I am notorious for having odd set-ups, ringing the crap out of my cars and them for someone else to jump in and ask how can I keep the thing on the black stuff (insert Gorilla driver joke here)
Mind you my last Strut front 5-link rear car were I played with this was a FWD. My last RWD competition car was… hmm 12+ years ago with a RA23 Celica, and I didn’t really “think” through that car, I just tended to do the “standard” mods that everyone else said you “should” do.

[greenamex2]

Originally Posted by glenn22481

Dennis,

just out of interest and perhaps to further fuel the current thread...

What is your rear toe-in and camber set at?

Does it lift a rear while cornering?

It's a live axle so, in theory, zero for both.

In reality it has about a 1/4 of a degree of negative, no idea how/why.

No it doesn't lift.

[Lukin]

Ben has noted (in this thread even) that the relative RC height should be a function of the yaw inertia. If you have a tank you might want a low FRC and a high RRC to get the thing to turn whereas in a small open wheeler I'd be suprised if either RC was above 100mm.

[glenn22481]

Originally Posted by Denis Bassom

It's a live axle so, in theory, zero for both.

In reality it has about a 1/4 of a degree of negative, no idea how/why.

No it doesn't lift.

All the top guys here cut and re-weld the axle tubes from the pigshead. From memory they are set at 7mm toe-in and 1deg- camber. If your regs allow this type of mod it's something well worth doing. One chap I know paid his engineering buddy to do the job 3 times untill it was how he wanted it, so for him it was worth the expense.

[phoenix]

Originally Posted by Denis Bassom

The lower links are pretty much level (err, now, more constructive interpretation of the regs this year), the upper links (which arent covered by either the Puhns or Staniforth books) are largely off with the fairies. They are VERY softly mounted because they are about one third the length of the lowers and rip out the mountings otherwise.

From the puhn book the virtual meeting pount of the lower arms is probably raising it a bit but the magic numbers are -

Front roll centre is 12cm above ground.
Rear roll centre is 23cm above ground (plus any effect of the lower arms).

How does this sound?

For info I am going to raise the body end of the panhard rod for the next race by 1-1.5 inches so it is just sloping downwards body to axle. Should be interesting to see what, if any, difference I feel. Will report back.

And the watts linkage is designed in case I do detect a difference.

I am surprised that the rear RC calculates so low, considering you have lowered the car four-five inches. I suggest you compute this again, including the lower links (which must now be 4-5 inches higher at the axle than they were before) as it is the point of intersection of the links that determines RC height. In my head I can only visualise this point as being much higher than before lowering.

When raising the chassis end of the panhard rod you should aim to have the rod still angled up, or at a mimnimum horizontal, when the suspension is at full bump. Will the 1.5 inch increase achieve that? If it does, you should notice a difference.

[greenamex2]

I'll have another look at the rear roll centre, I'm afraid I threw my toys out the pram when I spend an hour accurately measuring the front suspension points only find I couldn't save them properly on the demo copy of susprog3d and wasted my time.

Part of the reason it is quite low is because I corrected the lower arms with 'traction brackets', the rod attaches to the axle at the bottom of the casing rather than the top and Corolla's seem to have been designed to run a couple of inches lower than standard then got jacked up for some bizarre reason.

Out of interest, correcting the lower arms and adding 1/2 a degree of castor with no other changes transformed the car from understeer to oversteer. This is why I am now chasing geometry rather than spring rates to correct the balance.

Correct, it is unlikely that the 1.5 inch change in chassis attachment will achieve the 'ultimate goal', but I don't want to change things too much at first just in case it goes horribly wrong. In the end the car still usually wins it's class and there are a lot of changes planned over the winter, including the possibility of a watts or wobb linkage.

[Al Weyman]

If your regs allow this type of mod it's something well worth doing. One chap I know paid his engineering buddy to do the job 3 times untill it was how he wanted it, so for him it was worth the expense.

As I wrote the original regs for the ModProds then the answer is no, its not allowed but then again it will be if they amalgamate next season:-)

[greenamex2]

Originally Posted by Al Weyman

As I wrote the original regs for the ModProds then the answer is no, its not allowed but then again it will be if they amalgamate next season:-)

IF I get a new rear axle (another rule change) then I'll be upping the rear negative camber from 1/4 to 1/2 a degree. Checking the rear tyre temperature I don't think it needs any more.

As for adding toe in, I'm really not sure. I was hoping to play with this on the kit car but that would require an engine!!

[R59]

Denis,

1deg neg, 10mins toe in.

Trust me, I'm a doctor.

Rob.

[greenamex2]

Originally Posted by racing59

Denis,

1deg neg, 10mins toe in.

Trust me, I'm a doctor.

Rob.

Does that cause any issues on a solid axle?

Particularly given that splines for the diff are really 'designed' for a zero degree camber angle.

Or is it just big V8 Astra's that it causes reliability problems with

[Al Weyman]

Do what was rumoured to have be done on certain 'works' SDI's, run a Granada CV joint within the axle:-)

I definitely would not have the bottle to bend a Chevy axle with the 'legendary' GM break a halfshaft and lose a wheel C-Clip halfshaft retaining setup.

[greenamex2]

Originally Posted by Al Weyman

Do what was rumoured to have be done on certain 'works' SDI's, run a Granada CV joint within the axle:-)

Too much weight!

[GordonG]

Done it myself on a Ford English and a Morris Ital axle. Mind you I did only put 130bhp through it and the casing was comprehensively braced...

The splines will take a degree or so of misalignment without problem. THe whole things swimming in thick oil anyhow - imagine the amount of movement the teeth of the CPW see versus a 1 deg misaligned spline.

G

[R59]

If your axle has fully floating hubs, you're OK with that sort of a mod Denis.

My rear axle is not a reliability issue, and as "Strange" as it seems, it is equipped with fully floating hubs.

Had I gone for the Winters quick change axle, I could have had "cambered" axle tubes, and with a twist, I can change camber, and toe, in one movement (obvious really!).

Al, you don't mean those works Rover's were bent do you? No don't start the Volvo rockers issue again.... and like you, the Chevy diff is well known for letting those C-clips loose without the addition of any tweeks.
That's why I went for the Strange/Ford 9".

Rob.

[greenamex2]

Originally Posted by racing59

If your axle has fully floating hubs, you're OK with that sort of a mod Denis.

If I was to put my hubs in a bucket of water they would definately sink.

[Al Weyman]

DENNIS YOU ARE A FUNNY MAN!

Had to run the original Axles Rob so no choice. I do have disc brakes on both cars so figure if an axle shaft breaks the caliper may stop the wheel going walkies, never tried the theory though!

The 'Chevrolet' show car IROC promotional racer I am renovating in fact has a Ford axle (from new) with fully floating hubs, Strange axles and I think it has a SPool, not sure. Either that or a very tight locker!