Rear roll center change when lowering

[singlespeed]

With the McPherson suspension ,I know when lowering the chassis from the standard ride height, the roll centre at the front will change and end up lower as the front wishbone angle between inner and outer pivots and also steering axis will have all changed. So, roll centre adjusters can then be used to raise the roll centre and get the CofG and RC closer together again. Worked very well on the Audi which suffered badly when lowered.


This is actually on the Twingo, but the beams are the same design, but probably with different roll bar stiffness across the axle. What I can't find is info about the effect on the roll centre, which has a rear beam with programmed flex. If the rear end is lowered, does it change a bit, a lot or not at all? Confused and getting a fried brain.

.

 

[NorthloopCup]

It's more toe and camber changes that are the result of lowering the rear.

 

[singlespeed]

Yeah, I found the toe/camber usually change which can be corrected with shims.

I've thought about it before, but kind of discounted it as the axle/beam is pivoting about a line 90' to the roll axis. So, thinking if it does change, its probably the same/similar amount as the drop in ride height. So, the distance between CofG and RC should be simillar or the same.

Unlike the front which can end up way different.


A few nights waiting about aren't good for an inquisitive mind

 

[NorthloopCup]

Yeah, I found the toe/camber usually change which can be corrected with shims.

I've thought about it before, but kind of discounted it as the axle/beam is pivoting about a line 90' to the roll axis. So, thinking if it does change, its probably the same/similar amount as the drop in ride height. So, the distance between CofG and RC should be simillar or the same.

Unlike the front which can end up way different.


A few nights waiting about aren't good for an inquisitive mind

The front roll centres are the ones to focus on tbh as that's where the biggest gains in handling improvements come from. The extender pins like the vw/Audi boys use aren't especially good when combined with Clio hubs. They elongate the bottom hole on standard ball joints, so you just increase the load on them if you increase leverage.
This was one of the things that started me off with doing the roll centre corrector and bump steer corrector hubs.

 

[KevF182]

On twist-beam the rear roll centre height hardly deviates at all with suspension movement. The axle height is one parameter used to locate the position of it, along with the length of the arms and the positioning of the twist-beam relative to the mountings. Altering the axle height would therefore have a slight effect, but that's only possible if you change ride height through fitting different radius wheel/tyre - the axle height wont change with geometric suspension movement alone. Therefore there's nothing practical you can do to alter rear RC on the clio, its pretty fixed in the design.

Kev

 

[mikeh]

Have I missed something. How does camber, toe change with lowering on a trailing arm?

Surely it's fixed regardless of height.

 

[NorthloopCup]

Have I missed something. How does camber, toe change with lowering on a trailing arm?

Surely it's fixed regardless of height.

When you lower the rear the angle that the beam sits at is altered, so if the beam is machined with the camber and toe in mind for a set ride height it changes. Not dramatically, but it definitely changes.

 

[KevF182]

Have I missed something. How does camber, toe change with lowering on a trailing arm?

Surely it's fixed regardless of height.

as the hub rotates the angles it is positioned in (the verticle and lateral planes that set the camber and toe) change their relation to each other. Its only slight, but it will alter - eg camber moves toward positive (less neg) because the toe-in angle starts to detract from the camber angle as the car is lowered

EDIT - beat me!

 

[mikeh]

I'm not saying your wrong, but I'm struggling to get my head around that.

Where does the camber come from if both wheels are fixed on the same axis.

If one gained negative, surely the other would have to gain positive camber seeing as they are on a relatively solid beam....

 

[KevF182]

using a laptop? well try this...

position the laptop so that the screen is just pointing back slightly, a little past 90 degrees. Imagine the screen is your wheel and see that slight tilting as your camber angle. you can then imagine toe angle to be zero if you think of each side of the screen as pointing forward and back on the imaginary car. Now rotate the laptop onto its side so it sits like a book - see how the toe is now toed in but camber is zero? thats an extreme change because you'd not rotate the trailing arms by 90 degrees, but it makes the point surely?

 

[mikeh]

I didn't see Kev's post when writing mine.

That makes a little more sense, thanks! I'm guessing it's very slight, though?

 

[NorthloopCup]

I didn't see Kev's post when writing mine.

That makes a little more sense, thanks! I'm guessing it's very slight, though?

Yes mate it is, but the toe change is more noticeable than the camber change.

 

[mikeh]

Thinking about it - that might explain the discrepancies in the rear toe/camber I found when getting it aligned after fitting the ASTs.

Makes a lot more sense.

 

[NorthloopCup]

Thinking about it - that might explain the discrepancies in the rear toe/camber I found when getting it aligned after fitting the ASTs.

Makes a lot more sense.

The beams are also chocolate and do flex/deflect. The pure motorsport stiffening kit will reduce that a good amount and in theory, your geo should remain consistent.

 

[mikeh]

The pure stiffening and bearing kit appeals - a long way down the priority list, though!

 

[NorthloopCup]

The pure stiffening and bearing kit appeals - a long way down the priority list, though!

I've got the bearing kit - great bit of kit! Stiffening kit will be put on eventually.

 

[singlespeed]

The front roll centres are the ones to focus on tbh as that's where the biggest gains in handling improvements come from. The extender pins like the vw/Audi boys use aren't especially good when combined with Clio hubs. They elongate the bottom hole on standard ball joints, so you just increase the load on them if you increase leverage.
This was one of the things that started me off with doing the roll centre corrector and bump steer corrector hubs.

I bent one of the hub extenders on the Audi, just below the hub joint. Fortunately the taper in the hub wasnt damaged.
But, the 1.9tdi is a shed load heavier, was stiff as hell and went/stopped rather well

RS133 front uprights have a taper joint and the lower arms are cast, with the ball joint screwed into it and so far, a replacement ball joint has ment a replacement arm (RS133 known bugbear fault) So, I've been thinking of alternative arms to go on to (maybe increase the track and castor) and use a parallel ball joint with pinch screw type extensions.


Thanks Kev.
That probably explains why there's not much talk of it, as its a small change anyway and little can be done outside of a redesign.
I'm running 15s instead of the original 17s, so the axle height has dropped ~6mm

I think I'll stop worrying about the rear now and deliberate the PMS arms for the front :lol:

 

[NorthloopCup]

I bent one of the hub extenders on the Audi, just below the hub joint. Fortunately the taper in the hub wasnt damaged.
But, the 1.9tdi is a shed load heavier, was stiff as hell and went/stopped rather well

RS133 front uprights have a taper joint and the lower arms are cast, with the ball joint screwed into it and so far, a replacement ball joint has ment a replacement arm (RS133 known bugbear fault) So, I've been thinking of alternative arms to go on to (maybe increase the track and castor) and use a parallel ball joint with pinch screw type extensions.


Thanks Kev.
That probably explains why there's not much talk of it, as its a small change anyway and little can be done outside of a redesign.
I'm running 15s instead of the original 17s, so the axle height has dropped ~6mm

I think I'll stop worrying about the rear now and deliberate the PMS arms for the front :lol:

Interesting about the uprights! I'm sure something could be manufactured. I might have a look for a set of 133 uprights. Lol! You could convert to the 1*2 style hub and use the custom ball joints I use on the roll centre corrector hubs to gain track width.

 

[singlespeed]

Have you got a link to your hubs/extenders. I'm on a super rubbish internet connection and
waiting for the wrong picture to load is disappointing

RS Twingo uprights have taper lower ball joint socket, taper TRE socket and 60mm bolt pitch for the damper so 182 cup/trophy spacing. As for the relative lengths of the joint positions from the axle line and steering axis, I'd have to measure.

 

[NorthloopCup]

Have you got a link to your hubs/extenders. I'm on a super rubbish internet connection and
waiting for the wrong picture to load is disappointing

RS Twingo uprights have taper lower ball joint socket, taper TRE socket and 60mm bolt pitch for the damper so 182 cup/trophy spacing. As for the relative lengths of the joint positions from the axle line and steering axis, I'd have to measure.

They're in the group buy section mate and you need to be a member to see that. Have a search for BIGASH's thread mate as there's pics in there. I'll search for a link.

 

[NorthloopCup]

Here you go singlespeed

http://www.cliosport.net/forum/show...h-better-than-this-)/page263&highlight=BIGASH

It should take you to page 263.

 

[singlespeed]

Spot on, thanks for that

A pair of Clio arms , a pair of your spherical bearing holders and extenders, with the top end altered to a taper engagement for the RS Twingo upright. Possibilities for it working out if the inner bushes of the arms share the same spacing and the overall length is the same. I'm already sorting the bump steer with the PMS kit.

My hubs have 60mm bolt spacing and I've already got BC coilovers on, so the other option would be 182 cup uprights and hope the driveshafts and brakes swap over too.

As it stands, your setup may well work on the Twingo GT which uses pinch bolt LBJs. Although I'm not sure of the pin diameter.

 

[NorthloopCup]

Spot on, thanks for that

A pair of Clio arms , a pair of your spherical bearing holders and extenders, with the top end altered to a taper engagement for the RS Twingo upright. Possibilities for it working out if the inner bushes of the arms share the same spacing and the overall length is the same.

As it stands, that may well work on the Twingo GT which uses pinch bolt LBJs. Although I'm not sure of the pin diameter.

Altering the pin to a taper would be a pain mate tbh. You're better off getting some mk2 cup racer extender pins and checking to see if the taper matches.
How many splines are there in the drive flange? If they're the same as the 1*2, you could fit 182 cup hubs. The spherical joints are only done as part of the full hub kit you see.

The size of the hole in the twingo gt's isn't an issue to overcome, providing there's enough meat around them to accommodate being opened up (if required of course)

 

[singlespeed]

The Twingo hubs look like they are going to be stronger around the lower ball joint area, compared to the 1*2 hubs which look to have much less meat on them around this area. You can see one of the usual split ball joint rubbers and this was only 18months old and 4400miles, which has had me spending ages looking for and trying possible replacements instead of replacing the whole arm. However, I might start looking again, but for replacements with a parallel pin and pinch bolt, which could be get two birds with the one stone.


How many spline in the hubs, I'm not sure. I haven't had it apart (yet)

 

[MG cup]

Clio hubs have 23 splines and so does the JC5 gearbox.

You don't want to go away from that pictured hub above it looks infinitely better than a 1*2 hub just from that picture. Get mk2 cup racer unibal (spherical) holders (Marks are a copy of these) and tapered extender pins and try them in your hubs. If they don't fit then get your hubs reamed to the taper.

Then all you have to do is mount the unibal (spherical) to your wishbone however you see fit, use the PMS bump kit and you have the setup that Mark is producing but tailored to a twingo.

 

[NorthloopCup]

The Twingo hubs look like they are going to be stronger around the lower ball joint area, compared to the 1*2 hubs which look to have much less meat on them around this area. You can see one of the usual split ball joint rubbers and this was only 18months old and 4400miles, which has had me spending ages looking for and trying possible replacements instead of replacing the whole arm. However, I might start looking again, but for replacements with a parallel pin and pinch bolt, which could be get two birds with the one stone.


How many spline in the hubs, I'm not sure. I haven't had it apart (yet)

I'd just drill the taper out and run a straight pin in that setup mate. There's enough room to pull it up nice and tight with a nut at the top as well. Then it's just a case of the lower arms to sort.

 

[mikeh]

A slight sidetrack, but if you were concerned about the split ball joint rubber for MOT, you know that you can get ball joint boots for about 50p a pop, right?

 

[MG cup]

I'd just drill the taper out and run a straight pin in that setup mate. There's enough room to pull it up nice and tight with a nut at the top as well. Then it's just a case of the lower arms to sort.

I have to disagree Mark........ to remove a taper and then fit a parallel pin with a pinch nut is asking for trouble. Most RCA pins are tapered to stop elongating??

 

[NorthloopCup]

I have to disagree Mark........ to remove a taper and then fit a parallel pin with a pinch nut is asking for trouble. Most RCA pins are tapered to stop elongating??

I'd try and get the taper matched first, but if I had no joy I'd drill it parallel mate. A parallel pin doesn't need heat treatment if it's got plenty of meat to it, but a tapered one does ideally. You'd have to have the pin machined to be close tolerance though to reduce any movement. I don't think you'd get much elongation if it was designed correctly either. All just my opinion though.

 

[singlespeed]

Just been having a look for the Clio Cup uniballs. Are they easy to get hold of... I guess I'll need to get access to the hallowed vaults here to find some. However, searching the net has revealed some wonderful RC correction solutions which must have a huge placebo effect, just dropping the wishbone with a spacer and keeping the ball joint in exactly the same place :S

Drilling out the taper in my uprights will probably take this to 18mm or maybe even 19mm. How does this compare to the bore in the uniball?

I'll have a proper look after tomorrow, once I'm back home, but the list of jobs to do has been building:-
Replacement throttle body extensions to fit and sort the air filter which wouldn't go in at present.
Reset cam timing as the inlet was retarded to get it to idle on the plenum inlet
Mapping for the above changes
PMS bump steer kit
Replace the temporary fit VW expansion tank with a new Mk2 Clio one
Tidy up the battery wiring and mount the battery properly instead of a ratchet strap
Fit new wheels
Plus a few other jobs that my Mrs has saved over the past three weeks

 

[singlespeed]

A slight sidetrack, but if you were concerned about the split ball joint rubber for MOT, you know that you can get ball joint boots for about 50p a pop, right?

With them splitting so soon and quickly going from fine to perished and split, the problem has been the first you know is the joint is knocking. Then, its bend over time for replacement wishbones instead of £15-20 for a joint. The revised ones of late seem to be lasting a bit better, but there's some on my forum that have had three sets before MOT time and a shed load of us with at least one set of replacements!

 

[MG cup]

Here's some pics of a cup racer setup for the Clio. The RCA pin is tapered into the hub and parallel where it goes through the spherical (they're called uniball's by Renault for some reason)

holder. R-Sport in Wimbledon can get them from Dieppe or alternatively martijnk or @jp might have some.

The comparison between Clio 172 cup hubs and Cup racer hubs. I'm not sure if they will suit shock spacing wise but iirc the twingo is 4x100 PCD and caliper mounts are the same distance. That's the cheapest d best way to do it if you can mount the unibal holder on your bottom arm.

 

[singlespeed]

Thanks guys.

I can't see the pics here internet filter blocks lots of hosting sites
Back home tomorrow, so I'll look into getting. this sorted out over the next three weeks

 

[singlespeed]

Out at work now and can see the pics

It could be the angle of the photos but it looks like the Twingo LBJ is lower to start off with. The discs are the same 280mm and it looks like the bottom edge of the Twingo upright is a fair way down the swept area on the disc, whilst the Clio ones are near the inner edge of the swept area.

If it is lower, I doubt if it would be enough for you lot to get excited about, but it may be an improvement and a strong one at that.
I'm just hoping it doesn't give clearance issues for me, with the Clio Cup extensions, to fit inside 15" rims (that's if the taper is compatible)