Independent Rear

[aerobod - near CYYC]

As an Aerospace Engineer, I'm quite familiar with dynamic systems Sam. I agree camber gain (or lack thereof) is important. In the case of a standard double wishbone supension with an increasing load as the body rolls, the inner pivots movement decreases camber as the shorter top wishbone normally with a higher inclination than the bottom one will increase camber, usually designed for a net camber gain with suspension loading. I think we both agree on that.

What I'm talking about is the out-of-phase movement of the body relative to the suspension components at the point of grip loss when rear tyre slip angle has peaked. At this point the rapid dynamic changes can cause diffferent rates of movement between the inner and outer pivot points, causing a transient condition where camber can be potentially increasing or decreasing due to the different rates of rotation of the body around each axis as the car starts to skid. Rotation around the z-axis tending to continue loading the spring and maintain camber gain, but around the x-axis the loads decreasing at the tyre could cause a rebound of the suspension that lowers the camber faster than the levelling of the body pushing the upper pivot point out compared with the top one can increase the camber. The damping and anti-roll bar will provide some tuning/modification to the behaviour, of course, but without active suspension, all dynamic scenarios will be slightly different.

With the deDion any transient camber change at the limit is pretty well determined by any rebound of the loaded tyre causing a camber increase, as opposed to more complex movement of a double wishbone suspension, in my opinion (based on driving numerous cars with rear double wishbone suspension) the Caterham deDion setup gives more predictable and consistent behaviour, if not necessarily the ultimate tunable camber change of a double wishbone setup.

[Golf Juliet Tango]

 

https://www.caterhamlotus7.club/guides/technical-information/de-dion-or-irs

From decades ago. 

[Shortshift]

A very good find Stephen!  And written some years before CC did introduce IRS with the CSR development.

I'd be interested to know the weight delta between a standard S3 and an equivalent S3 fiitted with IRS (the Meteor IRS car, for example) and also to understand what difference there might be between sprung and unsprung mass in both design types. 

I suspect that the benefits offered by IRS in terms of ride comfort might be greater than any performance gain over the DD set-up - which has been proven to be pretty effective on these little cars.  So IRS for improved comfort, rather than outright (on-track?) performance??

James

[Wrightpayne]

Gratuitious opportunity to share some pictures of Ivan Dutton's climax engined Lotus Seven with an early dedion rear...

 

Pics taken at the last Sunday Scramble and yes, that is coconut hair door mat in the drivers footwell!

[SM25T]

CC built at least one IRS S3 badged R500 Evo. I looked at it years ago in a Surrey car dealer showroom for a club member who was working overseas. Decided not to buy it. It has been through the hands of an owner in Kent in recent years, but moved on now 

[bluenose]

Think it was Owain Davies who built a R500 IRS in 2008? He worked for Caterham Cars as a development engineer I think. The car used bits from a CSR I believe.

Nick

[SM25T]

That's the one. I remember his name from the dash plaque.

[John Vine]

Owain Davies

There's a blast from the past. He was amazingly helpful during my build (2008) answering endless queries about extra and unused wires and connectors on my R400D.

JV

[BenF]

Is there a term for an expert in one field (e.g. aerospace engineering) automatically assuming that they must then be an expert in a slightly related field?

They then explain the subject to someone who is actually a leader in the field at the highest level, who has been paid for said job for many years and is using their expertise to keep a roof over their heads (and buy a 7...) 

[simon_h]

Yes the term is often known as a 'caterham owner' 

[aerobod - near CYYC]

Thanks for the valuable commentary from the peanut gallery Ben

I'd like to understand how the traction variations that occur due to camber variation at the limit can be completely dialled out (as is the case with the deDion suspension). In track prepping both a C5 Z06 Corvette and Honda S2000 suspension with my son, both acknowledged as having some of the best rear double wishbone setups in the industry, we could not completely eliminate the effect. Multi-link systems with upper wishbones such on the Z4M I used to own and my son's FT86 have a bit more adjustability due to more variability in the bushings to give active toe control and adjust slip angles a bit on the limit to modify the effect of wishbone camber changes (perhaps the track link inner bush position and stiffness on a double wishbone suspension could be part of the equation here, too). Most people seem to just increase roll stiffness too much, with the consequence that you effectively don't have an IRS anymore.

My own home-built mid-engined rear suspension employees very long aluminium arms that wrap around the engine to try to optimize camber changes (upright component machining being limited by my basic milling and lathe capabilities):

[BenF]

Peanut gallery 

To be fair to James: you inputs on aero stuff are always interesting, was not meant to be a personal pop at you.

With regards to camber variation on the limit: I can only answer, as someone who had to define kinematics for various racing cars in the last years (but no longer, I work with something more ridiculous than racing cars).

Lack of driveability/traction is often blamed on suspension kinematics.  However, the first order effects on car handling are due to the global car balance, which is defined by the tyre size front to rear, the weight distribution, the aero balance and the  mechanical balance.  With a 7, you don't have to worry about aero balance and weight distribution is fixed, unless you are planning on running with ballast, which would be silly.  Tyre size is also fixed (ok, you can choose different rears...)

If you get these basics right, then you cannot go too far wrong, as long as the basic kinematics are in a sensible range.  On the limit, when a car starts to slide, the kinematics have remarkably little effect, because in the 0.2-0.5s when the car is starting to slide, the attitude of the car is not that different, so there is not much of a chance for the kinematics to affect the controllablilty on the limit.

When you move from racing cars, with very high roll and heave stiffness, to road cars, which have much lower roll and heave stiffness, it could be that these kinematics start to have more of an effect.  However, I doubt these are first-order effects and from my own personal experience, traction problems have usually been solved by a global shift in car balance (or downforce... not possible in this case).

Hence, going from De Dion to IRS, I would have no doubts.  The first-order effect I would want to know about, is that the mechanical balance of the car is more or less maintained when switching from De Dion to IRS.  I would be interested in knowing what the camber gain (camber change with wheel travel), but as long as it's within some sensible limits, there is not much to worry about.  The final part is knowing the compliance of the suspension.  Usually this is defined in a K&C test but I am pretty sure this has never been done for a 7 and if the design work was done correctly, they will have an idea about values anyway.

I'd be interested in knowing who did the design work on the IRS, but I can understand that Simon @ Meteor is not in a position to say...  there are lots of ex-F1 designers who have set up design studios doing this kind of work and I would trust them completely.

[DW199]

I'd be interested in knowing who did the design work on the IRS

Have you received the August edition of Lowflying?  There is a four page article that more or less answers this question. According to the article it was done by a bloke called Dominic who works for Porsche but I have a feeling this irritating video 

 illustrates the process.

 

  

 

[BenF]

Ah... I have but have not even opened it yet, still got Private Eye and Viz to finish first (not joking)

[Dan R]

#39 - the only thing in this thread that I am qualified to comment on is that these are two excellent publications!  

I also know where I can satisfy my desire for engineering expertise over a beer.  

[Simon.Rogers]

Hi Everyone.  Sorry been very busy on the phone and replying to messages.

Time for a catch up.

We have already supplied a second chassis at the time of the original build.  I am not sure if it has yet made it to the road as the chassis was having a full refurb = perfect timing!

The final iteration is a very small change.  The core of the upright is now being machined out of mild steel as a single item rather than just the bearing carrier and then all the webs and gussets hand fabricated in the jig.

The only other changes are simply a couple of mm shorter tie rods to allow greater toe adjustment and similarly, the top wishbone will have a couple of mm shorter boss for the rod end so again we have more range in camber.

Thats it.

The original design was pretty spot on.  Remember that car has been running for quite a few years and we use the same jigs.

Yes our car is on our suspension but we have had many people try who already have our suspension on their cars so for them it was a back to back test.

[Simon.Rogers]

Hi Stephen

The wishbones are all tig wellded - works of art if you ever see the fabrication and the basket is mig welded just like the metric chassis cars are now.

[Simon.Rogers]

There is no timing allowed on a track day.  Additionally, there is around 30 bhp difference between the 2 cars used.  Our test car also still had its windscreen in place.

Finally, although Rob is very committed in anything he drives I know he has the utmost respect for anyone else's car so would not (even if we asked) have pushed so hard.

I am very keep to actually measure the difference but the chances of having identical cars available on the same day is going to be slim.

[Simon.Rogers]

Hi Geoff

No we have not even considered this as one small change would mean it was a new design.   This is all about making the best product at a reasonable cost.  Considering where and how this was designed I don't believe we would see anything better.

We also made sure that we removed the additional expense of uprated brakes etc from the original - its certainly possible but should in my view be a brake upgrade not iRS upgrade.

[Simon.Rogers]

Hi James

The one thing I disagree with above is the sudden snap back of the suspension.  I don't believe this is related to the design of the suspension - this is from experience more related to the damping and spring forces.  When specifying the damper and spring it is essential to match these forces.

It is for this reason (and it only really happens when at the limits) that we recommend the progressive spring on the road and linear on track as when at the limit the spring rate changes and the rebound forces do not (they are linear at this point generally) and will not be able to match the increased spring rate - thats when you get the damper snapping back uncontrolled.

[Simon.Rogers]

Hi James

I thought the weight figures were in the article?

The iRS saves a small amount of weight overall - we were unable to weigh it ourselves as the car was a full refurb - we never had the same item to weigh.  I promise the first car to be fitted (end of September) will be fully measured.

However from Dominic's figures there was approx 1.5kg saved but 9.5 kg moved from unsprung to sprung.

 

[Simon.Rogers]

As below - Pe designed   - But we can not actually state that as fact.

Dominic has presented his design and process to a number of students at a couple of Universities.

The camber gain is very very little.  We measured it in dry build with the 3 d camera system fitted.

The car drives to all intent and purpose just like a 7 should.  The big diference is the significantly removed need to "brace" just after your front wheel has hit something rather sharp/big.

[SamC]

I've just read the article in LF. It looks like a well thought through, nicely designed and really well engineered solution.

Looking forward to seeing a few of them around in the near future!

[Shortshift]

"The iRS saves a small amount of weight overall - we were unable to weigh it ourselves as the car was a full refurb - we never had the same item to weigh.  I promise the first car to be fitted (end of September) will be fully measured.

However from Dominic's figures there was approx 1.5kg saved but 9.5 kg moved from unsprung to sprung."

Thanks Simon - good to hear both those figures!

Curious also to know - do you see the main benefit of IRS on a Seven as improved ride comfort or better (ultimate) performance?  I can see the weight savings (particularly unsprung) helping with the ride, but others on this thread have suggested the DeDion is hard to beat for performance - I guess that means on track.  I'm keen to know your views, given your experience with both systems.

James

[SamC]

James, I'll obviously leave Simon to talk about his experience with his system as he will know best.

However, as a point of interest (BenF will no doubt be able to explain more eloquently given his simulation background) ride is definitely a performance influencer but it's contribution is notoriously difficult to predict/measure/model.

It is very circuit specific in 'track' scenarios, for example bumpy tracks and ones that benefit from aggressive kerbing will result in a greater benefit, but I suspect somewhere 'billiard table' like Silverstone it will almost negligible. 

With the standard Caterham being really well dialled in in DeDion form, I suspect that will be the prime differentiator in terms of lap time, but as you say, Simon with his direct experience of both will be the oracle on that!

P.S. it's impressive to do a retro fit upgrade like that and pull mass out! Hats off!!!