105 Panhard Rod Conversion

[alfagtv58]

and the hacksaw blade is missing some teeth.

[Colin Byrne]

Quick update
got the car back together today and took it for a quick blast around the block,  must say all feels very promising.  Feels like the gearbox is finally sorted to, looks like a bent selector shaft was to blame for the problems I've been having with it since i put the car together.  Can't wait for Winton!!

[cjheath]

Be interesting to know how you find it now you've designed-out the rear wheel steering on roll that was such a major accomplishment of the original designers. As far as I can see, this is what gives these cars their forgiving character, and though the 105 setup doesn't do it as well as the De Dion rear, it still works pretty well. Perhaps under the controlled conditions of racing it's less important, but don't give folk the idea that this is necessarily an improvement for the road.

[Colin Byrne]

That's the first time I've  heard the term "rear wheel steer" in regards to a 105.  Are you refereeing to the axles ability to follow the arc of the lower trailing arms due to only having effectively one top trailing arm in the centre of the diff?  I'd never really considered this as design advantage.  I'm guessing, due to the angle of the rear trailing arms, there would be a counter steer effect from the rear on cornering.  From my experience and research designing a rear wheel steer system for a small Motorkhana type race car I found that at low speeds counter steer was defiantly an advantage as it added the vehicles ability to change direction but as the speed of the vehicle increased this could de-stabilise the vehicle significantly.  I remember reading that most modern cars that boast four wheel steer are electronically controlled so that at low speed there is a counter steer while at high speed the rear will start to parrel steer.  I guess the draw back to this, and the main reason I added 2 top trailing arms was to reduce compliance under heavy braking and acceleration.
And your correct about it not necessarily being a good idea for the road, anyone that knows my car knows it's not a "drive to work" machine, and hope I didn't give anyone the impression that this was the sort of thing you would want do to your classic 105!

[Sheldon McIntosh]

I remember reading that most modern cars that boast four wheel steer are electronically controlled so that at low speed there is a counter steer while at high speed the rear will start to parrel steer. 

Are there any modern cars that have it still?  Thought that was a fad that died out.  Low speed counter steer was intended for manuevres like parking, high speed parallel steering for things like quick lane changes.  Don't think it drastically improved handling though.  Porsche 928 had 'passive rear steer' in that under certain loads the outside wheel would toe in slightly.  I believe RX7 (the second shape, the one that looked like a Porsche 944) had a similar set-up.

That said, four wheel steer works fantistacly well on front-end-loaders.

[pep105]

Porsche 928 had 'passive rear steer' in that under certain loads the outside wheel would toe in slightly.  I believe RX7 (the second shape, the one that looked like a Porsche 944) had a similar set-up.

That said, four wheel steer works fantistacally well on front-end-loaders.

Yep series 4 and 5 RX7 had the passive rear wheel steer and the Peugoet 306 had it as well

Nice work Colin by the way

[cjheath]

That's the first time I've  heard the term "rear wheel steer" in regards to a 105. Are you referring to the axles ability to follow the arc of the lower trailing arms...

Yes. I saw the steering angle plotted as a function of body roll somewhere, along with those of the DeDion - sorry I can't recall where. But it was clear that the effect is that when you're reaching a degree of roll that's likely to equate to losing grip, you get a small degree of steering in the direction that follows the turn. I believe this gives the car a more "chuckable" feel. The DeDion does the same thing, but much better, as the steering stays almost dead straight until you hit about six degrees of roll, then the rear steering starts to come in quickly. It's affected by all the dimensions of the Watts linkage, including the centre piece, which isn't obvious until you think about what happens near the roll limits.

Of course, all these effects also depend on spring and rollbar rates too - go heavy and you get less roll - and independent of speed, unlike the mechanical RWS systems.

[Colin Byrne]

you get a small degree of steering in the direction that follows the turn

if by following the turn means that the rear toes out, as I  previously said, then by definition then the rear axle slip angle is reduced, following this then the lateral force generated by the rear tire must be  reduced.  At low speeds this is an advantage as it creates a reduction in the vehicles turning radius and a quicker change in direction  but at limit handling this reduction in rear grip in comparisons to the front wheel grip will promote significant over steer.  Coupling this with the instantaneous geometric weight transfer induced by having such a high rear roll center at the rear goes somewhere to explaining why when a 105 gets to the point of steeping out on the limit and is very hard to catch.  It was this snap over steer at high speeds that drew us to adding the pan hard rod in addition to trying to match the front and rear spring frequency's . 

Of course, all these effects also depend on spring and rollbar rates too - go heavy and you get less roll - and independent of speed, unlike the mechanical RWS systems.

I would imagine also  that ride height would have a significant impact on roll vs toe angle,

Thanks for brining this up Cjheath , this has made me think very hard about why these cars feel so fantastic on the road but when driven on the limit are very nervous, As I've said I've always considered the single top trailing arm as a design flaw rather than an advantage!

[alfagtv58]

So Colin, after Winton, whats the verdict??

[Barry Edmunds]

For what its worth Colin had a big smile on his face after his first couple of track outings at Winton so I guess that what he did leading up to the weekend really worked.

Barry

[Ads]

Thanks for the brief chat while my car was being scruitineered on Saturday Colin.  I had to get out of Benalla early Sunday, but dropped by Winton briefly, only to see flailing pairs of legs under your car- didn't want to interrupt!  Probably a good thing though, looking over your car instantly creates the "I'll just increase the credit limit" justification that I can fall for rather too easily when it comes to 105's!

Best of luck at Targa

[Colin Byrne]

Yea had a fantastic weekend up at Winton.  The car went really well and all the Mods did their thing.  The handling changes where fairly dramatic, finally the car had some serious rear end grip and was very predictable passing over the grip limit, I could even start trailing braking  into corners, definitely not a 105 anymore (it was understeering with the original front ARB!).  We had a small problem with one of the axles pulling out of the axle tube, the brake calliper stopped it coming out all the way but did a fair bit of damage to the calliper and the disc.  On inspection of the axle the press ring that holds the bearing into place had sliped down the shaft I think due to a combination of the tyre slightly rubbing on the inside of the guard and the extra rear end grip helped to pull it out.  We bashed it back into place and was able to complete the day.
   It's hard to say how much faster the car was going as I've never done the short track in that car before, but it felt great.  We made a few spring and shock changes during the day to balance the car out a bit and were very happy about where it was by the end of the day.  A few things to tidy up but can't wait for Targa now!!

[alfagtv58]

It's hard to say how much faster the car was going as I've never done the short track in that car before, but it felt great. 

The confidence you have in the car and it's handling is probably far more important at an event like Targa.  If you are tentitive about what it is going to do coming into or going out of a corner then it stands to reason YOU will choose to make the car slower through the corner.  If however you can predict the cars handling much better than before, you will be able to get through the corner much faster.

Probably not entirley true at a circuit, but somewhere like Targa where the fences/trees/dropoffs/bus shelters etc are a lot closer than at a closed circuit.

[pancho]

Hey Colin, awesome work here!! - tell me, how has this modification worked out now two years on ? Still happy with it or did you move onto another design?

[Colin Byrne]

Hey Pancho
yea still happy with the mod.  We had some trouble with the original design of the mount when we got to Tassie where the bumpy road caused some cracking isssues.  Those problems have all been fixed now and I've had no other issues with it since then.  I'm currently preparing for a Targa High Country and Next years Tas Targa and the only change being made to the Pan Hard Rod is the replacement of the rod ends with some softer nolathane bushes to help reduce some of the shock loads.   The mod defiantly allowed us to take the cars handling to the next level