A question on geo & tracking

[benjj]

I'm currently in the process of turning this



into this



in time for this



I have been advised to get a full geo and tracking adjustment done and there is a Porsche specialist locally (RPM) who can do the whole shebang for around £130.

Now this is where I may make a bit of a tit of myself.

I run two different sets of wheels on the Porsche. Both are 15'' 7J, in fact they're 2 identical sets of wheels, but on one I run 195/65/VR15 winter tyres, on the other set I run 215/60/VR15 sticky summer tyres.

The car has now been retired from normal daily driver duties in readiness for full rally prepping. I would say from here on in it will be wearing the skinny winter tyres 80% of the time as they offer a better overall package for rallying in general.

Does it matter which wheels I have the tracking done on? Will changing the wheels between winter and summer sets upset the setup?

I know, I know, I'm a bellend and there is probably an easy answer to this - I just don't happen to know it

[nta16]

seems a reasonable question as you're not just using the car for normal road use if one set of tyres lifts the car higher than the other it might make a difference

ask the person that's going to do the work

I do remember a few years back Porsche had problems with the geometry of one of their models (German car with problems ! and it's a Porsche, surely not !!) so they invested in training and equipment on geometry and as such at the time had the equipment and training better than most others

Edited by nta16 on Thursday 30th May 14:04

[john2443]

The tracking 'machine' rests on the rim not the tyre (it might hang from the tyre, but the contact is made with the rim) so I can't see that the tyre size makes any difference.

[benjj]

Thanks chaps.

I've just used one of those tyre calculator thingies and see there is a 0.79% or 5mm difference in rolling circumference between the two tyres.

I can't really find anything online that will help me work out the precise height of the top of the tyre from the road surface, I guess I'll have to do that myself the old fashioned way

Either way the question has prompted me to have the geo and tracking done with the winter rubber on. At least that way I know it is set up right for the most often used set.

[nta16]

well I'm hurt, do you never take notice of my posted links, have a look at all on this page (and the other three pages) for (IIRC) what you want plus a load more - http://www.carbibles.com/tyre_bible_pg4.html

no doubt you're sensible enough not to have old tyres but for others note the bit about age of tyres

ETA: see - Static loaded radius, outer diameter and rolling circumference

Edited by nta16 on Thursday 30th May 14:43

[benjj]

nta16 said:

well I'm hurt, do you never take notice of my posted links, have a look at all on this page (and the other three pages) for (IIRC) what you want plus a load more - http://www.carbibles.com/tyre_bible_pg4.html

no doubt you're sensible enough not to have old tyres but for others note the bit about age of tyres

ETA: see - Static loaded radius, outer diameter and rolling circumference

Edited by nta16 on Thursday 30th May 14:43

Not sure what you mean, you didn't post a link until half an hour after my last post?!

Anyway thanks, I'll have a read of that now with my late lunch.

My tyres (both sets) are 2012 dot codes so well fresh. Frankly I use them up so quickly that they never reach more than about 1.5 years old

[nta16]

benjj said:

Not sure what you mean, you didn't post a link until half an hour after my last post?!

A

I'm sure you've had the sheer joy of my many previous post and numerous links but just thought perhaps you don't recognised the PH name, note my car in my Profile

perhaps I've not put this link on PH for a while

whatever you have it now

£130 sounds a reasonable cost

obviously it's best done on a car that hasn't got too many worn parts and components fitted (I'm not suggesting that's the case with your car)

[benjj]

Have we met before on a rally, Nigel?

[944Nick]

Somewhat off topic, but isn't the start of 944 production after the cut-off date for the modern classics class on Le Jog? Are they more flexible than I imagined on that point or have I miusunderstood the regs?

[benjj]

Nope, not for LeJog and CRA events. They have:

Per 1967
68-74
74-84

HRCR rallies are the same but the cut off is Dec 31 1981.

Mine is a bit of an odd one though, that's the reason I bought it. It's a pre production car, made in Zuffenhausen in late 81, thus it qualifies for everything!

[velocemitch]

Don't you need 70 profile for HRCR Ben?.

Wouldn't imagine the tracking is affected by the tiny difference between the two wheel sets you mentioned, but why not take a pair of the other Tyres with you and ask them to see if it does?.

[nta16]

benjj said:

Have we met before on a rally, Nigel?

no never, I used to post here as 'SB-Nigel' and changed to 'na' but changed to 'nta16' to fit in with changing an advert on PH because the PH computer changed it mind on accepting names with less than three characters but then wouldn't accept my new name to place an advert because my email address was already taken on the previous two character name it wouldn't accept

[benjj]

velocemitch said:

Don't you need 70 profile for HRCR Ben?.

Wouldn't imagine the tracking is affected by the tiny difference between the two wheel sets you mentioned, but why not take a pair of the other Tyres with you and ask them to see if it does?.

Yep, still a requirement for 70 profile on the HRCR stuff, word is that its changing soon though. HERO / CRA happy with 65 profile though.

Doubt this car will go anywhere near the HRCR rallies though, mainly as it is getting the full treatment! A million lights, interior stripped to nothing etc. All the cool stuff that they don't like and I do

Am still going to build a car for HRCR clubman stuff though, just got to decide whih way to go.

[v8250]

Hi Ben, like the idea of a 944 on LeJog; splendid. Re' the geo/tracking Q?...it's quite sttraight forward. As mentioned above the set-up will be measured from the wheel/hub. The important point is for this to be a proper four wheel aligment where the datum point is taken from the rear wheels/hubs, and then dialled in/out at the front end. This is also useful to show that the back-end is 'true'.

The tyres will make little difference to these set-points but will naturally change drive characteristics of the car a little. More important will be the ratio/percentage difference and how this alters your Halda/Brantz unit for speed-distance timing. Here, you'll naturally need to re-calibrate for the tyre change...in fact you should calibrate for each event to be certain until you get the feel that they're 'spot-on'. Tell me, how/where are you calibrating? Using the 1/10th km markers on your local superhighway I hope

There are guys out there who don't bother changing tyres from M&S to summers as they find even summer events have some off road stages and they just get used to not needing sticky rubber. Also, tyre pressures are very important and will be worth playing with when 'hot' until you find the best all round pressure for you the driver and the car.

If you want to tinker with your suspension buy the Automotive Math Handbook by Forbes Aird, and, How To Build & Modify Suspension & Brakes by Des Hamill. Even though the 944 is a superbly set up car, both are excellent publications and give clear insight on how to get the best out of almost any suspension-wheel-brake set up.

Have fun Old Bean...look forward to reading about your LeJog'ing exploits 2013

[944Nick]

benjj said:

Nope, not for LeJog and CRA events. They have:

Per 1967
68-74
74-84

HRCR rallies are the same but the cut off is Dec 31 1981.

Mine is a bit of an odd one though, that's the reason I bought it. It's a pre production car, made in Zuffenhausen in late 81, thus it qualifies for everything!

Excellent. I did Le Jog in 1999 and 2002 and had a 944 S2 for 12 years from 1998 so will look forward to reading about your adventures in due course :-)

[velocemitch]

Ben, I believe we will be stuck with the 70 rule a bit longer, there was no movement from the MSA at the last Rallies committee meeting, mainly because Paul Loveridge still see's the 70's as the 'be and end all' for controlling the power of the Historics on the Stage's and refuses to accept Road events should be different. I think we would all like to see 65's especially on Cat 3 Cars, but not next year it seems.

I can't believe HERO get away with the stuff they allow on their events, (Lights, multi colours, interiors etc etc) appreciate they often run on a reliability trial permit not a rally permit, but most other clubs would be carpeted by the MSA for allowing it.

[interloper]

Taking something as modern, reliable and comfortable as a 944 on the lejog almost seems like cheating! Or possibly sensible?
Regarding the geometry set up, I don't think the slight differences in tyres widths will hurt. But I think its a pretty important thing to have done... This is mainly because the 944 can be adjustable all round, which means years of road use and abuse can put these cars out of alignment at all four corners, making it just a tad hairy at the limit.

[benjj]

velocemitch said:

Ben, I believe we will be stuck with the 70 rule a bit longer, there was no movement from the MSA at the last Rallies committee meeting, mainly because Paul Loveridge still see's the 70's as the 'be and end all' for controlling the power of the Historics on the Stage's and refuses to accept Road events should be different. I think we would all like to see 65's especially on Cat 3 Cars, but not next year it seems.

I can't believe HERO get away with the stuff they allow on their events, (Lights, multi colours, interiors etc etc) appreciate they often run on a reliability trial permit not a rally permit, but most other clubs would be carpeted by the MSA for allowing it.

Yes, understood on the 70 profile not being currently permissable.

Re your question on HERO taking perceived liberties, I have an opinion on that.

I'm not sure if its my place or not to comment, especially as Patrick and Tomas are friends of mine, but they are the polar opposite of Loveridge.

They really understand the fact that people want adventure, not to be hamstrung by red tape and some bloke in a dingy basement clinging on to his last few items of power.

They are also charging good money for their rallies and since the CRA merger are by far and away the dominant force outside the HRCR. They need to offer a 'product' for want of a better word that people see value in. I think they do that in spades.

If the MSA did try any funny business they would meet a great deal of resistance from competitors, I'd be at the front of that queue.

There is room for both to flourish so long as both sides accept the differences between the two - and frankly I only see grumbling coming from one side of this.

There, I've said my piece. Now I'll get back to nailing too many lights to the front of my car and ripping out the back seats

[benjj]

interloper said:

Taking something as modern, reliable and comfortable as a 944 on the lejog almost seems like cheating! Or possibly sensible?
Regarding the geometry set up, I don't think the slight differences in tyres widths will hurt. But I think its a pretty important thing to have done... This is mainly because the 944 can be adjustable all round, which means years of road use and abuse can put these cars out of alignment at all four corners, making it just a tad hairy at the limit.

You may be surprised at the differences between early and late 944s. Mine is ery early and does without PAS, TC, ABS, decent heater etc. It is very basic and has nothing in its arsenal that the 911s, Escorts etc don't.

Class 3 is an odd one though and not a class I've regularly competed in. It isn't unusual to see Porsche 911SCs going up against rubber bumper MGBs, late Escorts battling with Spitfires etc. There is een a chap I met on a rally in Germany last year who is using his early XR3i, I believe he's knocking around the HRCR scene too.

I kind of know what you mean though.

[benjj]

v8250 said:

Hi Ben, like the idea of a 944 on LeJog; splendid. Re' the geo/tracking Q?...it's quite sttraight forward. As mentioned above the set-up will be measured from the wheel/hub. The important point is for this to be a proper four wheel aligment where the datum point is taken from the rear wheels/hubs, and then dialled in/out at the front end. This is also useful to show that the back-end is 'true'.

The tyres will make little difference to these set-points but will naturally change drive characteristics of the car a little. More important will be the ratio/percentage difference and how this alters your Halda/Brantz unit for speed-distance timing. Here, you'll naturally need to re-calibrate for the tyre change...in fact you should calibrate for each event to be certain until you get the feel that they're 'spot-on'. Tell me, how/where are you calibrating? Using the 1/10th km markers on your local superhighway I hope

There are guys out there who don't bother changing tyres from M&S to summers as they find even summer events have some off road stages and they just get used to not needing sticky rubber. Also, tyre pressures are very important and will be worth playing with when 'hot' until you find the best all round pressure for you the driver and the car.

If you want to tinker with your suspension buy the Automotive Math Handbook by Forbes Aird, and, How To Build & Modify Suspension & Brakes by Des Hamill. Even though the 944 is a superbly set up car, both are excellent publications and give clear insight on how to get the best out of almost any suspension-wheel-brake set up.

Have fun Old Bean...look forward to reading about your LeJog'ing exploits 2013

Andrew you old dog, long time no speak

With all that brain power and experience we could do with a chap like you to informally work with the charity like I do, fancy a chat?

Edited by benjj on Friday 31st May 09:25