Staggered tire size?

[theDoktor]

I am investigating whether anyone has successfully staggered tire sizes for autocross with either the Mk6 or Mk7 GTI. Over the weekend I got to talk with and observe a local hot shoe running G Street quite successfully using narrower tires on the rear of his 2-dr. Mk.7 He's running the normal 245/40-17 RE71-R's on the front with 215/45-17's on the back. All are mounted on Enkei Fujun 7.5 x 17 rims. He's also running a Strano rear bar.

I got to watch all of his 8 runs, and I was impressed how flat his car was where my stock suspension had me up on 3 wheels to the point the Mini drivers were commenting... I'm running the same rims as his and 245/40-17's all around with completely stock suspension and spent the day trying to stay ahead of the back end of my car. By the end of the day my times were starting to get somewhat competitive, but no where near where I need to be. I know a rear bar is a high-priority item, but if I can't get the rear tires close to the temperatures the fronts are seeing, I'm going to be dealing with oversteer from the work imbalance, particularly with my driving style. Less rubber on the road in the rear doing the same amount of work might help with that.

I do have some concerns with the electronics if I change the rolling diameter of the tires between front & rear. TPMS might not like different diameters. I turn off stability control when on track, so that isn't a concern. Don't know about ABS though. Anything else I might have to deal with? I'm not talking about huge differences in diameter either- the 245/40's have a 24.7" diameter with an 8.6" tread width; the 215/45 also has a 24.7" diameter and a 7.2" tread width; the 205/45 has a 24.3" diameter and a 7" tread width.

Anybody out there with first hand experience with this? What have you observed at the Solo National level, Pro Solo or Tour and not necessarily just the GTI?

 

[RacingManiac]

Sounds like George Modlin. And yeah he's been running that setup for a few years now. I don't think its a bad way to go, also given Strano bar is a lot less stiff than most of the aftermarket offering. I am not running a staggered setup just for practical reason. The amount of tire wear I am seeing and not being able to rotate regularly is not very practical for my use.

I have no issue building rear tire temp as long as its not a freezing cold day like PittRace was. Also my car is on 3 wheels pretty regularly and I am not seeing an issue with that.

As far as ABS goes I don't know what VW's system can handle in terms of size difference, the colleagues I work with at GM say we typically keep them within 6% of the rolling diameter to not upset the system. So maybe use that as a guide.

 

[theDoktor]

Yup, it was George. That rascal is quick!
I'm trying to decide which way to go next- rear bar or tires. Ultimately I suspect I'll be doing both.
Years ago, a good friend who, for a while was an instructor at Bill Scott Racing, built a first gen Honda Civic street car/autocrosser back in the late 70's that had both the smaller tires in the rear and adjustable bars F&R that was an unbelievable handler. With the street tires available back then, he was regularly pulling well over 1G on a skid pad. I used a similar adjustable front bar design to his on my old DS X1/9 that worked well, but the car was out-classed by the then-new MR-2's. That project required a custom-made bar plus a bunch of custom-made machined parts to make it fully adjustable. I'm trying to avoid going that route if possible.

 

[RacingManiac]

What tires do you have now? If you don't have the current 200TW I'd go that route regardless.

I've generally been pushing people towards larger rear bar as the first thing after tires. My normal go-to recommendation is the 034Motorsport bar since it is adjustable and isn't too much softer than the H&R bar that I run. I feel like having that one click softer on the rear bar may give you slightly more flexibility in those instance where the car might just be too loose(IE when it is cold and wet).

H&R is fine when it is bone dry and warm(even when its wet and warm). But not being adjustable is an issue sometimes.

 

[theDoktor]

I'm running the RE71-R's- 245/40-17 all around. The 32psi pressure in the front that you recommended worked well. Ran rears at 36, and it was barely catchable. I'm only using the outside 2/3's of the rear tires even with -1.5 degrees of camber (just to the interior side of the center rib).

The 034 Motorsports bar is #1 on my list. Sure wish someone made a 26 or 27mm tubular rear bar- that solid bar is heavy! Planning on adjustable endlinks too.

 

[RacingManiac]

I think the Neuspeed is a 27mm tube no?

Interesting on the balance. I guess I have never driven the car on stock rear bar on autoX but it was never that loose even with my H&R...unless its freezing cold then its pretty sketchy....

With the bar I ran even pressure most of the time. Maybe try that first then.

 

[RacingManiac]

Neuspeed: http://www.neuspeed.com/331/2/0/3110/2502275-neuspeed-race-series-rear-anti-roll-bar-27mm.html

 

[theDoktor]

Looks like the 034 bar is next. It's adjustable- the Neuspeed isn't.
I'm also going to start tweaking my driving style. The stock GTI doesn't like the side-step technique on the slaloms....gonna try modifying my corner entry technique a bit too- in slow, out fast. I know I'm leaving a bunch of time on the table because of over-aggressive driving

 

[RacingManiac]

Yeah I had to learn to back off on entry. One as to not upset the ABS and 2 it just makes getting on the throttle later, which is where this car finds most of the time.

 

[victorofhavoc]

Looks like the 034 bar is next. It's adjustable- the Neuspeed isn't.
I'm also going to start tweaking my driving style. The stock GTI doesn't like the side-step technique on the slaloms....gonna try modifying my corner entry technique a bit too- in slow, out fast. I know I'm leaving a bunch of time on the table because of over-aggressive driving

Yeah I had to learn to back off on entry. One as to not upset the ABS and 2 it just makes getting on the throttle later, which is where this car finds most of the time.

I've gone to two events so far this year. I jumped on the direzza z3, just to be different from the typical re71 crowd, because why not. Both days were damp to wet and on stock suspension (no rear bar) and a 245/45/17 z3 I was pushing the tail out a lot. I slowed down my corner entry and with a lot of finesse could throw the rear out right after mid corner. When it was damp I was running 40 psi front and 38 rear, and 40/36 when it was wet. The front never plowed unless i really really pushed it, and overall I came 6th pax in a region with a lot of very strong drivers. I was also 2 sec faster than the next fastest in my class.

I've got the ST rear bar (25mm) on now, and will try that out. I'm not a fan of going to stiff on sway bars since it does reduce ultimate grip. I eventually want to run this car for time trials and the autox sth class, so I also got that bar since I plan on getting the bilstein damptronic suspension on, and that should take out a lot of understeer on its own.

Maybe it's the difference in my tires/surface, but I don't know how you guys are running pressures that low. I know the re71 have stiff walls, but i'm pretty sure the direzza z3 are just as stiff if not stiffer (a guy on same size re71 and wider wheels than me 8" vs 7.5" had similar pressure but his sidewall was bowing out where mine was straight...so just assuming z3 might be stiffer). I tried 34/34 psi and i was plowing early. Tried 34/38 and was plowing late. Finally tried 40/38 and kept the line well while allowing the rear to step out when i added throttle. This was all in the wet, so I think in the dry I'll be doing 36/38 as a starting point.

For slaloms, I've found that getting on the throttle sooner and letting off as I rotate has given me the smoothest runs. The turbo lag in this car isn't significant, but it is enough to notice in the slalom.

 

[Mike-E]

I was confused at first, then I figured out you Yankees call different size tires on the front & rear; Stagger. Those of us raised in NASCAR country, call different size tires on the left & right; Stagger. Done correctly, all a NASCAR driver has to do, is turn the steering wheel on the straights (car automatically turns left). That frees up the drivers hands for Texting, having a Coke Cola, McDonalds XL-fries and taking a Stanback (now available in tablet or powder).

 

[RacingManiac]

Both are correct and talk about the same thing. Just different direction of stagger.

You can have width stagger and diameter stagger too. In most cases we are talking about width stagger...

 

[victorofhavoc]

I had my first fully dry/warm event on the Z3. I installed the st rear bar after the last event and I think I jacked up my rear alignment. I went from 2 seconds faster than the next fastest in my class to being 1.5 seconds behind the same guy. I'm going to get an alignment this week before the event next weekend and we'll see how far off it is.

Also, I tried out 40/40 f/r pressure and that was a no go... Just plowed everywhere. Went to 36/38 and no Bueno. Tried 36/41 and things got better. Then did 32/41 and the car was rotating but the front out wear was huge and tire temp was higher on the outside than the middle or i side. I'm going to try 38/44 at the event tomorrow and then 36/44 if I need to go lower.

I think the 25mm st rear bar is actually less stiff than the factory bar, and I'm pretty sure I'll need to swap it out. Any thoughts on other bar setups?

 

[theDoktor]

I think the 25mm st rear bar is actually less stiff than the factory bar, and I'm pretty sure I'll need to swap it out. Any thoughts on other bar setups?

My two cents worth:
1. Keep making adjustments one item at a time & keep WRITTEN records of each change and the results. Multiple changes simultaneously will find you chasing your tail in no time. Stay with the ST rear bar for now; get a quality 4-wheel alignment next. You can achieve a surprising amount of negative camber in the front (where it is REALLY needed) by adjusting both the top strut mount and the lower hub-to-strut mounts.
2. The 25mm ST rear bar is much stiffer than the OEM rear bar. The next bar you might consider is the 034 rear bar. it is also a 25.4mm solid bar, but has (2) adjustment points to increase bar stiffness. You might want to consider adjustable end links with this bar. The OEM links are questionable when using the stiffest position on the 034 bar. You can also take out any pre-load in the bar with adjustable links. Another possibility if you get really serious about chassis tuning: you can tweak the corner weights by adjusting the bar end links.

 

[victorofhavoc]

My two cents worth:
1. Keep making adjustments one item at a time & keep WRITTEN records of each change and the results. Multiple changes simultaneously will find you chasing your tail in no time. Stay with the ST rear bar for now; get a quality 4-wheel alignment next. You can achieve a surprising amount of negative camber in the front (where it is REALLY needed) by adjusting both the top strut mount and the lower hub-to-strut mounts.
2. The 25mm ST rear bar is much stiffer than the OEM rear bar. The next bar you might consider is the 034 rear bar. it is also a 25.4mm solid bar, but has (2) adjustment points to increase bar stiffness. You might want to consider adjustable end links with this bar. The OEM links are questionable when using the stiffest position on the 034 bar. You can also take out any pre-load in the bar with adjustable links. Another possibility if you get really serious about chassis tuning: you can tweak the corner weights by adjusting the bar end links.

I've got everything written down, and yeah I effectively only make one change at a time. Sometimes there are variables I can't control (weather).

On the stock bar I was at a 4 to 6 psi differential and I'm pretty sure with the st bar I'd need to go to 12 psi diff to just get it back to stock rotation levels. FYI, on direzza Z3 I was running 36 front and 40 rear with pretty even tire temps on stock rear bar. Moving to the st rear bar, I had to run 32/41 and I probably should have gone to 32/44, but I ran out of air. Unfortunately at 32 front I was getting a lot of outer wear and heat and the "in pit" temps were at 151/125/134 from outside to in, so the front would like more camber and a bit higher pressure. Alignment is next to get more camber up front and some toe out.

From my personal experience with this bar I can say a several "possibilities":
It's not stiffer than OEM
My bar might be defective or structurally weak somewhere
I installed the bar incorrectly or messed up alignment somehow
The 12 degree temp difference between the last events and these affected the grip of my tires much more significantly than expected

really, any of these or none of these could be true.

I'll just have to keep tweaking and find where it all lands. I'm really hoping the STH class takes root, so I may end up going down that route sooner than expected, just to show that there's interest in it. The DSG arguably requires a tune, so i was always going to end up not in GS. We'll see how that affects time trials too...