85euro928

Has anyone take the time to determine the correlation between ride height and tire clearance? I'm asking because prior to installing my new wheels and tires my rear ride height was 172. With the new tires being about an inch taller I figure my rear ride height was about 185. Last weekend for laughs I raised the adjuster on the rear shocks one turn and this morning while going to work the rear tires rubbed. Granted I was pushing it through a tight turn with a dip at the bottom but I do this every day and never had it hit before? Is there more rear tire clearance with the car lower?

FredR

Thom,

For those of us that know something about wheel fitments on the 928 it would help if you can advise what wheel size/offset you have fitted, whether you have any spacers fitted and what size section/profile you have fitted.

Also, if you have experienced a tire rub what came into contact and where.

I believe the rear wheel geometry on your model is identical to the S4/GTS so clearance wise identical to the S4 perhaps. The S4 can carry a 295x30 section on a wheel with 60mm offset with absolutely no problems whatsoever and well clear of the fender and the suspenders. If the wheels have less offset the first contact point would be on the inside of the fender. I doubt those wheels would ahve an offset greater than 60mm.

Ride height is not normally a factor in this equation. I am a little puzzled as to why you think this is a factor given the photo does not suggest anything abnormal at first glance. In fact it looks rather well set.

With the ride height set lower invariably there will be more clearance relative to the side of the fender as the suspension compresses. Perhaps you can explain how the diamter of the wheel has increased by 1 inch- that sounds quite a radical change relative to the plus sizing concept.

Regards

Fred

Mrmerlin

If you raised the collar one turn you added about 4mm of ride height to the rear wouldnt you want to turn the collar lower to reduce the ride height?
What size tires and wheels and offset do you have now.

If you were doing this just for laughs did you take an actual measurement of the ride height after installing the wheels and driving the car before you made any adjustments.

when the rear wheels go lower or move downwards away from the car the tops of the tires will move outwards and vica versa

WallyP

The tires move in a path determined by the geometry of the suspension linkage. The path is not changed by adjusting the ride height - only the static starting point on the path...

85euro928

BTW, thanks for taking the time to think about this.

85euro928

Thanks

85euro928

This is interesting. Most likely my suspension has dropped while driving more than the 4mm I raised the car recently. And since it didn't rub before it is highly unlikely that me raising the suspension those 4mm had anything to do with the tire rubbing, this time.

However, this still doesn't answer my question. I guess I will just lower the car back down one turn and leave it at that.

Thanks for your help.

Mrmerlin

sorry I am confused at what your trying to do here Thom,
you said the new tires are 1 inch taller than the old ones that means that the car will be 1/2 inch taller than before,
and the top of the tire will be 1/2 inch higher than the old tire

so to compensate you lifted the shocks to increase the ride as well??
this sounds backwards to me.
BUT I am not really sure what your trying to accomplish, maybe to have more room above the tire?,

that may not be necessary.

though you will note that the inner edge of the top of the tire will be very close to the chassis,
and when the wheel is deflected upwards the top inner edge will move closer to the chassis.

NOTE raising the rear of the car up will also make the tops of the outer edge of the tires move outwards closer to the fenders

FredR

Thom,

First of all you have abrogated the plus sizing concept in that you have fitted a tire that is significantly bigger in diameter than designed for- this will increase the final ratio gearing which may suit you but suspect is not the best thing to do.

That you have fitted a 51mm offset and carry a 295 section tells me I am not too surprised you are rubbing and would defer to the experience of anyone running that combination. If my suspicions are correct you will see tell tale marks of rubber on the inner edge of the fender. Remember, when cornering hard the outer wheel tends to stand up as it flexes through taking more load and thus pushes the tire wall closer to the fender than in static conditions. Not sure of exactly how much increment we are talking here- not a lot but enough to make a difference I suspect.

Check that the fender flange is fully rolled back on itself [i.e. not at 90 degrees]. Also take a visual look at how close it is when static. You should be able to get a finger's width in there. If not- another warning sign.

If my observations are correct and you want to run like that then you may be a bit screwed given you really needed 60mm offset on the rears.

However all is not lost. First of all last thing you really want to be doing is running with the *** end up in the air- typical of 928's with tired front springs. It puts more weight to the front and increases the inherent under steer. It also reduces effective camber. Try to keep the running height differential [front to rear] stock but if anything keep the rear a bit lower.

Changing the ride height changes where the suspension arms sit relative to the horizontal andmore to the point, where the wheels are relative to the extent of travel when fully loaded. If full lateral load compression happens when the arms are horizontal that will not help either I suspect You should also remember that when you change ride height you also change camber and toe automatically- have you considered all this?

To get a bit more clearance from the fender and to take advantage of the extra rear grip you can increase rear camber and this will happen automatically if you lower the rear ride height but you also need to check/adjust rear toe which as you lower the car causes toe out if my memory serves me correctly.

On my S4 with a 285 section, 1.5 degrees of camber and 60mm offset I had something in the range of 10 to 15mm of clearance. You have lost 9mm with the offset, and have also lost 2.5mm with the 295 section and you probably have a stock camber [about 0.5 degrees- less after inreasing ride height?] which is also costing you a few mm if I remember correctly.

Do the arithmetic and I am not too surprised you may be rubbing.

With 295 section on my GTS chassis I was just about rubbing with the stock GTS spacers [38mm] so had to fit 24mm spacers which works well with those flared arches.

Trust the above gives you something to think about/work with.

Regards

Fred

dr bob

Following on and integrating some of the info shared:

You suggest that you raised the car by 4mm to try to stiffen the rear suspension. Be aware that raising the car does not, on its own, affect how stiff the springs are. A given spring, with the same amount of weight applied and with the same geometry, always be the same length. Raising the collar does not "preload" the spring, it only makes the spring/shock assembly taller and therefore the car sits further above the control arm.

Modern tires seem to like more negative camber than the tires available when the car was designed. There's a bit of camber change as the weight is moved from side to side during hard cornering, commensurate with the expected body roll. If you are going to DE or autocross the car, you'll undoubtedly want a little more negative camber with the newer-design and wider tires. Raising the car as you did changes the starting point for the camber change, and coincidentally raises the center of gravity. Assuming that there are no other suspension changes made, you'll start out with slightly less negative camber, and have slightly less throughout the travel, assuming the same body roll/weight transfer. With the CG raised slightly, you will have slightly more body roll though.

After all this, my suggestion would be to set the ride height to something safe and comfortable for your conditions and driving habits. Then set the camber slightly more negative at whatever that height might be, in consideration for the fender clearance and for the tires. High side loading will chunk the outside edges of the tires as body roll and camber change cancel each other out for the most part, and the side loading on the tire puts excessive pressure on the outside edge.

There are other factors, like toe, toe change with side loading, and effective toe and load direction change with the application of throttle or brake. So while it is an exact science on paper, there's some art to making the science apply under all driving conditions. I've modeled a lot of these factors and tried to apply the findings to the race car, but inevitably ended up tweaking the settings to make the car 'feel right' for whoever was driving it under the track and weather conditions on a particular day. The choice of tire and tire pressure settings is a huge component too.

85euro928

First I need to aoplogize as i'm writing this from my phone so it will be short. Second is that I need to say thank you to Fred, Stan, Wally & Dr Bob for the incredible amount of information you've provide me (us) with. There's a huge amount of information that will take me some time to digest.
I had no agenda when i asked my original question, I thought I would ask a simple question and get a simple answer but i guess i should know better by now that there are no simple answers with the 928 I need to clarify that the wheel/tire combo has been on the carcar sometime now and that since we rolled the fenders I haven't had any.problems and was happy with my previous ride height and handeling.
Fred, this wheel/tire combo has been used before which is why I bought them (ebay). I agree it's not the best combo but it works for me as my final drive is something like 2.72 and not 2.20 so the over sized tires actually work in my favor as first gear was too low and second gear too high with my limited low RPM torque.
Lastly (for now) I haven't been changing the ride height without consideration to alignment, I've had the car aligned twice since the new wheels/tires were put on and am scheduled to go again tomorrow for another alignment (which is why I was playing with the ride height).
Since the car handled fine before I will lower the car back down tomorrow morning (taking ride height measurement before and after) and then have it aligned.
I guess the answer to my question then is that ride height doesn't really affect tire clearance? I also need to read your replys again later as I'm a bit confused on the camberinfo. it appear Fred is stating that the cars prefer positive camber (2.0ish) while Doc is saying negitive camber is prefered, or did I read this wrong?

FredR

Thom,

I don't think any car since the model T runs a positive camber - apologies if I did not qualify my numbers with vectors. The stock camber setting is about -0.5 degrees- when you fit wider rubber you induce more grip and more deflection. As Dr Bob said earlier it is a very dynamic equation with many variables and most certainly not a one size fits all type of thing. Thus why you need to find what works best for your needs.

You have to remember that with alignment you are working with a "swings and roundabouts" type of scenario wherein some adjustments gain something and invariably lose something else thus a question of overall balance. Technology has moved on since our cars were conceived/specified and tire grip today is better than ever. With more lateral grip you tend to need more camber, but more camber can mean less braking grip. On the rear axle you need toe-in to enhance grip - less toe can improve the way the car drives out of a bend. I once set my rear toe to minimum spec to push this. Unfortunately I could not get the rear wheels to grip despite PSD- they [both] just spun when I minced the throttle. Went back to mid spec range setting and problem solved at the next event. I concluded that what had worked on my friend's race car in the States just did not work on our crrapply little autocross track- probably because we have loads of dust and thus grip thresholds are different. I was pleased that I could make such small adjustment, was able to feel/analyse the difference and correct it. We also have the additional complexity of street use and [in my case] very occasional track use [and the times when the two intertwine! ha ha].

For your needs I would target a rear alignment with at least -1 degrees of camber [I run -1.5 minimum] and rear toe set mid range. You will not have differential wear problems on the heel unless you habitually light up the rears at traffic lights- hopefully 928 owners are a bit more mature than that! I would also ensure that ride height keeps the same proportions as factory spec in terms of front/rear difference.

However, getting back to your original problem- you need to find out where you are rubbing. Just because the set up came off another 928 does not mean it will work on your model or for that matter, that it worked correctly on the car it came from. You can even have the same car with different drivers and one will find the problem and one never see it or feel it depending on how one drives and the nature of the problem.

With respect to oversizing, I would just never do that unless the car was outrageously undergeared- ie. you run out of revs in top gear. Not many 928's can do that and for sure no automatic variant that I am aware of can.

Regards

Fred

chitown928s4

So if i want more tire showing. I have to turn the collar one turn clockwise. In my sitch i think the rear is too low and the front too high. Is the front adjustable as well.

Jherriott

Maybe by *raising* the center of gravity you made the car less *tight* and more susceptible to G-forces. Therefore the car ended up leaning more easily?