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WideRide 86's Avatar
 
Join Date: Jan 2013
Location: NorCal
Posts: 116
Bottoming out in hard compression turns

Id like to know what the best way(s) to keep our cars from bottoming out in hard compression like Turn 6 at Laguna Seca (and Turn 8a for that matter). From what I can tell, my car never really hits or at least rides on bump stops under normal turning loads. A quick review of the forums turns up a lot of discussion of cutting bump stops to avoid suspension travel limitations on lowered cars and preventing unexpected oversteer, which I get. But I dont seem to be having that problem.

86 Carrera wide body with 23/31 Tbars, bilsteins valved to match, polybronze bushed, 16 wheels and the car is ~ 2950 with me in it. Its lowered, but not drastically. The rear is ~ 24 when I am in the car.

The car is reasonably stiff but Ive been in stiffer front engine cars that consistently hit and or ride on the bump stops (racing bump stops ala Fatcat motorsports). But I checked the travel with the car in the air using a jack under the TA and its incredible how high up in the wheel well it will go.

Besides the wincing that comes scraping the bottom, Ive actually gotten some damage the already reinforced ARB console failed and I have some damage to the paint on the wheel well. Sigh

So the questions are:
Are any of you limiting travel with bump stops? If so do you ever have undesirable oversteer as a result (re: all the trimming bump stop discussions)?
Is the car just too soft?
Any other ideas?

Id like to get this issue sorted properly (and soon) before my next two events, which are both at LS. I appreciate your collective input! Thanks.
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- '86 Carrera 3.2 wide-body
Old 05-14-2018, 08:42 AM
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2950?!
Old 05-14-2018, 12:34 PM
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I sounds like you do may not have the wheel rate/suspension travel combo that is optimal for that car and its use case.

As to the former, I will leave it to others to comment, because I have no experience with t-bars in 911s.

I can say that since you are up here in Norcal and since your car is a 3.2 at nearly stock weight, there should be an accessible recipe from any one of a number of area resources.

Jerry Woods Motorsports is one such resource, there are others as well. I suggest accessing resources with racing experience and/or decades of experience with air-cooled customers driving on track. There are several shops that have supported PCA track drivers for years, and there are known recipes. RMG, Don Wise, Sportech, Modderman's, Heyer Performance, and lots more. Of course one of my favorite shops is Reno Rennsport... deep experience.

As to travel and using bump stops as limiters... in my experience this is not a common approach with these cars. With the proper combination of parts and set up, there should be no need to use bump stops as travel limiters for most situations at local tracks. 911s don't like to hit berms really hard-- hopefully you are avoiding those types of jolts. You can do damage to things like your ring and pinion.

I have used Koni bump stops in other types of Porsche cars, along with stacker shims, but I don't think that should be necessary with a car like yours.

If you want to speak to some experts and get suggestions in person, several of them are likely to be in the paddock at Sears Point this weekend. Heyer, JWM, Reno Rennsport and a whole bunch of air-cooled experts will be racing under NASA sanction-- hopefully yours truly as well.

Many of us in the Porsche Racing Club will be in the garages right near the entrance.

Again, I strongly suggest being judicious with berms.
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Old 05-15-2018, 07:26 AM
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Do you have sway bars on your car?
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Old 05-15-2018, 08:39 AM
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Quote:
Originally Posted by Cory M View Post
2950?!
Haha! Yes it is still a street car with a mostly full interior. I guess you are implying that I need a stiffer wheel rate? For the class that I run in I am at the limit for weight reduction. I would need to remove a lot of weight to be competitive in the next class up. (excuses, I know).
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- '86 Carrera 3.2 wide-body
Old 05-15-2018, 12:09 PM
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Mike, thanks for the thoughtful response. I've learned a lot from you and others here on these forums over the years. I agree that there are plenty of tried and true recipes for track cars like mine and I definitely got input from as many sources as I could before I did my suspension upgrade last year, including folks on pelican, racers from PRC and PCA, and local shops. I selected 23/31 bars as this seemed like a reasonably stiff combo without being so bad on the street that I would not want to drive it to the track or the rare weekend ride.

Honestly, this combo is pretty great most of the time. Its really the dip at T6 and compression in T8a at Laguna (not the curbs) that are the problem. What I am hearing is that your 911 race car (and presumably others in spec 911 or GT classes) typically do not bottom out at these turns without adding height to the "standard" bump stops for the shocks being used. This makes sense as you are probably 600lbs lighter and running higher spring rates.

Thanks for the invite out to sears point. I've been out there for a couple of events while PRC was there and have walked the garages. Good luck out there and have a blast. Thank you again for your input.
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- '86 Carrera 3.2 wide-body
Old 05-15-2018, 12:38 PM
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Quote:
Originally Posted by kent olsen View Post
Do you have sway bars on your car?
Yep, adjustable on the rear, stock on the front. Oddly enough its the driver side (inside for T6 left hander) that bottoms the worst, so roll stiffness is not the primary culprit here.
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Old 05-15-2018, 12:43 PM
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Are you certain your rear shocks aren't damaged or worn out? I had a bad rear shock on my 911 street car and it would bottom out in a similar way. I swapped in some newer shocks and it solved the problem. If you do need new shocks you should consider custom valving to match your weight, spring rates, and intended use. Stiffer torsion bars, or coil overs, in the rear would help but that would make the driving on the street worse. Might not be too much worse if you went with 33's. Otherwise you should probably increase the ride height a bit and see if that solves the problem.
Old 05-15-2018, 02:29 PM
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Have you considered trying a modern style progressive bump stop instead of the oe hard rubber pucks?
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Matt - 84 Carrera
Old 05-15-2018, 05:24 PM
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Quote:
Originally Posted by Driven97 View Post
Have you considered trying a modern style progressive bump stop instead of the oe hard rubber pucks?
I am interested in learning more about these. It seems like spec miatas utilize progressive bump stops and seem to ride around on them most of the time Do you know of any specific style that seem well suited for our cars? Where to buy them etc? It seems like the advanced race shock tuners must be using them but I don't have any direct knowledge.
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Old 05-15-2018, 08:38 PM
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Quote:
Originally Posted by Cory M View Post
Are you certain your rear shocks aren't damaged or worn out?
The shocks were recently rebuild and specifically valved for the TB setup. Raising the ride height is on the short list of options and likely a part of any solution. Thanks Cory.
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Old 05-15-2018, 08:41 PM
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Quote:
Originally Posted by WideRide 86 View Post
I am interested in learning more about these. It seems like spec miatas utilize progressive bump stops and seem to ride around on them most of the time Do you know of any specific style that seem well suited for our cars? Where to buy them etc? It seems like the advanced race shock tuners must be using them but I don't have any direct knowledge.
Yeah out of my league. I do know most modern cars have quite long bump stops, and they're designed to be used as secondary springs, quite a contrast to our cars where they were a last ditch effort to protect the shock internals.

I did find a site that had force / displacement charts for all of their bump stops once (energy suspension maybe?) but quickly realized that I had zero idea what I would need.
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Old 05-16-2018, 07:49 AM
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Re: modern bump stops, Mike referenced the Koni ones above. Penske also makes good racing bump stops. Koni and Penske bump stops are very different from the old rubber ones that came on 911s years ago. The rubber kind don't really compress much at all. When your suspension hits a rubber bump stop in compression, your spring rate goes essentially to infinity in a very short distance - like 1/8" or maybe 1/4". Handling is badly affected. Koni and Penske racing bump stops actually compress. They come in different heights, like 1", 2", etc. and are progressive. So they gradually go from whatever your effective spring rate is up to infinity as they compress, but handling isn't as badly affected since it happens over a much wider range of motion.

I'd agree though with comments above that with heavy torsions like you have, it's a bit odd that you're bottoming. Could the shocks be worn or have the wrong damping settings? Let's see if someone with a similar setup can comment, or call an experienced racing shop in your area that is familiar with 911s. Jerry Woods was suggested, definitely a respected shop.

Scott
Old 05-17-2018, 04:31 AM
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I will ask around the paddock this weekend to see if anyone has used products like the Koni bump stops in your application. I used them on my 951. I think I still have some somewhere in my workshop.

It is of course hard to compare apples and oranges. I wonder how some of our race car set ups compare to your car. I think Spec 911 may come closest, but they weigh a lot less than your car (600 pounds) and likely have a lot higher wheel rates.

It may just be that your wheel rate, weight and ride height are not compatible with your suspension travel at those spots at LS.

We don't drive at LS much because of sound. The last time I was there driving in anger was 2011 RR IV. I think did some testing/DE driving there in 2016. I don't remember anything untoward in 6 and 8.

The turn 6 berm may have changed a lot since I have raced there... I'll ask about that as well since many in our group raced there in March. We have another race scheduled there in late June.

By the way, I think there are more TB choices out there than may be readily apparent.

Going to an infinite spring rate is never any fun, nor is damage from bottoming out. I hope that you can resolve this quickly. Kudos to you for running a 3.2 on track!
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Old 05-17-2018, 05:55 AM
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CW or CCW, is one track driving direction worse rubbing than the other?
CCW direction is where compression rubbing on 911 oil lines and tank fittings can occur (yikes)
Put marking paint on the wheel wells and swing arm bolts.
Are you using any wheel spacers?
I went with custom oil lines and a recessed tank fitting to get clearance for 315's
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Old 05-17-2018, 08:54 AM
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