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914World.com _ 914World Garage _ 914-6 weight and CG questions

Posted by: Bob Wolcott May 18 2005, 12:07 AM

Does anyone know what the weight, weight distribution (F/R) and CG height of the typical 914-6 is?

Also, what is the widest tire (16" fuchs) that can be safely installed on the rear of a 914-6 that is fitted with GT flares, while still allowing for full suspension adjustment?

Thanks,
Bob

Posted by: Brad Roberts May 18 2005, 12:10 AM

Welcome Bob!! You've come to the right place.

Great first post

I dont have the CG info here on this computer, but as long as this post isnt buried by tomorrow morning on the second page.. I'll answer your CG question.

Oh.. The largest I have seen is a 255 under a GT rear flare.

B

Posted by: Bob Wolcott May 18 2005, 12:19 AM

Brad,

Thanks for the reply. I plan to model the suspension into Mitchell suspension software (http://www.mitchellsoftware.com/) to get an idea of how the system reacts to various conditions.

Here are some pictures of my project. It is a factory six (original motor shown) with a 2.7 liter RS motor installed. I'm not really shooting for a race car, but I do want it to perform well (all chassis upgrades, brakes, etc...).

Thanks,
Bob

Posted by: Brad Roberts May 18 2005, 12:25 AM

You will find it VERY interesting. Most people (in the past) have been under the impression that our cars gain a lot of negative camber in bump, when in fact they "gain" very little. I have a Excel based setup sheet that several top Winston West teams use for suspension simulation. It works very well. The rear semi trailing arm setup isnt the greatest in the world... but works. The worst part I have found: The CG changes when rear camber changes. This was corrected with a different control arm that alows adjustment of camber at the wheel and not at the front of the control arm.

B

Posted by: Brad Roberts May 18 2005, 12:27 AM

Something else we have looked into recently:

Dropping the front of the front control arms down several mm's to help fight "dive" under braking. I havent tested this yet.. but it is on the agenda for this summer. The worksheets show a significant reduction in dive when we simulate this setup.

B

Posted by: Bob Wolcott May 18 2005, 12:29 AM

http://oregonstate.edu/~wolcottr/Porsche/

Oops, forgot to post the link :-)

Thanks,
Bob

Posted by: Brad Roberts May 18 2005, 12:37 AM

He he.. I have seen many projects in that stage. You will fit in fine with this group Just dont let the jackstands rust or you will never hear the end of it..LOL

Nice car Bob. Good start.

B

Posted by: Series9 May 18 2005, 01:05 AM

QUOTE (Brad Roberts @ May 17 2005, 11:10 PM)

Oh.. The largest I have seen is a 255 under a GT rear flare.

B

I'm going to try a 275...

Posted by: Brad Roberts May 18 2005, 01:10 AM

Joe,

somebody "must" pioneer this stuff..LOL Good luck. Show us how it goes.

B

Posted by: ken914 May 18 2005, 05:35 AM

I have 285's under the back of my /6. 285/50/15 to be exact.

The 285's are way too big in my opinion. The car came that way. I need to get it aligned but am going to get new tires first, as I think the 285s will keep me from getting the right camber in the back.

Attached image(s)

Posted by: James Adams May 18 2005, 06:59 AM

QUOTE (Brad Roberts @ May 18 2005, 01:27 AM)
Something else we have looked into recently:

Dropping the front of the front control arms down several mm's to help fight "dive" under braking. I havent tested this yet.. but it is on the agenda for this summer. The worksheets show a significant reduction in dive when we simulate this setup.

B

Porsche did it on the 930 - should work well.

QUOTE
You will find it VERY interesting. Most people (in the past) have been under the impression that our cars gain a lot of negative camber in bump, when in fact they "gain" very little.

That's the real reason that raised (RSR) spindles are desired on the front - they get you on the other side of the camber curve so that negative camber increases more in compression. Most people don't seem to understand this aspect of the raised spindle, and get hung up on the bump steer issue (which a raised spindle does not help).

Posted by: James Adams May 18 2005, 07:01 AM

QUOTE (Brad Roberts @ May 18 2005, 01:25 AM)
The CG changes when rear camber changes. This was corrected with a different control arm that alows adjustment of camber at the wheel and not at the front of the control arm.

B

Ummm - the CG can't change unless you are moving parts around. Do you mean roll center?

Posted by: slivel May 18 2005, 08:48 AM

When I still had steel GT flares on the car I ran Hoosier 275/45/16's in the rear. Fittment worked but it was close and I rolled the fenders.

On CG, my car with fiberglass Sheridan body has these proportions: 957 lbs front, and 1311 rear. Thats with me in the car and I weigh 150. Car is a 3.4 and has a 901 box.

Posted by: MattR May 18 2005, 12:28 PM

Why do you want CG? IMO its an overrated figure. Remember, a dumbbell has a central CG. Is it something the software calls for?

Im going to try to model the 914 suspension in Adams/Car when we get a copy for the FSAE team. Im curious how the decambered ball joints and everything affect the geometry.

Posted by: Bob Wolcott May 18 2005, 05:36 PM

I'd like the CG height so I can get a handle on the roll stiffness values that would be needed to maintain a reasonable amount of body roll under cornering. It will also help in getting a handle on squat and dive characteristics.

This will aid in selecting components from the ground up, sort of how I did it in the OSU FSAE program when I was there :-)

Thanks,
Bob

Posted by: Brett W May 18 2005, 11:19 PM

The CG is around 16in off the ground. The roll centers move a bunch in our semi trailing arm rear suspensions. Our suspension is not as bad as the 510 but it is pretty bad. You need to run a bunch of roll stiffness to keep it under control. The 914 semi trailing arm setup gains less than .5 deg of neg. camber per inch of suspension travel. The problem is the number of compromises you have to make to get a decent setup. The sweep angle plays a critical role in how things are move in bump and roll. The problem with the 914 stock suspension setup is everytime you change the suspension settings (camber and toe) you move the roll centers. Sometimes down sometimes up.

Another problem is the location of the wheel centerline in relation to the pivot point of the suspension. Moving the wheel centerline above the suspension pivot creates understeer under acceleration. The toe change contributes to roll oversteer, which can make the car hard to drive at the limits. Oh yeah you need a longer semi trailing arm.

The best way to set the suspension up (if you don't want to do it the way Sheridan did) is to put the pick up points in the optimal place and then cut the bearing housing off the trailing arm and re-position it to get the correct suspension alignment. Even with this method you are still limited to the semi trailing arm problems.

If you go up front, dropping the front mouting point isn't a bad thought and the effect is some anti dive. ON a street car this wouldn't be bad or on a car that is limited to a more or less stock ride height you could use this. But our cars operate at such a low ride height and our CG is so low (on race cars)that anti dive is not really necessary. All anti-dive is, is a mechanically induced binding in the suspension. Yes it can help in some instances but in ours it is not really necessary.

For the front struts, you just get the joy of limited camber gain. Most race cars have a problem with shock travel. On a lowered strut car it is hard to get enough without the strut bottoming out. But at the ride heights of a lowered race car you shouldn't have that much suspension travel anyways. My car is designed to use 1.5in in bump and droop. Of course I am not using struts and rear trailing arms either. Even when I was planing on running struts and semi trailing arms I was shooting for about that much travel.

One problem that you will get into with our cars and a production based body is the wheels hitting the inside of the fenders. With track width restrictions you can't get the tire/wheel far enough outside of the fenderwell to clear the body. This is obviously the case when the ride height is less than 3 inches. You will have to mod the body to include some sort of bubble, which in most cases is against the rules. By running something like a 16 inch wheel and raising the spindle you can really help get the car low, but you then run into the problem of fitting the wheel underneath stock body dimensions.

That is my useful post of the month back to the machine shop.

Posted by: groot May 19 2005, 07:16 AM

I love my job... seriously. I'm a vehicle dynamics development engineer (new position for me as of the first of the year).

I just spent two days learning about vehicle dynamics metrics and testing. I just wish I could stick my race car on some of the equipment we have. I've been testing steering and ride for a while now, but this training included a lot of other options we have to test vehicles.

Anyway... to the point of the post. The more I learn about steering and handling, the more complex the subject is. Each car we design has an "understeer budget" which lists out each contributor to understeer and its contribution to the total. So, some contributors create oversteer, but are countered by other components. The whole picture can end up way different than what one or two variables are doing.

Some examples of contributors to understeer/oversteer: roll steer, roll camber, tire compliance, lateral link compliance, bump steer, etc.

Posted by: groot May 19 2005, 07:19 AM

And there really are 3 different phases of understeer:
-Initial, non-linear, due to steering friction (on-center, 0-.1g or so)
-Linear range (.1-.4g or so)
-Limit range, also non-linear, but due to tire properties (what we're most interested in)

And, what you do to you suspension can effect each area differently.

Posted by: Bob Wolcott May 19 2005, 08:25 AM

Brett and Groot,

Excellent posts, thank you.

Bob

Posted by: d914 May 19 2005, 10:41 AM

when you guys get done discussing, just tell us in english what we are susposed to do to our track cars!!!! I have yet to gain a bit of understanding on the suspension stuff. Every time I get close someone confuses the crap outta me...Its either faster or slower its not turning or it loves gas coming outta a turn....JUST tel me what to do.....

Posted by: Brett W May 19 2005, 10:54 AM

Well Greg the first thing you gotta do is come to the track when there are people racing.

I don't like giving canned answers to peoples race car problems because every car is different. Each chassis is different so the same responses won't give the same effects. Are you referring to your old car or your new car?

Kevin has a great job. Make sure you develope the connections to hook a teener up to your testing stuff.

Posted by: groot May 19 2005, 11:40 AM

They test race cars all the time..... NASCAR, F1 cars (before we gave Jag away) even some FSAE cars, but I haven't been able to find someone willing to allow me to bring in my car.

And yes, Greg, it all depends. Sorry, but that's the truth. I've gone down a path addressing what I think are the biggest problems with the 914 suspension and so has Brett, Brad and others; but the problems and solutions are different. Some are successful, some are not. Having not tried them all, I can't say which is which. All I can say it what I think has helped my car, but even that is suspect because I didn't have real data on the car before I made the changes (since I only auto-xed it on street tires).

There is one universal blanket statement that applies to all track cars...... additional seat time will account for more improvements than probably any other single thing.