Right, but all of the suspension loads are behind the fuel tank. The problem in the 911 is chassis stiffness between the front and rear shock mounts, not between the front shock towers and the front bumper. Even with the fuel tank cutout there is tons of meat in the chassis to handle the minimal loads going to the front a-arm mounts.

I went and had a quick look at my car and I did a few quick calculations. Basically if a 3000lb 911 was taking a corner at 1.0G and the steering angle was at its max there would be about 200 lb of lateral force on the front of each a-arm. To me it doesn't seem that 400 lbs of lateral force at the front end of the car would cause much deflection. I could be wrong though.

We recently built a similar setup into our 912 project. I figure its an old car with a new front suspension pan, might as well keep it stiff.

Its made from a 3/8" 4-link kit sourced from eBay and DOM tubing from Metal Supermarket

http://forums.pelicanparts.com/uploa...1297839548.jpg
http://forums.pelicanparts.com/uploa...1297839557.jpg

Lifting a front wheel is probably more to do with the range of travel of a strut compared to the semi-trailing arm than the chassis stiffness, but still, a stiffer chassis means more drastic differences in front to rear roll couple distribution are possible, meaning more adjustability of oversteer/understeer balance. Even if you have rubber in the suspension, you can factor that into the effective roll stiffness of that end of the car. You still benefit from a stiffer chassis. That said, having monoball bearings makes chassis bracing more important because it is closer to being the "weakest link" or rather "softest link".

Much easier and possibly more relevant would be measuring actual deflection which would be pretty easy. Rig up rod/s to slip freely with some sort of collar to show point of maximum deflection.

People always talk about the structure deflecting but I've never heard any mention of how much.

I intend to loosen all of the tubes little and lift one wheel just above ground to see if some of the tubes stick and in which direction. If I have movement in the body there the system works...

What are the loads under braking? Where are they reacted?

:p

JR

The braking loads go to the front of the a-arms but assuming both wheels are still rotating during braking (no wheel is locked up) the forces will be balanced and not cause any lateral movement across the front end.

I don't know about your cars but mine always want to lock the right side early, so the forces are unbalanced. :D

Actually, I was thinking of a vertical force...

There has to be a reason Porsche did this on the 934 and 935, apart from the "cool factor".

JR

Cool project. What something is worth depends on what you compare it to.

There are no mysteries on this stuff as the 911 has been around for a really long time. All of the loads and directions and so on have been measured. All of the engineering has been done long ago.

Folks with reasonable experience from the high days of IMSA know what makes a difference and how much, so for a 911 race car, I like to rely on those resources. There are plenty in my area.

It is neat to see someone applying cleverness, talent and time to this kind of thing. Sure, it may not make a difference in AX or track lap time, or even in the feel of the car on the street... but then again it may in some way perceptible to the OP.

Very clever and thanks for sharing.

Well, strain is just deflection/total length, so they are pretty close. Aren't there stickers or something that change color with strain like the thermometer things?

I was afraid maybe this thread would go in this direction.... this deeply. Recall that when it counted....when Porsche raced these chassis, various piecemeal pieces were used over the years....but the 935's , arguably the most stressed and latest of the series using this chassis.... did use an X-brace. The cross-brace directly across the shock towers, also responds well to engineering principle when triangulated. So...no real issues as to "why".

My first minor critique on the home-brew.... was simply that there was a long-ish and flexible plate to where the X-brace and cross-brace attached. That could be improved with only a small change. That's all.

Actually, the braking loads go into the rear of the A-arms and into the crossmember unless you have a 935 setup, in which case the loads are carried in tension on the front A-arm attachment (maybe why the 935s did the X brace). The moment to angularly decelerate the wheel is a reaction force from the strut, so the top strut mount also sees some forward load and the strut sees bending moment.

In this case, the bracing would be more benefitial from the strut towers to the cowel, and bracing on the bottom where the crossmember hump is.

I agree that this negates most of the stiffness.

the flip side of this, is how much deflection causes how much effect...

If chassis rigidity (or lack of it) results in 1 degree of unwanted suspension deflection, what effect would that have if that offsets your wheel alignment spec. of –1 degree of camber or if it changes the toe by +/–1 degree? Depending on what you're doing at the time, it could be the difference between static and dynamic friction (slipping or gripping or directional stability), assuming the deflection is linear and consistent, that is.

Sherwood

Project painted and mounted in car.

http://forums.pelicanparts.com/uploa...1298652930.jpg

Looks good but you can REALLY get quite a bit more front stiffness if you involve all the strut tower not just the back bolts if you're not using a camber setup. Make something that places the load on as much of the tower as possible - remember you are now getting load from the front of the car! You need to have a way to brace the tower against the bar pushing back at the diagonal if one front corner starts to lift.

What I found in development of my brace was that if you created mounts that involve the entire front strut and position the bar between the two towers and have all points of tension meeting at those points you effectively stiffen the front of the car like no other.

Good move on the plates at the front of the car, however they should be mounted up a bit further to increase the load on the bars when the car starts or trys to flex. The plates should be strong enough to not allow any movement of the bar under a few hundred pounds of load. Also look into a way of reinforcing the center of the brace, with the solid threaded rod there it is actually easier to bend or deflect a solid bar than a tube.

Here's a shot of what I designed and built for my 935 & my buddy's 934 concerning strut tower chassis plates.

http://forums.pelicanparts.com/uploa...1298653642.jpg

Wow! That looks really nice! I will think of that for version II.

Hey brother - not to hijack the thread or anything but I've been working for a few months on how to do the brace right and why the factory did the things they did and why they did or did not work. I figure this might be appropriate for the thread so we can all learn a bit from my reverse engineering on the factory stuff.

The X brace on the factory 934's and 935's was there to effectively stiffen and reduce the the understeer characteristics of the 911 platform. The 934 benifited from a much pointier nose and better front traction and stability under braking or hard cornering thus allowing higher speeds & an advantage on the track.

The 935 was a different story. As with the factory cars I cut the front end of my 935 off and welded in a strong steel bar between the two hood pins. This no doubt weakened the tub structure and required a brace to unite the front end again besides the single bar which supported the front mounted oil cooler.

The 935's were the lightest and the most cut up of all the chassis and at the time they had a significant issue with chassis flex due to a lack of cross stability when the rear of the car was hacked to pieces to fit the rear seat intercooler and cage. Those cages didn't "X" as most modern cages do now in the rear seat area.

The 935 was known as a "flexi-flyer" because of this, and the huge rubber the car was shod with - not to mention the power and effect that has on even a standard chassis.

Overall the "X" brace on the 934 & 935 was there to prevent the front half of the car from picking up an inside tire when one corner tried to raise up, the "X" brace would push it down or just resist the rise and put more rubber in contact with the track meaning higher cornering speeds and stability.

This was in addition to the standard type strut bar going between the towers which would prevent the towers from expanding or contracting on each other. The need for a bar down to the brake booster area or floor pan wasn't needed because of the stoutness of the tubing used and the fact that the additional protection from strut tower collapse or expansion simply wasn't an issue when the "X" brace was executed properly.

Sure the extra bar isn't going to hurt anything - but in a race car when every pound matters, each part has to pull its weight.

There is eveidence on a few factory cars where the "X" brace would actually bend towards the passenger side strut tower...these things weren't for looks but do their job quite well.

So after 3 months of development and testing I know they work from the speed differences during cornering & overall traction increase not to mention how tight they make the front of the car feel.