-I'm not planing on going ahead of the shock towers with the cage. Would be a waste of tubing. and yes I'd like to keep the "nose" easily replaceable.
-Like I explained in previous posts......My race class (C-Sedan) has a 2000lb min. weight limit. I was underweight before as were many of my competitors. most just added lead weights to meet the required weight. I figured I'd rather increase the safety and stiffness of my car instead. So in my case weight is not so much an issue. Plus most of the tube is1.5"x0.095 wall tubing.....pretty light compared to what heavier cars (like a e36m3) are required to run. In the end I believe the increase in weight is worth the benefits.
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some body flex can actually help tremendously with traction in situations where there isn't that much grip (typical asphalt). More tubes means more weight, reduced visibility, and the chance of getting shaken up pretty badly. I wonder if there are any engineers that can weigh in on this stuff as I am researching building my own cage and want to know what the most effective minimums are. |
Let me tackle your comments....
1- since when asphalt a low grip situation? I think you got that one confused.
In any case you want a production sedan race car chassis to be as stiff as possible. Don't confuse chassis flex with body roll. You need the chassis to be a stiff as possible and leave all the work to the suspension. That's the golden rule! Just look at a nascar chassis or a grand am GT chassis or any car on a scca gt3 grid.
Why? you can control the suspension to do what you want. Chassis flex can be unpredictable and most important of all it actually can alter your suspension geometry during cornering!
Maintaining proper suspension geometry is VITAL. As it will induce understeer or oversteer during cornering. The first part of maintaining this geometry is maintaining the suspension attachment points completely static in all spacial directions. Up, down, left right.....they need to not move in relation to one another. Then the next step in maintaining positioning of the suspension parts.
For example, take the front shock towers. say you have sticky slicks on and 500+lbs springs (race setup) and you pitch the car decisively into a corner. If the shock towers flex inward or out ward you get a sudden change in camber which will immediately also affect your toe. Effectively this will either steer the car more or less into the corner than you are imputing and it might help induce understeer or oversteer. Most of all it will make the car a bit unpredictable due to the varying amount of flex and resulting geometry change.
This is exactly why strut braces can be beneficial.
This is also why when it comes to the suspension components the perfect RACE setup is a fully solidly mount suspension (NO Rubber or poly or even delrin) all the bushings get replaced with heim joints or spherical bearings of some sort. Even going from rubber to polyurethane bushings is an attempt at limiting the amount of "deflection" of the suspension parts. Solid (bearings) being the best option.
Take again your front control arm bushings. Under heavy breaking the front wheels are usually trying to resist forces trying to pull the front of the wheels outwards (toe-out). If you had stock rubber bushings you would observe the arms actually moving from their intended positions in high load conditions.......thus altering the suspension geometry....bad. The extra toe out might make the car darty/unstable during braking.
If all the moving parts are solidly mounted via bearings instead of busings all the suspension parts (strut, control arm, radius arm etc...) maintain perfect alignment and provide you with consistent, repeatable suspension setup and feel. That you can then adjust and improve.
look at any race suspension (DTM, WTCC FIA GT) you will almost never see "bushings" just bearings.
In the rear it is the same thing. Have you ever placed a jack stand on you rear subframe and lowered the car onto it? have you seen how much the subframe moves? imagine under heavy load the whole rear subframe will actually move around! This once again will drastically alter your rear suspension geometry and usually induce sudden oversteer.... This is why ireland eng. sells the rear subframe mount poly inserts...to limit this movement.......as before the more of a race setup to aspire to the stiffer you get ......rubber>poly>delrin>solid. Ireland also sells solid rear subframe bushings for this reason.
Which brings us full circle. If now you have increased the spring rate from stock (thus transmitting more force to the car and absorbing less in the spring) if you have also increase the stiffness of the subframe mount bushing or made it solid (like me) then where is ALL that force going to go?
Into the chassis!
So the stiffer the springs and "bushings" or lack of them the more you "work the chassis", This usually result is a twisting in a front to back relationship. THis further can alter the geometry of the car or alignment front to rear and create handling issues.
So in recap. In a perfect chassis you want the chassis to be able to maintain proper suspension attachment points at all times. This allows the suspension to work properly and most of all CONSISTENTLY. This enable you to compare apples to apples when working on your setup and make improvements.
I THINK what you are eluding to in a low traction situation is....
Wet asfault? Rain...In that situation you want to increase the weight transfer on the car and in a sense "soften" or slow down the weight transfer. You want to do this via suspension tuning and not rely on "chasis flex". Generally you'd want adjust the sway bars (smaller or remove them completely) and reduce the spring rates. This is the correct way to deal with this situation. The problem with using "chassis flex" to achive this result is the "spring effect" . For example take a kitchen sponge. imagine this is your car, take each end and twist them in opposite directions. Now let go! What happened? It violently snaps back into it's original shape. Your car is the same way. Imagine your going thru a series of S curves (chicane) once you chassis is loaded in one direction and have taken a set (flexed in one direction) then you have to suddenly change the cars direction it does the same thing the sponge did in your hand and snaps back.....this usually results in a snap oversteer or sudden spin... It's basically like a big spring. Except you can't dampen it with shocks! So the goal then is to eliminate flex and allow the suspension to control the car. Your basically removing variables from the equation.
hope this makes sense....
As far as Reduced visibility?
Not sure what your referring to exactly? This is a race car not a street car. pretty much every race car out there has the standard "x" across the down tubes. I'm not doing anything that different here. Also I have 17" longacre panoramic rear view mirror and it is surprisingly effective. So basically my car is no less visible than any other race car on the grid.
As far as cage design I rule of thumb is that while taking suspension pickup points and load inputs into the chassis into account, you want to think about "load paths" of the forces inputed into the chassis. Where they are coming from and where they are going. You want to shoot for TRIANGLES. Proper triangulation of your tubes is key.
So it's really important to keep the front strut towers exactly where they supposed to be. It's important to keep the rear subframe static. Then it's important they do not move in relation to one another. This is the reason behind integrating the rear subframe mounting points into th cage.
The first people to do this was BMW Motorsport with thier DTM e30M3's
here is a few other examples of this in '02 race cars. But it doesn look like either is sunken into the chassis like the DTM cars did. I just copied what DTM cars were doing.
look closely at these race car subframe location....it's all been done before! I just had a different design about what to do with the loads or tubes.
[img]http://www.skidmarkracing.net/photos/cage18.jpg.medium[/img]
In my personal opinion the cage in any production race car should be as extensive as the rules allow........ when you see pictures of what some WTCC, ETCC, BTCC and DTM cars have done with the cages in the production chassis you will be amazed at the complexity of thier cages. Especially AUDI's
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68' 1602
98' ///M3 Sedan
88' ///M3 Sold *
06' ///M3 Competition Pkg Sold *
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