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mcgee
Re: Race car rear ride height, what are you running?
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From my understanding of the text, the actual maths behind determining the Roll centre height and roll steer are complicated (They do not go into the equations in the book).
What is presented is a simplified approximation of Roll Centre height and roll axis (Roll steer).
This is done by the roll centre as discussed already (front mount and upper rear mount, vertically above the axle) and the "swing arm" which is derived from the slope of the front 3/8ths of the spring (Q to R), transposed to the roll centre (C to O). The slope of this line then determines roll steer and the end point (C) is the Instant Centre (length C to O = the 3/8 spring length L).
In the text SVSA (Side View Swing Arm) or the single point at which the suspension will rotate around.
I hope that helps.

Posted on: 6/14 7:24
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mcgee
Re: Race car rear ride height, what are you running?
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No, but the drawing is an extract from a book which does talk about it.
I am 100% curtain that is not a representation of a physical link.

Posted on: 6/14 5:53
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mcgee
Re: Race car rear ride height, what are you running?
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Im not sure I get your last discussion point. I don't think the 'swing arm' discussed in the diagram is a physical link. More of a virtual equivalent to a control arm to make visualisation of the roll axis easier.
I'm sure with more links (upper for example) you would change to roll centre height. The axle would now rotate around different pivot points (or worst case bind if they do not work together). I would think that an upper link of the same length as the 'swing arm' would be a good starting point.
Overall roll centre height adjustment would be better managed with a watts linkage and slippery link between the axle and leaf spring.

Posted on: 6/14 2:48
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mcgee
Re: Race car rear ride height, what are you running?
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Quote:

L18_B110 wrote:
get's a bit confused talking about roll centres and roll steer all at once like that. But assuming you're mainly talking about roll centres and roll axis - all roll centres are dynamic things, they never stay in one location (height) in any production suspension configuration. Sliders are probably worse than hangers for maintaining roll centre height because they fix the roll centre to the body. As the body leans while cornering and gets closer to the ground, so does the roll centre The roll centre height changes exactly witht he height of the body at that side. With a shackle as the body leans, the spring compresses and (typically) lengthens, pivoting the rear hanger back and upward, giving back some of the lost roll centre height. But changing shackle angles also change spring rate... And that is the main advantage of sliders - consistent spring rate.

If you're worried about changes in roll axis during cornering, the strut front end is going to be the main player - they have notoriously poor dynamic control of roll centres.

On the idea of front and rear eyes at the same height - leaf spring setups always handle best with the rear eye higher. And it wouldn't stop twist anyway - any time there is roll, the twisting of the live axle has to cause leaf spring twist. Its just inevitable.



There are a couple of good book that describe what I was trying to say better then I can type it, but in an essance we are in agreeance if I have read the above correctly. Lower front mount point is better.
Milliken and Milliken, Race Car Vehicle Dynamics has a small section on Hotchkiss setups.
If found the below photo of the page on the web. Good book to read if you have the time.
If the below is take to be correct, then the roll centre height will not be effected by sliders as it is a function of the front spring mounting point and the Upper rear mounting point!
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Posted on: 6/14 1:06
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mcgee
Re: Race car rear ride height, what are you running?
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Benny,
Agreed this would in theory produce a roll neutral setup, whilst going straight. But these cars are far from ideal stock and given the spring is an arch, as it compresses you will still likely end in either roll understeer or roll oversteer at some point in the corner, if that make sense. I.e the roll axis will change angle as the car rolls. The only way i know to avoid this is to use rear sliders, in lieu of shackles.
I guess the idea is to optimise an element of the corner to produce the best characteristic, potentially at the expense of the other aspects. As you know its always a compromise.

I think most people extend the rear hangers because that is the only option they have.
Rear hangers will help to ensure the angle change on the rear is minimised, thus keeping the spring at a more constant rate. Given that it stiffens/softens with changes in rear spring shackle angle.
We have a little luxury with the coupe (should the rules allow) to play with both front and rear easily. therefore playing with the front can minimise the impact of playing with the rear shackles only and may help fix an underlying issue. I guess it depends on what you are trying to fix or optimise for.

Posted on: 6/12 10:06
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mcgee
Re: Race car rear ride height, what are you running?
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Im not sure how much travel you would need in both rear wheel bump (worst senario i guess, in roll i would have a guess that 45mm at the centre is plenty).
I would have thought this bump senario would be limited by correctly setting the rear shock abosrbers high speed bump or if you are really worried, I'd add a small/stiff/progressive bump stop in the centre.
This should allow enough freedom in roll and control the amount of bump movement (These bump stops could be incorporated into the shocks like Bilstien do).

If we are talking a 1200 coupe and your regulations allow, I would look at lengthening the front spring hangers as this will point the rear roll axis down further and produce better roll geometry (roll understeer if i remember correctly) and also increase the rear ride height.

Posted on: 6/12 3:53
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mcgee
Re: 3J Improved Production coupe build
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I had a small win last night.
I finished the Body harness and tested all the elements. Wipers, lights, indicators, starter motor all work as advertised. Dorian timer turned on (forgot to check reverse switch though). The start button lights up, I am hoping this goes out when the alt is charging but won't know that until i get to start the car properly.
Hopefully I'll finish the wiring in the next week. I am so close I finishing this build.

Posted on: 6/12 1:32
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mcgee
Re: 3J Improved Production coupe build
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The Motec Gear is expensive but you know that when you buy into it.
Top gear and the support is excellent as well.
What I liked about it was the software for logging, I convinced my wife to let me spend the extra for this factor.
I weighed up Link/Haltech/Racepak/RaceTechnology combos but couldn't go past the Motec combo for the extra $$.
If you have this combo you can work around the M84 restrictions and maximise the usage of the inputs.
For example the M84 has locked inputs for MAP and MAF, I won't use MAF and have water pressure sensor into this input.
I have no intention for the ECU to make engine adjustments for either MAF or Water pressure, however I can pass the raw signal through to the Dash and allow the dash to convert MAF to Water pressure and then log this data.
Because of this I have, at this point, saved the need to upgrade the Dash to have the 12 I/O function enabled.
In my setup MAF = Water pressure, User 1 = Front Brake Pressure, User 2 = Oil Temp, Gear V = Steering angle.

Posted on: 6/11 5:04
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mcgee
Re: 3J Improved Production coupe build
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Quick update,
I'm sure I have said this 100 times but the wire loom is taking some time to complete, but I really want to get It right.
All the wiring back from the firewall is now complete and tested.
It hasn't been without some teething issues though.
Firstly, I put the ECU power pin in the wrong place which took a while to diagnose (face palm).
Secondly, I had some issues with the Shift light Module (SLM) where it would flash event though I wanted the lights solid.
This was diagnosed as a problem where ECU, Dash and SLM were on the same CAN bus and the ECU and Dash were trying to both use the SLM. When I turned off the SLM functions on the ECU it worked properly.
I've been playing with the ECU/Dash software and have all the channels passed to the Dash now, each time I turn it on I learn so much about how to use it, how to pass extra channels to the dash and maximise spare ECU inputs.
Last issue is a dodgy Master Switch, that sometime won’t turn on (This will have to be replaced - but for now it's ok for testing).
Today I will work on the engine bay part of the loom. The work spent on the other side of the firewall has meant that this should be pretty easy. Each sensor has it's own pins, no wire splicing on the engine bay side.

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Posted on: 6/9 23:13
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mcgee
Re: 3J Improved Production coupe build
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Cheers,
I should have some better progress shots tomorrow.
Hopefully with the dash panel reinstalled.
Getting very close to finishing then onto log book time.

Posted on: 6/1 11:05
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