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Re: Race car rear ride height, what are you running? |
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Home away from home  
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2002/3/22 14:50
From St. Louis, MO USA
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Tom, Often we would cut the floor/trunk pan above the H190 pumpkin (for travel) and make a removable cover out of aluminum. Make sure it has nothing to hit. Peter
Posted on: 2013/6/12 4:33
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Re: Race car rear ride height, what are you running? |
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Home away from home Joined:
2005/3/2 7:09
From Canberra
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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: 2013/6/12 10:06
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Re: Race car rear ride height, what are you running? |
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Home away from home 
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From Melbourne
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thanks for the info tom.
very much appreciated!
Posted on: 2013/6/12 13:07
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Re: Race car rear ride height, what are you running? |
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No life (a.k.a. DattoMaster)  
Joined:
2002/8/6 2:24
From Brisbane, Australia
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Quote:
mcgee wrote:
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.
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.
Posted on: 2013/6/13 0:20
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_________________
"if you're not on the edge, you're just taking up space"
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Re: Race car rear ride height, what are you running? |
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No life (a.k.a. DattoMaster) 
Joined:
2002/11/26 0:38
From Las Vegas USA
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Peter, modifying the floor is what we'll likely do. My friend who built the D-sports racer I ran years back and did my road race bike frame can cut the floor and weld in a modded piece that would take care of the clearance issue.
As for the whole what to do with the spring mounts it's simple................get rid of the leaf springs. The vintage class and the SCCA class are now both GT-Lite. I could run a full tube frame car if I so desired with 4 link rear and dump the struts and use A-arms on the front. I have no plans to do so though as this is allegedly a budget car.
Tom
Posted on: 2013/6/13 2:02
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Re: Race car rear ride height, what are you running? |
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Home away from home Joined:
2005/3/2 7:09
From Canberra
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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! 
Posted on: 2013/6/14 1:06
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Re: Race car rear ride height, what are you running? |
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No life (a.k.a. DattoMaster) Joined:
2002/8/6 2:24
From Brisbane, Australia
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I thought roll centre datum line was through the eyes of the spring, not eyes of the chassis pivots. I'm sure I've seen tech drawings in suspension books like that, but it looks like my memory might be failing me there.
I also thought adding an upper link as per that diagram affected roll centre height. Such that you have to treat it like a 4 link rear end to calculate roll centre witht he front half of the leaf spring forming the lower arm?
Posted on: 2013/6/14 1:43
Edited by L18_B110 on 2013/6/14 2:01:17
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_________________
"if you're not on the edge, you're just taking up space"
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Re: Race car rear ride height, what are you running? |
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Home away from home Joined:
2005/3/2 7:09
From Canberra
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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: 2013/6/14 2:48
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Re: Race car rear ride height, what are you running? |
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No life (a.k.a. DattoMaster) Joined:
2002/8/6 2:24
From Brisbane, Australia
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Registered Users
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There is no indication from the drawing to suggest that is an imaginary arm. All the imaginary datum lines for the leaf spring pivots, roll center height and roll axis lines are drawn in broken lines.
ps, no freedom in IPRA rules for front eye location other than within the elastometric bushes.
Posted on: 2013/6/14 4:15
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_________________
"if you're not on the edge, you're just taking up space"
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Re: Race car rear ride height, what are you running? |
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Home away from home Joined:
2005/3/2 7:09
From Canberra
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Registered Users
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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: 2013/6/14 5:53
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