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Learning how to build Lotus Seven replicas...together!
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PostPosted: December 30, 2014, 12:34 am 
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The voice of reason
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it's all about picking the best set of compromises that works best with the specific package you've got.


This may be the same thing, but also picking compromises that work with each other. I haven't done any parking lot events so can't speak much about them. I run 1/8+" of toe out on my FF. It's front steer and I've never checked to see how much Ackermann it has. The toe-out keeps it responsive and it is stiffly spring compared to some.

I don't like having to use much steering angle to corner. Once you have the front wheels turned much it's hard to get the back wheels doing much work. The angle of the front wheels is subtracting from the angle of the rear wheels. Then using throttle you are pushing against the drag component of the turned front wheels.

I think a reason the toe out helps is that as you start turn in, the inside wheel starts generating drag quicker than the outside wheel and that helps tug you into a cornering attitude. Not sure how big that effect is, but it seems to make a difference.

John, I don't understand your explanation for using buried roll centers. May work for you and it is not unheard of, but I don't think it's a common choice.

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PostPosted: December 30, 2014, 2:38 am 
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i guess the sub ground roll center is a european thing, if you look at most top line open wheeled vintage racers, then you will see the lower arms are either horizontal or up at the wheel end and down at the chassis end, given that the lower ball joint is on the centerline of the a arm or above the centerline so the real axis of the instant center must be a horizontal or below ground.

when you use the standard alford & alder upright then you either go for a below ground or the inboard a arm mounts must be quite high in the chassis, lotus has the arm an average of 1-1/2" from the bottom of the chassis and generally to pass scrutineering you would need 3" of ground clearence, (you have to roll over a house brick) that would put the arm pivot at 4-1/2" above ground, using a wheel diameter of 22" that puts the spindle hight at 11", the distance then from the spindle to the trunion is 4-1/4", if you subtract that from the 11" of spindle hight then the trunion pivot is 6-3/4" to the ground.

when you consider the diameter of the tires they used, this gives a wheel radius of around 12" and most are actually running 2" of ground clearence, this would put the distance from the ground to the lbj at 7-3/4" and from the ground to the chassis pivot at 3-1/2", there is no way that an a arm in this config would give an above ground ic and therefore will not give a roll center above ground, thats a 4-1/4" down slope.

springs should be the softest that will support the chassis at ride hight, while allowing enough travel for the chassis to absorb imperfections in the road surface, the chassis should be controlled by the differential between the sprung and unsprung weight, the shock should control harmonics.

however that would be a perfect world and we don't live there, so the shocks are used to do two jobs, control the spring rate and dampen their action, this makes me wonder if all this fuss about resenant frequency in spring choice is acurate when you have to limit the frequency with stiff shocks and don't say it controls the roll rate as that is done by the sway bar.

maybe i have a different view of things but the frequence is the natural rate at which a spring wants to contract and recover and people go to great lengths to select this but then bolt a stiff old shock to it to slow this rate down.

you really do want that wheel to react up and down as fast as you can whilst preventing deflections in the chassis, all you want the shock to do is stop osillation.

toe out, so the car turns in well, good for you but how easily does it turn out, and how stable is it in a straight line if it hits a bump? these are critical things to consider when you need to go from lock to lock really quickly and with considerable self centering to get you back to the mid point in a bumpy old parking lot.

i also consider that you need redundent travel in droop, too many cars are picking up the inside tire, seems to be a lot of access weight to have 4 wheels and only use 3.

maybe i'm a dinosoar and the world has moved on?

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PostPosted: December 30, 2014, 10:51 am 
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John, I'll get the numbers off my FF for you. I had to spend an entire day re-arranging to get my engine crane out last week to bring in my 302. So now I may be able to get enough room in front of the car to read a ruler :BH: .

If both your wishbones are level you get a roll center at the ground. Once you start to tilt the upper arm downwards towards the car, that starts to raise the roll center. SO it takes effort to keep the RC even near ground. The higher the lower arm is from the ground, that also raises the RC.

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PostPosted: December 30, 2014, 11:48 am 
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Marcus, it is true that if the lower control arm is parallel to the ground, the instant center will be on that line extending across and outwards from the car, but if the lower control arm is not parallel to the ground, tilting down towards the chassis, this will put the instant center below ground, (unless you put an extreme angle on the top arm that pulls the ic so close to the center of the chassis), the roll center will also be below ground.

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PostPosted: January 12, 2015, 2:19 pm 
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so having calculated the suspension geometry i have now got to see if i can rationalize the measurements to easily measure and manufacture on the chassis itself, for instance, if the inner pivot of an "a" arm needs to be 4.396 from the ground, this is not really convenient to measure, so you could go for 4.373, this would only be an inaccuracy of 26 thou but then the hole in the lug may have 5 thou tollerance on the dia. and may be off center by 25 thou, we could be 56 thou out and there are two of them on one side and two on the other,what would this effect elseware in the geometry as far as roll center?

if the chassis hight from the ground must be 3" and the chassis tube you wish to attach to is 1' square tube but the suspension design needs the pivot at 4" then you must take into account the hiem joint dimensions, if you place the hiem on its side and wish to use misalignment spacers either side of the hiem to increase travel, this will mean a 1/2" hiem will be .531" higher than the chassis (1-1/6" o/a), if you place the hiem vertically, then the lugs will be off set to the tube, complicating the shape of the lug and concerns as to what the weld path would be.

these all have to be taken into account as well as the outer end of the "a" arm ball joint which is a fixed point dictated by the components you wish to use and tire diameter to ensure your roll center is where you want it in bump and droop, its o.k. in the suspension program but to reproduce in the shed with limited equipment is a whole other thing.

now if you have access to a water jet then some of the complication of the lugs goes away or if you are not tied to a specific ride hight this can be changed as in the example above, the chassis could be raised or lowered to accomodate the inner pivot/bracket/lug required. (yes i know, i said that word).

the criticalness becomes all to obvious when you can't get the toe change where it is within .015 per inch of travel or you are welding the lugs on the frame and are trying to place them accurately, even making a jig for the location points only adds more tolerances to the mix, therefore i will try to make the smallest decimal .25" of any measurement in the hope that i can restrain them within .125" at worst.

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PostPosted: January 22, 2015, 5:07 pm 
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Can this program or Vsusp be used in a rear sla suspension to measure inner cv joint plunge?


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PostPosted: January 23, 2015, 3:27 am 
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this could probably be adapted by using the track rod inner and outer end settings for the drive shaft but i haven't tried it

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PostPosted: January 23, 2015, 12:37 pm 
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In thinking further about this, you could as you suggest, consider that your half-shaft is an outer steering link. The inner and outer links are defined by the location of the cv joints. But you will have to write some equations to map the path of the outer cv as controlled by you suspension arms. This will be "sort of" the average of the motion of the two ball joints. This path would then have to be compared with the circular arc of the outer cv joint as controlled by a non-plunging half-shaft and their respective lengths compared.

This would seem a significant check considering the limited plunge typically available.


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PostPosted: January 24, 2015, 2:18 pm 
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I've been meaning to get back here for a while & comment on something but I've yet to find the time to dig very deep into why so I can explain it effectively... but the upshot of it is...

Regarding my earlier post about how to "correct" the anti-dive results.... ignore it... it's wrong.

But, it's wrong not because that method wouldn't work.... it would.... IF the program was calculating anti-dive the proper way to begin with... it's not... the method used by the program to generate the anti-dive number is being done in a way that I've never heard of, and to what I've been able to calculate, isn't even close.

My 1st clue was when I changed the tire size for the suspension I've been looking at in the program... I adjusted the outer ball joint/rod end "Y" locations & changed the tire diameter... I left all the chassis location points the same... effectively, I put on a different tire & then "reset" the chassis height.

Why then did I get a different result for the anti dive?

Where the outer ball joint is located shouldn't matter... at least not the way I've understood anti-dive to be calculated for the last 20+ years.

As they say, a picture is worth 1000 words....

Image

So, then I went out to the shop & drew a "mental picture" of the side-view instant center of the car... sure enough, it should have been somewhere close to ground level.

Armed with that knowledge, I looked at the chassis measurements, pulled up a trangle calculator, & went to work... a few minutes worth of calculations set the SVIC at 129" foreward & 1/2" above ground... that being the case, for the anti-dive to be -30% would mean that my car's CoG would need to be barely an inch off the ground... which it's not.

By the "correct" calculations, my anti-dive is about -1.5%.... a HUGE difference.

Bottom line... if you're using this program for a front suspension, ignore the anti-dive calculation... it's not even close.

The formula in the above image will give you the correct figure... of course, you first need to locate your side-view IC... which you can do with any triangle calculator you can pull up via Google search & your A-arm chassis mount locations... it takes a few minutes to plug in the numbers & solve the various triangles.. but much quicker than drawing it out on paper (for me anyway)... Once you do that you can either plug the result into the formula above, or (what I find easier) skip the trig problem & just do another triangle calculation to find the angle of the 100% line... then divide that into the SVIC-to-front tire contact point line angle(the angle noted as 5.4deg in the above image)

(SVIC line angle / 100% line angle) * 100 = Anti-Dive%

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