Jag V12

I am impressed by the amount you have acheived in such a short time, from planning to basic layout in a month, faster than any itallian car manufacturer i know.

Well if you can throw yourself at this and achieve what you have done in a month, I think you would be a pro golfer within 3 months.

Well atleast you can now go around and welcome the new neighbours to the neighbourhood. lol

 

I am enjoying your fantastic project, keep up the good work.


I have a couple of thoughts (hopefully helpful) based on structural engineering, I hope any mechanical engineers out there will please jump in and correct some of my base assumptions...

To explain myself better I need to draw some sketches and write plenty more, but given the fanastic speed you are working at, I reckon I'd better speak now, because you'll have built it by tomorrow!



So to the points:
1) a stated by Trevor, the coil over shock units will have quite a short throw and so the dampers may not be effective.
2) If the dampers are oil filled, then having them flat (or near flat) might not be ideal

HOWEVER
3) what I am really concerned about is the stress level on the inclined strut. Just applying structural principles of "resolving the forces"... the force in the strut is:

F = W / cos A

where
F = force in strut (and connection welds)
W= corner weight of car that that strut is attached to... (MUST be multiplied by a factor of safety to allow for bump shock...)
A = angle from the vertical.


Say your corner weight is...
car 800Kg, with 40% front 60% rear split, divided between two wheels = 170Kg
factor of safety for bump say x3 (MIN) = 510Kg design load per wheel/suspension strut (peak).

now apply formula
A = 0 (ie: strut vertical and above wheel) Force in strut = 510Kg
A = 10 degrees from vert Force = 518Kg

A F (Kg)
0 510
10 518
20 543
30 589
40 666
50 793
60 1020
70 1491
75 1970
80 2937
82 3665
84 4879
86 7311
88 14613
90 8325521661585730000 !!!!!! (theoretically infinite)

The wider the angle between the direction of the force and the restraining strut... the less effective that strut is. To provide enough vertical force, more and more force is induced in the strut and its connections. I would NOT reconmend the connection detail you are suggesting as the forces would be amplified badly, as the struts are too flat.


The horizontal strut set ups you will have seen on race and supercars look similar to your sketch, but are subtly different to avoid this problem... SEE SKETCHES ATTACHED

Use of a push rod and load rocker controls how the forces are moved around and are much more effective. There is still amplification of the forces, but in a lesser degree.



HAVING SAID THE ABOVE, your car will be very light so with a few sums and some very good welds your own arrangement is theoreticall workable. But I fear you are adding the some very highly stressed connections to components you REALLY DO NOT want to come undone!

Better to look for another idea... say a traditional inclined coil over from the lower wishbone, mounted within the wishbones?


Yours,

David
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Okay, I know what I want to explain, but I'm not sure how best to do it... There are a lot of effects at play here, load, leverage, resolving of forces.

If you want to understand your design, you have to be able to break it down in your mind and look at parts of the design in isolation, really think about where the forces come from, in what direction and how that will be countered.

FIGURE 1: my awful drawing of ZONDA suspension.
FIGURE 2: I have added some arrows to indicate load
FIGURE 3: here it gets messy... read the notes and move onto my next post...
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So why does this work, but I say your version doesn't?
Stictly speaking your version would work (theoretically), but it would transfer the forces differently and be inefficient and very highly stressed. So it is not a good solution.

Back to forces...

FIGURE 5: We need to look at direction of forces...


While the Zonda and others use a rocker (modified top wishbone) to press directly down vertically at the connection to the hub, transfer the forces through that rocker and then into a road spring... your proposed design attached a road spring as a steep angle to press (more or less) back against the hub. Because the spring is so flat it would generate little down force incomparason with the force in the spring, but significant sideways forces. Hence the cnnection would be highly stressed.


I hope I have explained this clearly. Really I'd rather draw loads and loads of diagrams, but I don't have a scanner at home so that isn't an option. If I am not making sense please ask questions and I try to clarify.





DISCLAIMER: In this modern world I must cover myself... I am not a engineer specialising in automotive design, I am merely discussing basic structural principles that could be applied in order to understand some of the issues. This is not a full or detailed explaination of the engineering design. I have not checked or analysised any part of your proposed design and can not take any responsibility for this.



Yours,


David

PS: I'm glad to see some diagonal bracing (triangulation) being added to your chassis... alway a good thing.
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No, not really. My engineering knowledge is more structural than mechanical. However I'll have a guess:


Once you have a feel for you finished vehicle weight you (we) could then have a go at working out the corner weights, from that a few sums would give the basic spring forces needed to support the car.

The spring length (laden) is going to have be governed by the shock absorbers your have there. ie: the spring (LOADED) will have to be the distance btween the spring pans with the strut say 1/2 extended and the adjustable pan at 1/2 range. From there you'll have to work back to get the length of the unladen spring.

SPRING LENGTH (laden) = [ UNLADEN LENGTH - (rate x load) ]

turn the formula around

UNLADEN LENGTH = [ SPRING LENGTH LADEN + (rate x load) ]

(Check that the UNLADEN LENGTH is greater than the fully extended length of the damper... otherwise the spring will go slack and mis-seat every time you go over a jump!)

rate in mm/Kg, or mm/KN (KiloNewton), inches/lb - depending on the units used...
load expessed in same unit
...so if you get springs rated in inches/lb you will have to work out your loads in lbs (or convert Kg into lbs...)


That will get the car sitting roughly at the correct right hieght (fine tuning done by adjusting the adjustable spring pan). UNFORTUANTELY this is where my engineering knowledge starts to run out. The rate of the spring you choose will govern how hard the suspension feels. I can not really advise on what those rates should be in numbers, I fear trial and error may be required.

Given that your car is for road use and is very light... you'll want the rear (heavy end) no more than moderately stiff, but the front may actually have to be suprisingly soft, so that you get the suspension to follow the road surface and not skip. Stiff is okay on a race track, but roads are full of bumps and dips and over hard suspension becomes less effective, as the tyres skip and jump.

For my little Fiat X1/9 the front springs were shortened, but were still quite soft, the damping was set correspondingly soft, tyre pressure were low 18psi and it worked really well! Because the nose was so light (NOTE: x1/9 = 880Kg kerbweight - maybe something similar to your special?)



OKAY I HAVEN'T DONE THIS BEFORE, but I'll have a GUESS... say we want 4" (100mm) suspension travel on bump say at 2G (2 x gravity vertical acceleration)

say 800Kg, 40/60% split >> rear 60% x 800 = 480Kg

so each rear corner 240Kg (standing still)
look at a 2G bump, 2 x 240Kg and the corner load at 100mm travel is 840Kg


STANDING CAR = 240Kg

load in spring (vert effect) = 240 * 300mm / 200mm = 360Kg (vert) = W

Spring is lying at and angle of 40degree off vert so F = W / cos A = 360 / cos(40) = 762Kg

Hence the spring needs to generate 762Kg at its standing laden length...



PEAK LOAD

Wpeak = 480Kg * 300mm / 200mm = 720Kg

Fpeak = 760 / cos(40) = 1524Kg

Hence the spring needs to generate 1542Kg at the bump spring length (fully closed)...



SUSPENSION travel = 100mm to bump (closed) at the wheel.

Spring travel = 100 / ( 300/200mm) = 100 * 200 / 300 = 66.7mm (vert)

Spring closes 61mm from standing to bump (peak load)
(SEE FIGURE 7 - actually I'd have to go further into the geometry as the end of the spring strut would travel on an arc - but this is near enough for a guess)


Rate peak - normal = 61mm / (1542 - 762)Kg = 61mm / 762Kg

Spring rate 12.49Kg / mm OR 0.21mm/kg






SO back to earlier formula... UNLADEN LENGTH = [ SPRING LENGTH LADEN + (rate x load) ]

If strut standing is 300mm between spring pans (assumption)

UNLADEN LENGTH = 300 + (0.21 x 762) = 461mm




So based on all the above guess work you need a 460mm spring rated to 0.21mm/Kg of the REAR SUSPENSION. The front would need another calculation.

BUT THIS IS A GUESSTIMATE, NOT ACCURATE... we would have to work with real figures to get a starting point... then I'd stress we are talking a starting point! I reckon you'll still need to try a couple of sets of springs to get the thing to feel right.

ASSUMPTIONS MADE (ie: engineering guesses!):
1) weight of car 800Kg
2) weight distribution 40/60
3) geometry of rear suspension (length of wishbone, angle of strut, connection point of strut)
4) geometry of strut, open, standing and closed length
5) required suspension travel.


ANYHOW - I hope I have showed roughly how we could go about roughing out the spring rate. Frankly trial and error might be just as quick!




DISCLAIMER: the above calculation is an EXAMPLE ONLY and does not represent real data and can not be applied to any particular car. This is for discussion purposes only. I am not a mechanical engineer, I am merely discussing engineering principles tha might be applicable for this subject.


Yours,

David
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OKAY anybody fancy a SILLY practical experiment? I am used to dealing with beams and steel, but normally on retaining walls and other things that do not move. Cars do move and so are subjected to dynamic forces which I am not quite sure how to quantify.

We therefore need a silly two man experiment.

EQUIPMENT
1 x car (any type)
1 x driver
1 x passenger observer
1 x kitchen scale (prefereably good quality digital or dial type)
1 x 1kg weight (eg: bag of sugar)


METHOD
1) Secure the bag of sugar to the scale (bluetack?) and place the scale level on the floor in the car.
2) Check the measured starting wieght of the test weight (bag of sugar)
3) The driver now needs to find one of those big dips/bumps in the road that is covered in scratches, where other people have bottomed out their cars.
4) Make a few passes to establish the maximum safe speed to take this bump without actually breaking the car, but it still wants to be enough to really sit it hard down on its suspension.
5) Make the measurement runs and read the peak weights for the 1Kg bag of sugar.
6) Stop the car and recheck the basic weight of the test load.

Assuming you have a 1Kg test load, if the peak weight seen is 2Kg then the car has just pulled a 1G vertical acceleration (ie: 2G - 1G static gravity). (Everything is subjected to 1G anyhow due to gravity). Likewise 1.5Kg is 0.5G...


Now for the tricky bit, put the scales and sugar back in the kitchen without getting caught!


This would actually be useful as it will give a feel for the range of forces NickT's road springs will be subjected to AND the forces the chassis, suspension and all things will be subjected to. If (for example) a big road bump doubles the weight of a bag of sugar, it will do the same to 400Kg of Jaguar V12... double it effectively to PEAK load 800Kg.


Oh and if anybody actually knows the answer of the assumed design G forces that a production car should be able to handle, please speak now and save a lot of messing about. Thank you.


While we're at it... any Nissan Skyline owners out there? What's the biggest latteral and braking G forces you've recorded on your dashboard Xbox computer thingy? (preferably not induced by hitting something solid...)



Answers on a post card...to
I CAN'T BELIEVE I JUST DID THAT
WE'RE JUST BIG KIDS
INTERNATIONAL


This is how science moves forward. Silly experiments!


David

 

Hey Nick,
come on, I am busting to see the next step!!!!

Mark

 

Nick, she's lookin' good buddy. Really enjoying this thread.

 

Looking good mate!

Keep it up.

 

+1

 

Looking good Nick !

Will it be ready for dream machines ?

Rgds

 

No f'kin idea, but congrats on post #1000..!!!!

 

brilliant thread mate, lovin it.....