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MG Midget and Sprite Technical - Front spring rate

Hi Racers out there. I am interested in front spring rates that people are using on racing sprites and midgets. I have just measured some front springs that I expected to be in the 450lb rate but were in fact much softer. Any help would be most helpful. Suspension tuning is a cross between a black art and science and also has a lot to do with feel and personal preferences so I will be interested in what you all have to say about it. I also understand if people dont want to talk about as it might be their edge. Thanks
s page

The trick with any live-axled car is to control things from the front. In simplistic terms this means stiff front, soft rear.

However the Spridget is a wee bit odd in that its front Instantaneous Roll-Centre is subterranean, whereas the rear is at the centre of the diff (assuming no Panhard Rod or Watts/Mumford link). This means that it will naturally develop Corner Entry Understeer. Couple this with the tramping effects of a live axle under Lateral Load Transfer (cornering) and you also get stabilising Mid-Corner Understeer.

Thus when playing around with suspension settings you need to ensure that you don't:

1. Increase the Understeer (no serious racer likes this).

2. Cause Corner Exit Oversteer (which makes drivers a tad leary!).

The upshot of this is that you need to be aware of what you are doing with the CoG:Roll Centre relationships and minimise any Diagonal Load Transfer whilst cornering. Additionally you want to minimise any changes in tyre footprint (ie camber change).

As a long time racer, personally I prefer to control the suspension with Spring Rate and only fine tune with ARBs. This is because ARBs introduce cross-coupling effects that ultimately lower grip and traction ie Diagonal Load Transfer. Increasing spring rate will NOT increase ultimate Load Transfer but WILL increase the rate at which the Load Transfer occurs (thus stabilising any manoeuvre entry) as well as resisting body roll (and the chances of lifting a wheel). However, go TOO stiff and you will make things worse!

Spridgets have a relatively short lower A-arm which obviously gives less lever effect on the spring so, unlike double wishbone set ups, you require greater Spring Rates to get the same effect.

For a Spridget racer you need to be looking in the order of 600lb front Spring Rates around 63/4" - 7" free length (to drop the front CoG closer to the Roll Centre as well as to build in a little extra Negative Camber). With these, going mad on ARB rates will be disastrous so don't go above 11/16" (unless you want to lift a front wheel and love terminal understeer!).

You will also need to check your rear Spring Rates - std springs are too stiff on a seriously lightened racer and will cause it to hop about all over the place. On ours I had to remove the bottom 2 leaves. Be aware, however, that if you do this you will need to add extra clamps to the spring forward of the axle since this part of the spring provides axle location; softening here will exacerbate any tendency towards axle tramp (the part of the spring aft of the axle is what provides the springing action).

THEN you need to match your dampers to the vehicle's Corner Weights and Spring rates - all the monotube conversions I have seen are poor in this respect and do not provide ideal Bump:Rebound damping, especially the rear conversions. If staying with lever arm dampers then you will definitely need the fronts upgraded to take account of the increased Spring Rate.

If you want to read an in-depth study of the hows and whys of suspension set ups I would point you towards 2 excellent books by Carrol Smith (ex US Race Engineer and leader of the Le Mans Ford GT40 project):

"Prepare to Win"
"Tune to Win"

Mr Smith's books should all be in the Library of any serious Race Team.

Deborah Evans

Deborah,
I have never before seen all of this so clearly and concisely stated for a Midget set up. Thank you, your post is definitely going in my bookmarks.

Norm
Norm Kerr

I love it when a girl talks about roll-centres! ha ha.

Malcolm
M Le Chevalier

TY Norm :)

Just to add:

WRT the front suspension of a Spridget:

Owing to the lower A-Arm length and inclination when unloaded, the Instantaneous Roll Centre is actually below the track surface. Thus the Front:Rear Roll Axis is inclined towards the front. Couple this with a high CoG and you have a larger Roll Moment at the front than at the rear. This increases the Lateral Load Transfer beyond what you'd expect with a Double Wishbone set up, along with increasing camber change, and results in Corner Entry Understeer. In other words the car wants to roll around its front upon corner entry and, because the suspension deflects, the Roll Centre moves lower, so the faster you go into the corner the worse the Understeer becomes.

With a racing Spridget the trick is to reduce the inclination of the Roll Axis and reduce the Front Roll Moment. You do this by lowering the car (and reducing weight). This will bring the GoG closer to the Instantaneous Roll Centre thus reducing the Roll Moment (as well as dialing in some negative camber). However lowering the car will cause an increase in the A-Arm inclination and actually lower the Roll Centre further! It's a case of swings and roundabouts I am afraid, and a juggling act to get the best compromise, hence the free spring lengths I stated.

Increasing spring rate will resist vehicle roll and associated camber change (and hence alteration to the car's tyre footprint), but go too far and the Understeer will start to increase again! Going psychotic on ARB size will, aside from increasing Diagonal Load Transfer (which reduces ultimate grip) also tend to make the car lift a front wheel upon exit of high speed corners; this again will increase the Understeer.

It is also beneficial with the Spridget to set the Front Camber to around 21/2 degrees negative overall at loaded ride height, since this will counteract the Camber change on the outside loaded front tyre in a corner. However, again, go too far and you will make things worse and the car will become twitchy on the straights.

In an ideal world you'd bin the prehistoric front suspension and fabricate a double wishbone set up with idealised arm lengths (as the Class A cars do).




WRT the rear suspension:

The reason a live-axled car develops Mid-Corner Understeer is because of the effects of Axle Tramp. Specifically, as the car stabilises in the corner, there is Lateral Load Transfer (this applies to ANY car when cornering). Thus the outboard side of the car is more heavily loaded than the inboard side. Thus there is more spring deflection on the outboard side.

On a live-axled car the extra spring deflection on the outboard rear cart spring causes the spring to take up an 'S' shaped curve. This means that the effective springing length shortens relative to the inboard side, resulting in the axle skewing INTO the corner. For most people this is a good thing because it stabilises the car mid corner - most drivers get rather un-nerved by mid corner oversteer! However it does slow the ultimate corner speed because it introduces a degree of tyre scrub over and above the slip angle that is required to get the car cornering. Additionally, as you come out of the corner the axle will skew the other way, tending towards Corner Exit Oversteer which drops your corner exit speed because you loose traction. Watch poorly set up Spridgets getting on the power too early out of a slow corner, such as a hairpin, and they will be fishtailing as they enter the next straight

Adding extra spring clamps forward of the axle reduces this tramping effect and means the car becomes more responsive. I do it by heating the clamps until cherry red then clamping them up on the spring as tight as I can before allowing them to cool, contract, and develop a greater clamping load.

You might think Anti-Tramp Bars may be the answer here but they are not because, by their very nature their swing arm is different to that of the spring. The result is that they fight against the springing action so the car will not take its set properly in the corner and will hop around all over the place on a bumpy track.
Deborah Evans

Deborah -

Can you please try and be more analytical and accurate with you replies. You're bringing the tone of the forum down with your frivolous and thoughtless responses :-)
Andy Hock

Hi, Thanks Deborah , Food for thought.
s page

Hi S Page.

Bear in mind that what I have said really applies to lightened full race cars running on 71/2" slicks.

While the basic tenets don't change, going for a different tyre type will require some fine tuning.

What Race Series are you planning to compete in?

If you need help with chassis set up drop me a line.
Deborah Evans

Hi Deborah,

What ride height (Sorry not sure what would be a mutually agreeable ride height) where there ride height of a Midget with std suspension takes the roll centre height below ground height

Cheers

Spencer
S Deakin

Spencer,

Anything from std to lowered. It's down to the short length of the A-Arm coupled with its inclination.

The only way to really cure it would be to use longer fabricated top and bottom links.

The 1500 is in many ways worse. While raising the ride height did raise the front Roll Centre marginally, it stayed subterranean but the Roll Moments increased. The raising of the rear ride height, while it increased the Roll Axis inclination, actually worsened the situation (coupled with the extra weight) because it increased the rear Roll Moment significantly, leading to problems with increased Power Off Oversteer as well as a reduction in the dialed in Mid-Corner Understeer which can lead, in extremis, to quite violent Power-On Oversteer.

As I said earlier, setting up a Spridget is a series of compromises to get the best effect.
Deborah Evans

So just to be clear...

At std ride height you consider the front roll centre height (FRCH) to be below the ground plane

Cheers
S Deakin

Exactly.
Deborah Evans

Someone (David Billington?) has already done this analysis on his (road) car... see encl (and apologies if I've wrongly assigned it).

A


Anthony Cutler

Hi
Just to add to Deborah's excellent response to the spring rate question
I tried 600's in my hillclimber and although excellent on smooth surfaces, on a bumpy hillclimb they are a bit skitty accross the corners and have found 550 with a 3/4 bar a tiddle more complient to irregularities but still gives good rear traction
To stop roll-in understeer and mid corner understeer I extended the front spring mounting bracket up into the body as far as it would go with a very noticeable difference in handling. No understeer and heaps of drive out of the tightest bends
Just my 10 cents worth Willy


William Revit

I agree with Deborah - with the caveat that if you're using road tyres as opposed to slicks (and I count AO48R's as slicks for all intents and purposes) then you might want to drop the spring rate a bit - I used to use 400lb springs on my old (dearly departed) FISC car (as did Dave S) when we ran 155/70 B320 Bridgestones
James Bilsland

I am curious to learn more about how Willy's modification (raising the rear spring's front mount) helps handling:

Based on the layout in Anthony's link, the rear spring is originally mounted, angled "down" at the front. Does reducing this angle (as per Willy) reduce the cornering effects that Deborah outlines in her second post?


Follow-up question for Willy: what is that horizontally mounted (yellow) damper's job? It doesn't seem to be on an angle that would allow it to be stroked when the axle moves up and down, but that might be a trick of the photo? Or, is it used for another purpose.

Norm "at suspension school" Kerr

Norm Kerr

Raising the forward spring mount as Willy suggests effectively changes the angle of the forward part of the spring such it deflects less under Later Load Transfer. Thus the 'S' shape it will attain will be less than on a stock set up. Thus the amount of axle skew (tramp) will be reduced so Mid-Corner Understeer will reduce.

I have no idea about Willy's damper though!
Deborah Evans

The layout posted by Anthony above didn't show a front view, or how the front suspension geometry changes as it is stroked, so...

I just went out and measured the camber angle on my Midget (removed the springs and sway bar, shimmed the jack stands so that the inner A arm mounting bolts were dead level, stroked the suspension from full jounce to full rebound, and measured the angle of the disc surface at those two extremes, and when the upper control arm was horizontal (a sort of "approximate ride height", for comparison purposes).

The camber seems to stay the same from full rebound (droop) to horizontal, but it goes about 1 ~ 2 deg negative from horizontal to full jounce (stroked all the way "up" into the fender until the spring pan was a good bit into the rubber stop).

Granted, these are pretty rough measurements using a carpenter's level and a digital caliper, but I thought that it was interesting to see how the camber went progressively negative as it went up.


Norm



Norm Kerr

Hi Spencer,
did you ever get around to modelling my Mumford design?

cheers
Brad
Brad 1380

Hi Brad,

No I have been flat out on the front, I had to 'commit to metal' for my front telescopic dampers and lower wishbones so I had to check and double check before spending money!

Deborah,

Do you have the hard point data you used to determine that the FRCH was below ground, did you draw it out or use a suspension kinematics program

Cheers

Spencer
S Deakin

No worries Spencer, just interested in how it works out on your modelling program.

Still pondering Deborahs explaination of how the suspension all works together, taming axle hop would be my next biggest improvement, so i think i need to look at the anti tramp side of things again or perhaps just slow down a bit.
cheers
Brad
Brad 1380

OK then
The raised front spring mounting point works as Deborah has explained -
By raising the front mount the diff (wheel) more or less moves straight up and down as the body rolls and compresses the spring instead of walking forward as the spring is compressed and back on rebound when mounted in the original position
This eliminates the feeling of understeer as the car enters into the first half of a corner

I don't own this car anymore, it was just a test car for the car I'm building now and mods were agricultural to say the least. It ended up with rockerarm front suspension with inboard mounted shocks a feature to carry over into the new car with telescopic Bilsteins all round
The yellow shockie,another Bilstein, you weren't supposed to see is to control the rear axle. I hate Panhard bars and tramp rods and firmly believe the axle needs to move around a bit to get maximum traction. The yellow shockie in the pic. is mounted to the bottom/passenger's seatbelt mount on the side of the tunnel, goes out through the bulkhead and the top of it is mounted on the top, passenger's side of the diff banjo housing. It completley eliminates any axle hop in any situation but still lets the axle move around to find grip, It works a treat, full hook standing starts and all
It was a fairly soft 50/50 shochie, from memory a 5

Cheers Willy


William Revit

For a positively outrageous sum of money I can (I think) provide stupendous hand crafted lever arm dampers which will provide optimised damping, in both bump and rebound. Oh sh!t! I might have let the genie out!
Clive Berry

This thread was discussed between 17/04/2011 and 20/04/2011

MG Midget and Sprite Technical index

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