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MG Midget and Sprite Technical - Drag racing Midget

Fitted a new turbo cam and also a stronger clutch.This is an Escort Mexico unit,same as the one used by Rob Newton.Went racing at Stratford last weekend and managed a 13.007 sec pass with 106.5 terminal speed.Still got problems with traction on launch,I'm wondering about softer front springs to help weight transfer.
steve cowling

Impressive... represents a 0-60 time less than 5s.

What's the weight/power/torque?

A
Anthony Cutler

Hi Anthony,thanks,yes 0-60 about 4.1 based on eighth mile time.the car weighs about 700kg, power is around 200 bhp plus 25 bhp NOS shot,torque I guess about 180 lbf ft.60 ft times are still slow at about 2 secs and I need to get it to hook up properly.
steve cowling

Steve, I doubt if softer front springs will help, but altering the valving in the front shocks will. Many drag racers here in the US used to use front shocks that had a 90/10 rating, 90% resistance on compression and only 10% resistance on extension to help lift the front and then help hold it up to increase the weight transfer period. I suppose that with some trial and error you could modify the valves in the lever shocks to achieve the same sort of balance.
B Young

That's very helpful,I did wonder about softer shocks.Regarding the springs,I was thinking about lower rates combined with longer free length,to keep the same ride height,but allow more movement and hence weight transfer.
steve cowling

Steve, the longer free length would be of little use as the droop of the suspension is limited by the bump stop under the upper shock link. You could cut down the bumper a bit to gain an extra inch or so droop but I wouldn't remove it all together. Make sure that the shock isn't bottoming out at that level or there would be internal damage to the shock.
B Young

I might say something stupid but I will try anyway.

Tyre's!!!!!
Sure more weight on them will help (by creating a bigger contact patch) but the problem is loss of traction.
So a stickyer compound must do the trick.
Might even be as simple as less (or more) pressure
Onno Könemann

Hi Onno,you're right of course,but I'm already running 205 wide soft compound Toyo 888 at 14 psi,effectively a road legal slick.If I go lower on pressure,the contact patch is bigger,but the tyre may spin on the wheel rim,and I do worry a bit about doing over 100mph on an almost deflated tyre.......
Regarding the bump stop,I agree,cut it down but don't remove it.I reckon from static ride height to full bump I might get 3-4 in.wheel movement.The American book I have talks about needing 6 in.but on a slightly bigger vehicle!
steve cowling

I got.an insane idea that ive been bothering bill young on all summer

Duel ignition....

Here is what i want to do .... Set up a crank trigger ignition only 4 degrees retarded and hooked up to a switch on gear shift ... Once you shift to 2nd the switch is activated and switches the crank trigger ignition to the dissy

So by having the crank trigger retarded by 4 degrees you not applying full power to the rear wheels thus giving you more traction

Thats my therory anyway

Prop
Prop

You need serious help Prop!!

You really really need to worry!!!

That is so ridiculous on so many levels

The main one being how does retarding the timing increase traction??????????????

Traction is increased or decreased as a function of tyre grip!

Do you want me to go on?
Robert (Bob) Midget Turbo

Hi Prop,people use something similar as a rev limiter and a flat shift system,but unfortunately one of my self set rules in this project is no driver aids,so I just have to learn to control my right foot............
steve cowling

For gods sake Steve please dont encourage him!!!
LOL
Robert (Bob) Midget Turbo

Haha

I did mention it was an insane idea didnt i ...hahaha

Im sure my treminalogy is a bit off,.... What im thinking is with the the crank trigger timimg retarded by a small amount you wont have as much power / torque so that would reduce wheel spin, then once you shift to 2nd gear a switch is activated and the ingition is switched to the dissy which is properly tuned so you would have full power and torqued


So in effect the crank trigger retarded timing is actting like a traction control... Until 2nd gear is activated

With so many of my ideas incorperated into the car I simple cant understand why my car isnt driving yet...hahaha

Only from the mind of prop does great ideas spring eternally forth
Prop

Guys, all Prop is proposing is to use a second ignition timing system to change the timing to reduce torque in the lower gears to lessen wheel spin then when a higher gear is selected the timing would change to the higher torque setting. A lot of effort for basically an engine without enough torque or horsepower to need that kind of limitation. The technoloy exists and is used by some pro drag racers in some classes, but those guys are dealing with 800 to 900 horsepower and above and usually are forced to run a maximum of a 10" wide slick tyre by class rules. I can't think of why any Spridget would ever need such controls except for someone like Prop who seems to want all the tech he can put into the car for bragging rights. LOL Why else would you build a custom 10 gauge console?
Steve what size tyres are you running anyway? I'm running 205 60 13's and with my paltry 130 rated bhp and 3.43 rear gears I can't really break them loose without really trying hard. Never have raced it on the quarter mile though.
B Young

Steve, are you running an lsd/locker/welded diff? I would actually look to soften up the rear rather than the front, and if it's a dedicated drag racer then I would look into playing with the front shock valving as Bill said. Most of the fast dedicated drag 'street' cars over here run 90/10 front shocks, it's not great for handling but certainly helps weight transfer on launch.
AndrewF

Bill,I have the same width tyres as you but around 200 bhp and 4.1 gears.
Andrew,yes it's a TranX lsd,and I do have adjustable rear shocks.I also have my copy of a Caltrac traction bar system on which stops rear spring wind up but also makes the rearend very stiff.
steve cowling

"I'm wondering about softer front springs to help weight transfer."


"I was thinking about lower rates combined with longer free length,to keep the same ride height,but allow more movement and hence weight transfer."




The amount longitudinal weight (load) transfer a vehicle experiences is NOT a function of either spring length, spring movement, chassis movement, or of spring/damper rate.

It occurs because the vehicle's CoG is necessarily located ABOVE the surface of the track/road/dragstrip.

Longitudinal load transfer is a function of the vehicle's AUW, CoG, wheelbase, and the amount of Longitudinal acceleration the vehicle generates at any given point.

It is defined as:

Longitudinal load transfer = (Vehicle Weight x CoG height) / Wheelbase


Thus, changing springs and/or dampers will NOT affect the AMOUNT of load transfer unless the change in ride height significantly alters the vehicle's CoG.


However, sping/damper DO alter the RATE of load transfer.



On a race car you want to minimise the Transition Period during which the load transfers (because the changing vertical load on the tyres, aside from any camber change, will affect acceleration/braking over a finite period of time which destabilises the car) so you use high spring/damper rates (stiffer = faster load transfer). Indeed, race cars are ALWAYS over-sprung and over-damped compared to road cars.


It is a curious fact that when load transfers longitudinally (but not laterally or diagonally) the vehicle's total grip INCREASES because tyre grip is a function of vertical load applied (as well as Co-efficient of Friction)- the loss of vertical load on one axle is mitigated by the increase on the other. Thus the faster you can get the load to transfer the better the vehicle's overall traction.

Softening the front in rebound (as mentioned above), or softening the rear in bump, will effectively lift the front as the load transfers (causing the vehicle to 'squat') and increase the period during which the load is transferring. This will mean that the vehicle will not achieve it's maximum traction at the rear axle in the minimum time! - Effectively, because you are increasing the time it takes the load to transfer, you are increasing the period between when the load starts transferring and when it is complete; thus the tyre 'sees' less vertical load over a longer period of time and therefore develops lower traction for a longer period of time.

The only reason I can think of why this is done on some US drag cars is down to poor suspension geometry that allows too great a camber change at the rear wheels (which degrades traction by effectively decreasing the tyre's Co-efficient of Friction) - by slowing the load transfer you slow the camber change thus reducing the camber induced loss of traction over a longer period of time.


Camber change shouldn't be a problem in a midget with it's beam axle, relatively short suspension travel and associated flattish camber curve.

I would be tempted to fit mono-tubes to the rear and stiffen them up a couple of settings higher than what I would use on our circuit racer - you don't have to do any cornering (and drag strips are relatively smooth surfaces) so over-stiffening the rear isn't an issue.



Deborah Evans

Deborah, the shock rates mentioned were commonly used for drag race cars that were already raised significantly above the original body height, raising the center of gravity. When accelerating they tended to lift the front of the car because of the torque reaction from the rear axle and the racers found that they increased the loading on the rear tyres when the instant center of gravity was transfered rearwards and upwards thus they wanted the front to rise quickly and lower slowly as the torque on the rear axle diminished. Most were using parallel leaf springs and live axles in the rear with A arm type front suspension designs which were common on American cars of the period, 60's and 70's as was the usual practice on most British cars of the period. No camber change at the rear wheels at all. Typical starting attitude just off the starting line for a lot of stock class drag racers such as the Camaro shown in the photo shows the rise of the front of the car. A well set up car with enough torque could actually lift the front wheels off the ground effectively placing the total vehicle weight on the rear tyres and increasing traction.


B Young

The ultimate understeer!
OK, who's going to be first to get that to happen in a Midget :-)

Steve - what's your weight distribution? What changes have you done to shift it? Our K is 53-54% on the rear - the bigger figure is if it's me driving :-)
It has really good traction off the line as a result, best 1/4 time in the 13's.

Limiting torque in 1st gear is not a silly idea. You can spend a lot of time wheelspinning getting sub-optimal traction. It takes a lot of BHP further down the track to make up for that lost time at the start.

Paul Walbran

Id be curious as to how your rear suspension is set up.... Have you got ladder bars.... How long are they... Perhaps lengthing them out somewhere closer to the door area, that would certainly tran sfer the something something force out plus elevating the rearend a few inches giving the tires more bite
...kind of like a childs play ground equippment of a sea saw... Aka lever and fulcrum

Just a thought... For basic insight, get a high school physics text book... What Deb is discribing is a pure classic example of potintial energy vs kenetic energy transfer...its an easy read and the math is pretty simple...might even check with a book store or public library... For an abridged version but that would give you some great insight on suspension geometery math and , science.
Do you know if your rear axle is winding up and flexin the leaf springs in a S shape


Prop
Prop

Deb,thank you for setting me right,I need to digest all of that a bit!I do have tube shocks on the back,set up fairly stiff.
Paul,I don't know the current weight distribution,but it's on the list.I'm picking up some fibreglass wings tomorrow which should be lighter,and looking at putting some ballast in the back of the boot.
Pop,the Caltracs transfer axle wind up to force the front end of the spring down and prevent the S shape.
steve cowling

Prop
My nephew runs a 5.0 litre(fuel injected)v8 cortina at the drags and runs reduced fuel pressure at the starts with fantastic results.
His car runs with raised ride height at the front with soft rebound frot shocks and soft compression rears.
Admittedly it has plenty of rubber on the ground at the rear, but his problem was different in that the thing lifted both front wheels off the deck (100% weight transfer) and he couldn't point it where it had to go.
After a lot of testing he has found the right amount to cut the fuel pressure for starting off and now it leaves the line with the left front about 3" off the ground and the right front just on enough to get some steering.
To me your timing reduction idea has some merit BUT ONLY if the vehicle has that much traction that it is causing problems.
If the car has too much wheelspin there is a suspension or more likely tyre choice problem that needs to be sorted before thinking of reducing engine power as all that will do is make it slower instead of FASTER
Cheers Willy
William Revit

Steve, if you do get to the point of lifting the front tyres you should have an actual locked rear diff as any differential in rear wheel speed will turn the car rapidly with the front tyres unloaded. Most of the US drag racers run a simple spool which is a solid connection between the ring gear and the axles.
B Young

Be gentle with me Deb,but here's what I think:
There are 3 separate ways of increasing downforce for a given car weight.
1.Static position of CG.
2.Torque at rear wheel rotating car about rear axle,transfers weight from front wheels onto rears.A closed loop effect,more torque=more weight transfer=more torque.A wheelstand puts all weight on rears.
3.Inertia resisting acceleration works through C of G,the higher the CG,the bigger the vertical component acting on the rears.Hence the old high riding drag cars.
Allowing the front of the car to rise on launch will only affect 3,by raising the CG?
Bill,if I ever make a wheelstand I'll have other problems!The reasons for avoiding a spool are it's street driven, and also if a half shaft let go mid run the resulting spin could turn into a roll.
steve cowling

Went back to the strip today and am deighted to have finally run 12.975 sec with a terminal of 106.2.Following advice on here,I loaded up 25kg 0f ballast right at the back,and gained about 0.3 sec overall (the track wasn't very grippy today)
steve cowling

when the traction is enough (and the tires are screwed physically to the rims, so they don't spin), and the power is enough, then the next thing to happen is this: the pinion simply lifts the entire car up, around the rear axle centerline, walking up the ring gear


not too much risk of getting to that point in a Midget though...
":o)

Norm
Norm Kerr

Hmm,a Midget pulling a wheelie.......that's something to think about Norm.
steve cowling

I'm at the drags in mine in a couple of weeks. Our MG club rents the place out once a year (Kwinana Motorplex). We do 3 autokhanas around the pits area, 3 sprints around the entrance roads and 3 drag races, with all the timing gear,lights etc.
I find in my 'a' series road car, that I need quite a bit of wheelspin to keep the revs up. I drop the clutch at about 4000 revs, and set off like a moto crosser. It also breaks traction in second when I change gear. I have done 2 of these events, and have always been paired up with a local race midget. On the last race, I got too much wheelspin off the start. It was a weird feeling. I had the initial push back in the seat, then when it broke traction completely, it felt like I had broken something. It was a good chase once it hooked up. Another 20 feet and I would have had him.
My quickest time is 15.65, with a 660 foot time of 9.98. My fastest terminal speed is 89mph. The only race I have lost is that last one. It's actually one of the quickest cars in the club at the strip, but then most of the other members are typical MG owners. a bit older than me, driving MGB's, and maybe not quite as willing to rev their cars as high.
Shawn

Haven't posted for a while,but made a bit of progress.Went back on the RR a few weeks back with more gas on,and saw 200 bhp flywheel as before,then 260 bhp with the gas.Now running a pair of M/t drag slicks on the back and finally beginning to sort out the Caltrac traction bars.Managed a 1.89 sec 60 ft time and a 12.37 sec quarter mile with 110 mph terminal.Sorry Norm,still can't get the front wheels in the air though.
steve cowling

700Kgs is heavy for a racer - my all steel body road car with a 5 speed, rear roll bar (with diagonal) fire extinguishers, passenger seat weighs at 719KG with 4 gallons of petrol. I'd have thought you'd have a weight under 600Kg. Ballast in the boot might help the zero to 30mph time but after that will probably slow you down all the way to the line.

Do you have a photo of your Caltrac copy you can send me?
Daniel Thirteen-Twelve

Yes Daniel,I agree.The rear axle and turbo setup add to the weight,but the real culprit I suspect is the lashings of underseal on the car.
steve cowling

This thread was discussed between 22/09/2011 and 30/09/2012

MG Midget and Sprite Technical index

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