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MG Midget and Sprite Technical - Tyre pressure revelation

Didn't expect to be sitting here with a daft grin on my face thanks to the frog tonight. I was scanning some FCC/IOW Frogeye literature to put on the www.healey-frogeye.co.uk website when I came across its first service record. Something I'd not noticed on it before were the words 'tyre pressures checked: front 18 lbs, rear 20 lbs'.

Blimey I thought, that's low. Still, worth a go eh? So I dropped 6 and 4 psi respectively out of the front and back to get there, then just for devilment (and to confuse the experiment ha ha) I turned up the front AVO dampers from softest to 4 harder.

Off for a blast.

Bloody hell!!! What a transformation! It's like a different car. Obviously the ride is softer and much more comfortable, but the handling...! Where before it was edgy, over-reactive and always threatening to oversteer, it's now smooth, predictable and maintains its line through the bends. Much easier to control the line on the throttle, much easier to keep it on the straight over bumpy roads and generally just a lot more stable and chuckable. The feeling of it being about to bite me on the ar*e through every corner is gone. All for the sake of a few psi and a damper tweak.

Excellent. Fingers crossed for an MOT pass in the morning a weekend of froggy hooning :)

Jordan Gibson

Good luck Jordan.....! Let us know.

Mark.
Mark Boldry

Jordan
I always thought those pressures were for crossply tyres only, but if it works for you then that's got to be a good thing.
Ian
Ian Webb '73 GAN5

Interesting - I didn't know that about crossplys Ian. The car was indeed on wires at the time, but I don't know if they'd have been crossplys with the car having just been built up. Do such low pressures apply to tubed radials too perhaps?

Ah well it's a vast improvement anyway so I'll see how it goes.
Jordan Gibson

Ian.

As the IOW (Healey Frogeye) was produced long after the Crossply tyre was popular, the tyre pressures that Jordan has discovered is correct for that car on Radial tyres.

Mark.
Mark Boldry

Jordan - great it works for your set-up.
Be aware though that your mpg may plummet - and you may get advanced tyre wear towards the shoulders of the tyres over time.
Steve Clark

My frog has fairly standard suspension with the addition of front ARB and polybushes. I run standard wires with 155/13 radials and I have always found the best pressures to be 18-20F and 24R. Anything in excess of that (especially on the front) makes the car jittery.
SA Wood

What is the weight of an IOW Frog? I presume they are a good deal lighter than an all steel MK IV Sprite, even one without bumpers!
Guy Weller

Talking of which, what good accurate tyre preassure gauges do people recommend as I would imagine that the one on my woolworths foot pump is probably a tad unreliable.
By the way (according to my woolworths gauge/pump) I have absolute stability at high motorway speeds with preasures of 26 front and 28 rear although I do have a frontline front suspension which gives extra camber and also I've a little more toe-in than usual.
G Lazarus

Jordan, Gary and all,

What tyre sizes do you have?

I am currently running 165/65/13 and found I get best grip and handling around town at about 22/24 F/R

On the motorway I have them quite a bit harder for hi speed stability at about 28/30

Also running frontline front suspension and telescopics at the rear.
Bob T

Good points Steve. I don't do massive mileages so the tyre wear hopefully won't be an issue for a while. I could do without sucking up any more fuel though! I'll keep an eye on both issues. There may well be room to go back up a psi or two and still keep the improvement whilst mitigating mpg and wear.

Guy I'm not sure what it weighs with the Type 9, I keep meaning to run it over one of our lass's weighbridges to find out. The brochure claims 'a kerb weight of under 625Kg' for a standard non-Supersprite IOW.

Gary I use one of the RAC digital gauges. Bought in Aldi for about a fiver I think.
Jordan Gibson

165/65/13 here too.
Jordan Gibson

BTW Tanya has weighbridges beacuse she weighs wagons a lot, not because the bathroom scales can't cope :o)
Jordan Gibson

Did upwards Jordan, Dig upwards.
Nick

I too am on 165/65's - just as an aside, I did some trackwork and took along one of those infra red thermometer thingy's. I ran at 28 psi, and the difference in temp between the outer, centre and inner of the tread was negligable. (Frontline, lowered, bigger anti roll bar, polybushes etc.) My wheels are also stiffer 4.5" wires. I run at 26 all round on the road with no issues. If I want comfort, I drive the Subaru!
Shawn

I put the pressure up to 30psi on all four 145/80/13s before a long trip down the highway. If i had a compressor i might experiment with lowering that a tad for around town.
S.A. Jones

Mark B - it passed! :oD
Jordan Gibson

Congratulations Jordan. And it was good to meet you last Saturday too.
G Lazarus

Gary, you might find your pressures are vastly different to what the footpump tells you. I have a Clarke one that over reads by around 6 or more PSI, I pump up with this and then check with a digital gauge I got from Halfrauds

Mark
Mark

33/34 psi on 175-65-14 when loaded with bagage(like the Gaydon weekend) for motorway use.
I just dont like driving with soft tyres.
A de Best

625Kg is quite light so the low pressures would make sense. Arie, your car must weigh in at close to 850Kg hense a better feel with higher presures.

Jordan, well done on the Pass... :-)

Mark.
Mark Boldry

Great now more confused than ever.

I am running wires with tubes on a 72 sprite with completely std suspension 165/70/13 at 22psi car feels ok, I was previously running 28 psi but was told that's too high so very interested in what pressure I should run.

Thanks in advance

Shaun

S Riches

Jordan, apologies I misread your thread ...didn't take in that it is IOW Frog, my stupid confusion. Ignore my posting please.
I've got Frogeye things on the brain right now since my first Frog came into my possesion 3 weeks ago !!

Ian
Ian Webb '73 GAN5

I believe some tyre manufacturers' rough and ready start point for pressures is kerb + occupants expressed in lbs, divided by 10. So a Frogeye with fuel and occupants might come out around 1800 lbs / 10 = 18lbs. Fairly close to the 18/20lbs the factory recommends. This formula seems to bear up if you apply it to lighter cars or heavier ones.
f pollock

Shaun.

At the end of the day, it really is down to you and how you like the feel of your car. 22 front and 24 rear is a very good combination for local driving with your Sprite and tyre sizes, I would increase to 24 front 26 rear for longer motorway work.

F Pollock.

I have never come across that formula but it makes sense.

Shawn has the right idea with tyre testing and temperatures across he width of the tyre, however for circuit work it is always advised that the pressures are slightly higher than one would use on the road, so the 28psi quoted would make sense.

Mark.
Mark Boldry

1800 / 10 = 180
R T Jakeman

This thread was discussed between 29/05/2008 and 04/06/2008

MG Midget and Sprite Technical index

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