[sweex]

I’m thinking of jacking up the rear of my car (and put it on axle stands), remove both rear wheels, and then get an assistant (my son) ready to put his foot on the brake pedal.

If I then put a long breaker bar across the wheel studs (I’ve got one that can be fixed on, using the wheel nuts) and then start to rotate the wheel slowly while my son slowly puts his foot on the brake pedal, there will come a time when I can no longer rotate the wheel (unless I were daft enough to put my whole 100 kg weight on the end of the bar)

My question is, if I know the length of the bar and the force I apply, how does this translate to the kg.f value that the brake will get on the MOT test ?

Put another way, if I know the approximate value that the kg.f typically is for the car and wheel to pass, how much torque would this equate to on the wheel ? (this is a more sensible question)

EDIT – just to clarify, if I do it both sides then I can tell how much balance there is, but I also need to know how this torque method would correlate with expected kg.f

[sweex]

By working the other way from engine torque, I think that I can answer my own question 😯

Using data of another car that I have, engine has 225 N.m max torque, TG ratio = 1.03, diff ratio = 4.39 (so overall ratio = 4.5217) then max wheel torque in TG = 225 * 4.52 = 1017 N.m

The max Force at tyre edge in TG = max wheel torque in TG divided by wheel radius (to tyre edge, 0.3 metres in this case)

So max Force at tyre edge in TG = 1017 / 0.3 = 3391 N = 345 kg.f

So all I need to do is measure the radius to tyre edge, get the nominal kg.f pass value for the wheel, convert that to Newtons, multiply that figure by the radius, and that’s the torque to apply and see if the brakes can hold it ?

[Stealth]

Hi sweex & welcome yo the forum.

Why not call a local MOT site. Lots of sites have automated brake rollers nowadays which can give you
a print out of individual brake performance ?

[sweex]

Thanks for the welcome 🙂

There is an MOT place I’ve been going to for about 35 years, but it’s 20 miles away now (I moved house 5 years ago). I still go to them, but the car in question now has no MOT so unless I book it in with them, I can’t take it there.

I trust them implicitly (it’s an old place with analogue scale brake meters, and I sit in the car when it’s up on the lift to do the steering and brake stuff) and if there are any problems they show me. I’m very wary of going to anybody else, I once went to a place very local, not even a mile away, and I just felt like they’d rip me off given half the chance.

EDIT – basically, finding a good garage that you can trust is like finding a GP that you can trust, or a good dentist.

[sweex]

[sweex]

[user=21050]sweex[/user] wrote:
[quote]So max Force at tyre edge in TG = 1017 / 0.3 = 3391 N = 345 kg.f[/quote]

For the sake of fastidiousness, the above value would be for both wheels on the driven axle, so max Force at [b][u]one[/u][/b] tyre edge in TG = 345 / 2 = 172.5 kg.f
I did think that 345 kg.f was high for one wheel in TG 😀

But otherwise, the conclusion is the same ?

i.e. measure the radius to tyre edge, get the nominal kg.f pass value for the wheel, convert that to Newtons, multiply that figure by the radius, and that’s the torque to apply and see if the brakes can hold it.

[Author]

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