Brake balance...? 15" wheels all round.

[Victor Meldrew]

From what I have read and heard the Elise is pretty good as standard for brake balance. Its only when you start messing around with different size disks and calipers that you start to get too much the front end bias. I had an interesting conversation with Kelvdon today when I ordered the hard top about a remotley adjustable brake balance conversion that mounts onto the back of the stock padals, standard rod feeds a balance bar awith two new master cylinders.
Also I notice Elise parts are now doing a conversion plate to allow you to run the larger 295mm disks on the rear.

The end game is to be able to use 15" ACB10's all round. If you change the diameter of the tyres, you change the torque that the wheel generates, you also change the speed that it turns at for a given speed. I am guesing that this will also have a direct effect on brake balance as well as the change in diameter will be different from front to rear. Lots of variables, would be handy to have a proper adjustable brake balance to compensate, I dont like the idea of a basic bias valve as they can only cut pressure to the rear end and from what I've heard you actually want more at the rear.

Has anyone heard of this conversion? any thoughts?

[Rich H]

Depends on the rolling radius of the wheels I would think - what tyres will you have?

You will get 2 effects I'm guessing - the wheels will be rotating faster for a given speed but the brakes will have less torque to deal with while slowing down as the distance from the road to the hub will be shorter. Faster rotations will give better cooling too.

[roadboy]

Hi John,

Bored at work?

In my experience it will make no noticeable difference to the brake bias running 15" rears. Your theory is sound enough though.

I agree with needing more rear bias though and I think fitting 295s on the back would be a great idea.

Personally I think the adjustable bias setup is overkill and just more stuff to go wrong/mess about with.

/2p

Dan

[robin]

The difference will be minor (assuming you're running 16" rears at the moment).

Braking force (assuming there is sufficient friction) is divided down by the ratio of the distance from pad centre to hub centre and contact patch to hub centre.

So given that the calipers aren't changing, and assuming you're running a 225/45 or similar you'll be dropping 1 inch out of about 8.5, i.e. your rear braking force will increase by 8.5/8.0, or about 6%.

I doubt you'll notice it.

If the ACB10s are actually higher profile than the standard 225/45/16 then this will make the increase smaller.

Cheers,
Robin

[alicrozier]

John,
Do you have standard front brakes? You're right that standard brake balance is pretty reasonable (but crucially the optimum balance is dependant on weight transfer and hence available grip, many factors governing this but tyres and probably most importantly the track conditions). Standard setup is pretty good in the dry.
For example take a standard Elise, Lotus design the system so it won't lock up the rears at maximum weight transfer (R rated tyres, dry, grippy track). It'll have a big saftey margin too (so the average muppet doesn't lock the rears and end in a firey death on the way to Tescos). Hence they can stand some extra rear bias and in wet conditions could really do with much more...

If you go with the bigger brakes on the rear AND 15" wheels there is a danger you'll have too much rear bias (in the dry with you're ACB10's!) but have a potentially better balance for wet conditions. In that situation a bias valve to reduce rear bias in the dry might be required.

Also don't underestimate how you as the driver can affect the brake balance. Personally I'd leave it standard and increase rearward bias when required through technique...

edited to add:
For sprinting/hillclimbing I'd want a car that has some margin and is friendly at the limit. Doune is probably not somewhere you want to lock up the rears...

[Victor Meldrew]

Ali,

I have 295mm AP's on the front at the mo. I was considering moving these to the rear with the new Elise parts upgrade mountings and going up to 304mm on the front.( I have been told by a reputable source that they will fit) Possibly four pot calipers on the front but most likely sticking with stock calipers.

As far as tecnique goes, I am hopeing to get two days in April with Walshy to work on my transition from braking to turn in.. still not comfortable with that..

[alicrozier]

OK that'd work, standard calipers. I don't think you'll need bias adjustment. I'd get any mods done before the Walshy days so you get the most from the training.

For technique on brake balance (before turn in) think about where and how you downchange.

[fd]

Agree on the setup philosophy Ali, it's not like circuit racing where you can probably get a good bias setup because of the amount of time you have to experiment . . . I set the bias on the caterham to be 'just safe' and have a wet and dry setting . . . that's not optimal but it keeps you out of the armco . . . oh and Doune isn't somewhere you lock brakes but for different reasons . . . the one place where you really need to brake you're climbing a 30 degrees hill at 80mph . . . you can brake as hard as you like and the wheels don't lock . . .

As for turn in and brake blending . . . I think you can do a lot worse than Macintyres at KH, great corner to practice, plenty space to screw up, nice shape of corner which is obvious when you get it right . . . in two hours you'll get 100 attempts at it . . .

[tut]

Duffus, Macintyres, Butchers and the Chicane are a great combination, and equal to anything in the UK, but if the circuit could be extended to 2-2.5 miles with similar corners making the most of the topographical landscape, it would take some beating.

tut