The curious case of the Atom 4 brakes

[plip1953]

@plip1953
Sound good
I am curious about your driving report.

4. An AP proportioning valve in the rear circuit. Also recently added.

Do you have forgotten to order the bias adjuster?
Please show the partNo. of the valve.

And most crucially, we have eliminated any and all air in the system. Don't underestimate how important, but not especially easy to achieve, this is!!

Yes, it's not easy to bring all air out the system when you have AP-master zylinders. The alcons are much more easier.

Yes, it's not easy to bring all air out the system when you have AP-master zylinders. The alcons are much more easier.
[/quote]

Following yesterday's outing, we at Cadwell Park we were surprised by the degree of effect that the change in rear master cylinder actually had such that having we ended up with the brake bias needing to be set quite a long way to the rears to prevent front lockups. Our ideal is still to be able to run as close as possible to 50:50, but it's not really a problem if it ends up needing to be set somewhere else other than in respect of limiting further adjustment to account for, say, wet weather conditions. For now I think we'll leave is as 0.7" front and 0.8125" rear, but could end upback at 0.7" front, 0.75" rear or even 0.75" front and 0.8125" rear.

The AP lever type proportioning valve wasn't used or needed to be used at point.

The brake master cylinders on the Atom 4 are Tiltons and incorporate their own bleed valves, which is handy, but the clutch master cylinder is an AP.

[Curves Junkie]

The brake master cylinders on the Atom 4 are Tiltons and incorporate their own bleed valves, which is handy, but the clutch master cylinder is an AP.

Oh, sorry my mistake - I dont known that. And what series?
I had thought brake from AP and MC from AP too. I had changed my MCs at my Atom3 from Tilton to AP front, rear and clutch.

[plip1953]

The brake master cylinders on the Atom 4 are Tiltons and incorporate their own bleed valves, which is handy, but the clutch master cylinder is an AP.

Oh, sorry my mistake - I dont known that. And what series?
I had thought brake from AP and MC from AP too. I had changed my MCs at my Atom3 from Tilton to AP front, rear and clutch.

Both brake Master cylinders are Tilton Series 74. As well as having their own bleed valves, the design lends itself to relatively easy fitment of the Corten-Miller pedalbox brace.

[Andy G]

Ive got the race brake set up, and things are definitely much better, but adding Performance Friction pads has improved things yet further. Had them on my Lotus 311 and loved them.

[reg]

Took the new setup (RS14's, brace, springs, up one size on both MC's) and headed to Bedford to meet up with a load of other Atom's. After going ok at Anglesey I was really looking forward to it, now with a solid pedal. It didn't go very well. On the first lap, first corner, I locked the rears...not the best start and completely took me by surprise. Wound off the bias and carried on. After two or three laps things didn't improve so ended up with the bias completely to the front. Despite this every time I leant slightly hard on the pedal it locked Ended up spinning...erm....twice at the hairpin trying to slow it down in the morning session, I guess at least one thing was ticked off the to do list . The brakes needed a leap of faith before, but now they were just scary, good initial bite then as soon as you asked for a bit more you felt the rear becoming very lively, then smoke and not much in the way of slowing down . Spent the afternoon just cruising around, it was like driving on ice and I was totally fed up. On the road the setup with all the mods seemed ok, but it's impossible to really push it. I thought it was solved. Not so much.

Ordered another master cylinder, this time another size up for the rear and a brake bleeding kit. In the meantime had found a very slight leak from the washers on the MC banjo, the old washers had been refitted but not annealed so that was likely the cause.

The weather has been crap, and tbh I was so fed up with it I just left it alone for a while.

Fast forward a few months and I have decided not to faff with the standard setup any further. I am sure that with another MC and bleeding it again it will be pretty good. The trouble is, I don't want pretty good. Decision made to move to AP5000R calipers which are sized appropriately front to back, unlike the stock setup. It's a different league of caliper (similar to the Alcon level on the 3.5) and is correct for the application, not a cheap compromise.

I am going to go the route of using standard factory discs and bells. I opted for the 'track' setup ( ) originally and they are not exactly stressed after 3000 road miles and 3 track days. In the future there's the option of bigger discs, fully floating, not to mention ABS. But the first step is obviously the most cost effective and apart from ABS should offer most of what I want....which is actually not much.....

Brakes that match my old 3.5

[robfitz]

I had similar at Pembrey but could mitigate by moving the bias forward. I’d left the MC standard. Contemplating doing similar on the callipers though.

[reg]

Some pics. I'll update again once we have some progress

[Charlie]

Interesting them being the “R” with I believe 152mm mounting centres as opposed to the cp5040 calliper with the normal 130mm radial mounting be interested to know what was involved in getting them to fit?

[reg]

They have new mounts.

[Charlie]

They have new mounts.

Are they specially fabricated?

[Corten-Miller]

Yes we designed everything from the ground up on the brake kit including the rear brackets to allow for the new mounting centres, it was a unique approach as we didn’t want to modify anything on the rear uprights.

[Atomboy4]

Took the new setup (RS14's, brace, springs, up one size on both MC's) and headed to Bedford to meet up with a load of other Atom's. After going ok at Anglesey I was really looking forward to it, now with a solid pedal. It didn't go very well. On the first lap, first corner, I locked the rears...not the best start and completely took me by surprise. Wound off the bias and carried on. After two or three laps things didn't improve so ended up with the bias completely to the front. Despite this every time I leant slightly hard on the pedal it locked Ended up spinning...erm....twice at the hairpin trying to slow it down in the morning session, I guess at least one thing was ticked off the to do list . The brakes needed a leap of faith before, but now they were just scary, good initial bite then as soon as you asked for a bit more you felt the rear becoming very lively, then smoke and not much in the way of slowing down . Spent the afternoon just cruising around, it was like driving on ice and I was totally fed up. On the road the setup with all the mods seemed ok, but it's impossible to really push it. I thought it was solved. Not so much.

Ordered another master cylinder, this time another size up for the rear and a brake bleeding kit. In the meantime had found a very slight leak from the washers on the MC banjo, the old washers had been refitted but not annealed so that was likely the cause.

The weather has been crap, and tbh I was so fed up with it I just left it alone for a while.

Fast forward a few months and I have decided not to faff with the standard setup any further. I am sure that with another MC and bleeding it again it will be pretty good. The trouble is, I don't want pretty good. Decision made to move to AP5000R calipers which are sized appropriately front to back, unlike the stock setup. It's a different league of caliper (similar to the Alcon level on the 3.5) and is correct for the application, not a cheap compromise.

I am going to go the route of using standard factory discs and bells. I opted for the 'track' setup ( ) originally and they are not exactly stressed after 3000 road miles and 3 track days. In the future there's the option of bigger discs, fully floating, not to mention ABS. But the first step is obviously the most cost effective and apart from ABS should offer most of what I want....which is actually not much.....

Brakes that match my old 3.5

Hi Reg

Good account on whats happening with your braking set up, im due to just have the springs fitted to mine in a few weeks but now im wondering if my might get the same issues on track when im next out a few weeks later!

Last thing anyone wants is lock up and loss of confidence in the car, how much were the AP5000R calipers for the set!

Cheers

Daz

[plip1953]

Hi Reg

Good account on whats happening with your braking set up, im due to just have the springs fitted to mine in a few weeks but now im wondering if my might get the same issues on track when im next out a few weeks later!

Last thing anyone wants is lock up and loss of confidence in the car, how much were the AP5000R calipers for the set!

Cheers

Daz

If your front/rear balance is ok right now it will be just the same after fitting springs. The OE master cylinder sizing is 0.625" front and 0.7" rear. What a number of us have done is increase the size of both master cylinders (which essentially reduces brake pedal travel but increases the amount of effort that needs to be applied), but not necessarily by the same amounts, and it's these differences that have been causing a few balance issues here and there. The type and mix of brake pads can also make a difference, but RS14s all round should be good.

See how you go after having the springs fitted and let us know :-)

[reg]

Hi Reg

Good account on whats happening with your braking set up, im due to just have the springs fitted to mine in a few weeks but now im wondering if my might get the same issues on track when im next out a few weeks later!

Last thing anyone wants is lock up and loss of confidence in the car, how much were the AP5000R calipers for the set!

Cheers

Daz

You’ll be ok with just springs and pads, I think the challenge is when you start to play with the master cylinders? It’s not the case for every car as mine is setup as per quite a few others. Could be some air trapped still, could be another larger rear mc is the fix. Either way I just wanted to draw a line under the OE calipers. It’s not like there’s much support for them when you do have issues and it’s a day to return the car each time to Ariel. Smaller rear pistons to me makes sense as do calipers made specially for non servoed lightweight cars, rather than big brake kits for road cars. The only factory option is to go with carbon, I’m not keen on the cost of those or for a car used mostly on the road.

[plip1953]

I agree with reg that a line can now be drawn under the regular AP calipers that Ariel in their wisdom chose for the Atom 4. But only once anti-knockback springs have been fitted.

The original reason for trying out different master cylinder sizes came prior to the realisation that springs were the answer, and was primarily intended as a means of reducing pedal travel, which it does - bigger master cylinder equals less pedal travel, but as said previously, you will need to apply more braking effort (ie press harder to achieve the same stopping power).

The current Ariel AP caliper option comprises 4 identical calipers and therefore the same piston sizing on both front and rear. This is quite unusual and wouldn't be found on very many cars at all, but the Atom is rear engine, rear wheel drive and so most of the weight is on the rear, and therefore a higher proportion than normal of the braking effort needs to be applied to the rear. Nevertheless you still need more on the front than rear and that gets taken account of in the selection of differing master cylinder sizes.

With the higher spec AP 5000R caliper option there is the opportunity of two different piston size combinations and it therefore probably makes sense to have the smaller piston version on the rear. And in that way it might even be possible to have the same size master cylinders both front and rear - which wouldn't be a bad thing at all.