TR4/4A TR4A rear brake upgrade

[Darrell_Walker]

I've been reading about using larger cylinders on the rear brakes (stock is 0.70", the upgrade is 0.870" (7/8")). This appears to increase braking power on the TR6, without causing the rears to lock up early. Has anyone done this on a TR4A?

 

[PeterK]

Usualy this is done after changing the fronts to Toyota 4-pot calipers. I have this setup on my 4A but it's not on the road yet.

The larger bore rear cylinders are from a Morgan.

 

[Darrell_Walker]

I thought maybe I should explain my thinking a bit more. I realize that, assuming you can lock a wheel, that the total braking (for that wheel) is limited by the tire. With stock size tires on my car, I can lock the fronts fairly easily (if I want to). I used to have larger tires, and it took more effort, but was still possible. I also realize that you don't want to lock the rears first (unless you like that kind of thing). I've never (that I know of) locked the rear wheels. I also realize that the rears contribute less to braking than the fronts. What I don't know is if there is still some stopping power back there. It would seem so, since when using the engine to break as well, it feels like there is more stopping power, but that is subjective. I also can't figure out if the fact that the TR6 is slightly heavier, and that the extra weight is biased to the front, means that there might be more or less braking needed at the rear.

 

[Andrew Mace]

Usually the heavier nose would mean that the brakes would (should?) be biased more to the front. The rear tends to get "lighter" on braking, so effectively less braking would be needed in the rear. As an example, 1968 and later (Federal) Spitfires with dual-circuit brakes usually had a straight 50/50 master cylinder, but the original dual master cylinders for GT6s were more like a 70/30 -- 70 front / 30 rear -- via stepped bore, IIRC. I stress original as I don't think that those stepped-bore cylinders are currently available.

 

[Darrell_Walker]

Thanks, Andy. That would seem to imply that the 4A could benefit from even more from a little more brake in the rear.

 

[DNK]

Don't know if that is correct. With more will they lock up quicker?

 

[Andrew Mace]

I probably should have mentioned that the 70/30 was 70 front / 30 rear; I will amend my above post.

 

[TR3driver]

With stock size tires on my car, I can lock the fronts fairly easily (if I want to).

IMO that indicates there is a problem with your rear brakes. Contaminated or glazed linings is most common. You might start by cleaning the drums thoroughly and installing new or relined shoes.

Even with the less effective handbrake on the 4A, you should still be able to lock the rear wheels with it. Can you?

PS, it's also possible for tires to get old and not grip as well. Have you tried rotating them?

 

[AEW]

just my rambling thoughts...

I believe there is a lot of physics you guys are ignoring... some of the tire web sites go into great detail on patch area, rubber/road friction, and on and on.

Locking up the wheels only means the friction coefficient is exceeded. Randall has the best advice "there is a problem with your brakes".

I think even the 10" down to 9" rear drum change relates to Triumph having sorted out the balance issues way before TR6.

Though it's only my opinion, improvements in calipers and pads gain mostly in friction material heat management (not necessarily just dissipation). And this is all race car stuff. Is that what this car will need to do?

 

[tdskip]

Hi Darrell - as Randall said it sounds like your rear brakes should working harder than they are.

The Toyota calipers and Morgan rear cylinders is a fairly proven combination - I run it on my TR6 and TR4s. I didn't do be/after testing but the combo works well and holds up to an aggressive driving style.

 

[Darrell_Walker]

I don't think there is anything "wrong" with my rear brakes. If I adjust them a little too tight (dragging a bit), there is no problem in locking them up (before the fronts). I can lock them up with the hand brake. I suppose if I continue to apply pressure after locking the fronts, I'll get the rears locked up, too. But of course the best braking force is before locking up, so I was just wondering if a little more force in the rear would be beneficial or not. I understand why manufactures bias the breaking to the front (beyond the weight transfer issues), just as they bias towards understeer.

BTW, I don't believe the Toyota front calipers really change anything, they have nearly identical piston area as the stock calipers. The larger pad probably resists fade better, but that isn't the case I'm talking about. The question is, can a TR4A benefit from more rear breaking over stock. There seems to be some evidence that a TR6 can.

 

[Murieta]

In such discussions, some fundamental facts are often omitted, I mean mass transfer upon braking and frame torsion.

A lot of guys like big tyres at the rear and they always use more pressure than in front tyres. In fact, rear tyres should be less inflated for two reasons: gives better grip and the tyres support less weight at the rear. so set the car at 2 kg/cm2 front and 1.8 rear. you'll be very surprised. AEW is right: tyre grip is essential.

Out of racing conditions, big tyres are just show-off. 165.80.15 on 4.5" wheels are perfect for any car up to 220 bhp (personal experience on a modded AH 3000 MkII). That is if you know how to drive "period style" which is avoiding the brakes thru downshifting on straight lines and drifting thru curves to alleviate torsional constraints on the frame. If you have reinforced your frame like in works cars, it's a different situation.

TR6 accomodate 185.80.15 on 6" wheels and it's an absolute max. Above that we're talking surfing and very poor handling (weight divided by contact surface).

It's just a version of the classic gambit: modernize the car to drive as usual or optimize the car with respect to period and learn to drive accordingly. Your choice. I go for period driving. Adrenalin comes up faster. Just try an Austin seven in a winding road at 75 mph. Don't need to reach 88 to see some serious sh**.

 

[Andrew Mace]

Many manufacturers have long "set up" their cars so that they will a: tend more towards understeer and b: tend to lock front brakes first, both for similar reason: the "average" driver apparently copes much better with the front end needing more input via steering wheel than he/she does with the rear end breaking free in either a rear wheel skid or rear wheels locking situation. At least that's what I've always heard!