Separate handbrake caliper
Discussion
I cant find the link anywhere...But provided you have enough clearance from the wheel...
You can actually convert most racing disk callipers to take a handbrake cable..
Basically...space the two halves of the calliper appart, fit a steel paddle between the pistons and backside of pads on both sides, join the two paddles with a cable...hey-presto a road legal handbrake..
I have seen it done on many kit cars....I cant imagine the handbrake is ever very good...But it would work...and is much lighter than adding another calliper!
You can actually convert most racing disk callipers to take a handbrake cable..
Basically...space the two halves of the calliper appart, fit a steel paddle between the pistons and backside of pads on both sides, join the two paddles with a cable...hey-presto a road legal handbrake..
I have seen it done on many kit cars....I cant imagine the handbrake is ever very good...But it would work...and is much lighter than adding another calliper!
TT Tim said:
Hi about going the Cerb' route, or come to that my 300zx and have a drum handbrake?
Rear calipers for the 300zx can be picked up dirt cheap!
Why would you want 4-pot on the rear, are you not in danger of upsetting the brake bias, and overbraking the rear?
Tim
Indeed! How big are your front brakes going to be then to compensate?Rear calipers for the 300zx can be picked up dirt cheap!
Why would you want 4-pot on the rear, are you not in danger of upsetting the brake bias, and overbraking the rear?
Tim
This big 
Hopefully (bracketry permitting) I'm going with the same setup on the rear as I've already got on the front. As standard the Griff has 273 rear and 260 front. I'll have 300mm front and rear which should move the bias back from it's current point which is now too far front biased IMO. In theory I'll still have a more front biased set up than standard but with much more feel.
Hopefully (bracketry permitting) I'm going with the same setup on the rear as I've already got on the front. As standard the Griff has 273 rear and 260 front. I'll have 300mm front and rear which should move the bias back from it's current point which is now too far front biased IMO. In theory I'll still have a more front biased set up than standard but with much more feel.
HRG said:
Hopefully (bracketry permitting) I'm going with the same setup on the rear as I've already got on the front. As standard the Griff has 273 rear and 260 front. I'll have 300mm front and rear which should move the bias back from it's current point which is now too far front biased IMO. In theory I'll still have a more front biased set up than standard but with much more feel.
Can't help but think that that doesn't sound good, rear calipers are smaller for a reason.Having said that I'll be interested to see how you get on, someone has to try things to see if they work or not, so good on you.
Tim
The reason why the bias is towards the front is that it is very dangerous to have a brake bias so that the rear brakes lock up first. Remember the issue with not heel/toe on down changes and the rear wheels locking up with the torque and causing a quick visit to the scenary. Same thing happens when the rear brakes lock up.
If anything the favoured upgrade is to go bigger on the front and keep the rear standard on the back. This moves the brake balance forward - some reckon it is too rear balanced as normal anyway - which is a big improvement. The bigger disks do help but the whole setup is just better balanced.
Also bear in mind that with bigger rear and front callipers, the master cylinder may not be able to flow enough fluid so the pedal through becomes very long and it can feel as if there is no resistance. The calliper sizes will also change the balance and setup because the balance is based on the ratio of cylinder sizes. Nothing that money can't solve but not cheap as it may require a custom master cylinder to get right. I had to change the master cylinder on the Wedge because of this problem - the pedal would go almost to the floor which made things like feel very difficult to experience.
As you say it is an experiment but it does go against collective experience and wisdom. I wouldn't want to drive a Griff that was more rear biassed than normal. The thought of a rear wheel lock up during an emergency stop and the resulting spin are a little disconcerting to say the least.
If anything the favoured upgrade is to go bigger on the front and keep the rear standard on the back. This moves the brake balance forward - some reckon it is too rear balanced as normal anyway - which is a big improvement. The bigger disks do help but the whole setup is just better balanced.
Also bear in mind that with bigger rear and front callipers, the master cylinder may not be able to flow enough fluid so the pedal through becomes very long and it can feel as if there is no resistance. The calliper sizes will also change the balance and setup because the balance is based on the ratio of cylinder sizes. Nothing that money can't solve but not cheap as it may require a custom master cylinder to get right. I had to change the master cylinder on the Wedge because of this problem - the pedal would go almost to the floor which made things like feel very difficult to experience.
As you say it is an experiment but it does go against collective experience and wisdom. I wouldn't want to drive a Griff that was more rear biassed than normal. The thought of a rear wheel lock up during an emergency stop and the resulting spin are a little disconcerting to say the least.
Edited by shpub on Wednesday 1st October 18:06
Sorry, perhaps I'm not making myself clear... My fronts have already been upgraded and in my particular case they have shifted the bias a long way forward. I need to redress it by upgrading the rears too, however I'm actually reducing the ratio slightly so that the system will be mechanically balanced 300/300 instead of the std rear 273/260 bias.
Pedal travel is most definitely not an issue at the moment. If anything I may actually need to reduce the pressure as they're phenomenally powerful
Pedal travel is most definitely not an issue at the moment. If anything I may actually need to reduce the pressure as they're phenomenally powerful
HRG said:
Sorry, perhaps I'm not making myself clear... My fronts have already been upgraded and in my particular case they have shifted the bias a long way forward. I need to redress it by upgrading the rears too, however I'm actually reducing the ratio slightly so that the system will be mechanically balanced 300/300 instead of the std rear 273/260 bias.
Pedal travel is most definitely not an issue at the moment. If anything I may actually need to reduce the pressure as they're phenomenally powerful
The size of the disk is not the same as the amount of force that can be applied to the disk by the caliper.Pedal travel is most definitely not an issue at the moment. If anything I may actually need to reduce the pressure as they're phenomenally powerful
You may have had 260 on the front and 273 rear disks but my betting is that the fronts were larger CALIPERS than those used on the rear. It would not have been biased to the rear, but what you are heading towards is a 50/50 front and rear, and as Steve has said, this is not ideal.
I've just checked and they are single piston rear calipers as standard, so no where near as much force as the fronts, ironically the single piston rear caliper is the reason that the rear isk is larger as there's more area for the caliper to act upon.
Tim
Edited by TT Tim on Wednesday 1st October 19:50
Std Griff front fare is single piston too... And with the brakes I'm running (mine are designed for the track) there is way more braking force available at the front. I'll quite happily discuss swept areas and line pressures but on my particular car I need a rear upgrade 
IF it ends up mechanically over-braked after the conversion I can easily swap the rotors for some smaller ones and bring the caliper closer to the disc to accommodate it, and if the line pressures aren't as I expect then I can fit a proportioning valve.
So, back to the bracket, who's actually done it?
IF it ends up mechanically over-braked after the conversion I can easily swap the rotors for some smaller ones and bring the caliper closer to the disc to accommodate it, and if the line pressures aren't as I expect then I can fit a proportioning valve.
So, back to the bracket, who's actually done it?
Why not fit a proportioning valve first and experiment?
The griff setup uses a acceleration valve not a proportional one (although it does the same job of preventing the rears from locking up first). That would give you a good indication of balance.
The other factor to take into account is the weight transfer under braking which moves the weight forward. This increases the grip on the front which is why the front end up doing most of the work and reduces the weight at the rear which in turn reduces the grip and the amount of braking before the wheels lock. With a light car like the TVR, this can often result in an 80/20 split where the rears are doing very little because of the weight transfer.
I would fit a proportioning valve - you will need one if you go with 300mm discs etc simply because the braking capability of the calliper has increased and will need to be significantly reduced - and see how you want the balance set up. That would then let you work out what the effect of the 300mm upgrade you are planning will be. The actual performance of the brake is based around the gripping power of the calliper which depends on the disk diameter, pad size and the pressure applied. The pressure in turn is dependent on the ratio and capacities of the master and brake cylinder and the mechanics of the pedal. All this can be worked out to give you some idea of how much the rear braking power is changing which is critical as it has the least amount of grip available from the tyres.
Changing the rear brakes are a lot more critical to get right compared to the front. With the front, the better the braking, the more weight transfer and the more grip which allows more braking. A nice positive reinforcement. With the rear, it loses grip with better braking.
Might be worth talking to the Griff racers. I am pretty sure they have left the rear callipers as is.
The griff setup uses a acceleration valve not a proportional one (although it does the same job of preventing the rears from locking up first). That would give you a good indication of balance.
The other factor to take into account is the weight transfer under braking which moves the weight forward. This increases the grip on the front which is why the front end up doing most of the work and reduces the weight at the rear which in turn reduces the grip and the amount of braking before the wheels lock. With a light car like the TVR, this can often result in an 80/20 split where the rears are doing very little because of the weight transfer.
I would fit a proportioning valve - you will need one if you go with 300mm discs etc simply because the braking capability of the calliper has increased and will need to be significantly reduced - and see how you want the balance set up. That would then let you work out what the effect of the 300mm upgrade you are planning will be. The actual performance of the brake is based around the gripping power of the calliper which depends on the disk diameter, pad size and the pressure applied. The pressure in turn is dependent on the ratio and capacities of the master and brake cylinder and the mechanics of the pedal. All this can be worked out to give you some idea of how much the rear braking power is changing which is critical as it has the least amount of grip available from the tyres.
Changing the rear brakes are a lot more critical to get right compared to the front. With the front, the better the braking, the more weight transfer and the more grip which allows more braking. A nice positive reinforcement. With the rear, it loses grip with better braking.
Might be worth talking to the Griff racers. I am pretty sure they have left the rear callipers as is.
All good points, but as I don't have enough mechanical braking force available at the rear at the moment I would have thought a proportioning valve is merely going to make matters worse?
I've still got the standard kit for the front and rear so I suppose it's possible to do some proper maths with swept areas etc. to understand the ratio when it left the factory, but I don't know if there are any line restrictors in circuit to provide a pressure differential.
So, if we were to throw theory at this rather than my usual empirical approach what information would I need to calculate it correctly and is it available?
I've still got the standard kit for the front and rear so I suppose it's possible to do some proper maths with swept areas etc. to understand the ratio when it left the factory, but I don't know if there are any line restrictors in circuit to provide a pressure differential.
So, if we were to throw theory at this rather than my usual empirical approach what information would I need to calculate it correctly and is it available?
How do you know that you don't have enough mechanical force at the rear? What are the symptoms? I think that is where most people are struggling with understanding.
As for theory it is basic hydraulics to calculate pressures etc and clamping forces. You need cylinder diameters and a calculator for that. From that you can use basic lever theory to work out moments and so on. There are a couple of books that go into the theory of brake upgrades. You can also calculate the weight transfer and that is really enilightening as it really show how little the rear brakes actually contribute apart from stability. If you look at the spec for proportioning valves they will often tell you the pressure reduction and so you can feed that into the equation.
As for theory it is basic hydraulics to calculate pressures etc and clamping forces. You need cylinder diameters and a calculator for that. From that you can use basic lever theory to work out moments and so on. There are a couple of books that go into the theory of brake upgrades. You can also calculate the weight transfer and that is really enilightening as it really show how little the rear brakes actually contribute apart from stability. If you look at the spec for proportioning valves they will often tell you the pressure reduction and so you can feed that into the equation.
Edited by shpub on Thursday 2nd October 10:37
Good old fashioned seat of the pants driving. She'll understeer on the approach to corners, plus there's no squat at the rear whatsoever. I can feel that all the retardation is coming from the front.
On my particular car with the brakes I currently have the fronts are overwhelming the rear. There's also the issue of feel, the fronts are lovely now, but you can't really tell what's going on at the back, but that's another discussion altogether.
On my particular car with the brakes I currently have the fronts are overwhelming the rear. There's also the issue of feel, the fronts are lovely now, but you can't really tell what's going on at the back, but that's another discussion altogether.
Hmmm interesting. First thing is that upgrading the rear brakes will do nothing about the lack of rear squat. That is due to weight transfer as the weight moves forward. The front dips as a result. The improved braking will increase the transfer and make everything feel that it is centered on the front. The cars are designed to understeer into corners which is why many use trail braking to provoke a bit of oversteer to compensate. Geometry settings and in particular toe can also affect this as can tyre pressures etc.
I would check the static balance. Get the car on axle stands and apply the brake slowly while someone else turns a front wheel. Use a lever or wheel brace to increase the test force. When the wheel stops, hold the brake position and see if the rear wheel will move. If it does, then the balance is in the front. Gently apply the brake and see how much it takes to lock it up. That will give an indication of where the static balance is. If the balance is wildly out then I would suspect a fault so it would be worth bleeding the brakes and checking that the callipers are working correctly.
I would check the static balance. Get the car on axle stands and apply the brake slowly while someone else turns a front wheel. Use a lever or wheel brace to increase the test force. When the wheel stops, hold the brake position and see if the rear wheel will move. If it does, then the balance is in the front. Gently apply the brake and see how much it takes to lock it up. That will give an indication of where the static balance is. If the balance is wildly out then I would suspect a fault so it would be worth bleeding the brakes and checking that the callipers are working correctly.
Interesting... I'll have a look at that and also do some maths 
I don't think there's a fault as such at the rear as it was all rebuilt last winter, I just think the fronts are far more efficient than the rears and they're not coming into play.
I've got the formula from the bible so I'll have to go and do some measuring and work out the original, the current and the proposed ratios.
I don't think there's a fault as such at the rear as it was all rebuilt last winter, I just think the fronts are far more efficient than the rears and they're not coming into play.I've got the formula from the bible so I'll have to go and do some measuring and work out the original, the current and the proposed ratios.
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