56 replies to this topic

[Stu-7]

Had a great day at Bedford yesterday. My mate who was circulating with me in his S1 Honda, said my car looked very unsettled and squirmy under heavy braking, such as into the first corner of the lap.

I am wondering if this is because the car is over-braked at the front and the rears can't cope?

So, I have looked into what's available on the market, and the only 2 piston caliper I can find is by hi-spec:
http://www.hispecmot.../vx220_rear.htm





This has the handbrake mechnism built in as well. I think if i fit it to the VX, I will need new handbrake cables as well?

Has anyone else carried out this upgrade? How difficult is it to change the handbrake cables?
Any thoughts on whether I would need to alter brake bias post fitting?

[siztenboots]

What about a brake bias valve and what is your brake setup at the moment

[Stu-7]

For some reason, I can't seem to access the "edit post" function. Anyhoo, I forgot to say, I'm running the AP 4-pots on the front, with Pagid RS14's. Std VXR rear calipers with RS14's.

[colgy]

How about using your original front 2-pots on the back? The discs are exactly the same so I can't imagine it would be too difficult to fabricate some sort of adaptor. You could also try speaking to Plans, as they sell ali bells (295mm) which make quite a difference supposedly - minime had them on his vx.
Try reading these posts here and here. Seems like the build quality of the Hi-specs is a little dubious; I've spoken to people (non-vx'ers ) who have had problem with their calipers too.

Edited by colgy, 19 February 2007 - 07:41 PM.

[siztenboots]

How about using your original front 2-pots on the back? The discs are exactly the same so I can't imagine it would be too difficult to fabricate some sort of adaptor.

Would need the handbrake to work somehow if using the fronts. Hispek are the same as the Eliseparts (Geary) . Tempted to go for some ali-belled bigger rotor fronts, and maybe try and get some lighter ventilated ones for the rears.

[2-20]

There is a guy who put bigger pistons on the caliper of the Viper.
We share the same rear caliper.

HERE

[colgy]

How about using your original front 2-pots on the back? The discs are exactly the same so I can't imagine it would be too difficult to fabricate some sort of adaptor.

Would need the handbrake to work somehow if using the fronts

D'oh! Forgot about that

[Stu-7]

Has anyone else not considered doing this upgrade? Can't believe I'm the only one. I don't think a brake bias valve will be effective as the rear caliper simply isn't powerful enough. Any thoughts?

[siztenboots]

Has anyone else not considered doing this upgrade? Can't believe I'm the only one.

I don't think a brake bias valve will be effective as the rear caliper simply isn't powerful enough.

Any thoughts?

Planning ahead for future upgrades, I was thinking about

FRONT: TD PR1.2 16" R888 , lightweight possibly 4 stud hubs ali belled two piece brakes and curved vaned plain oversized ~320mm rotors, Hispec ultralite 4 calipers and RS14 pads , adjustable ARB
REAR: TD PR1.2 17" R888 , as above

1 way Nitrons to compliment the higher grip from tyre / wheel combo ,

Stage 2 custom lightweight exhaust full system 3" straight thru , de catted , with cat MIL eliminator

Lots of reasons to move to 4 stud, rather than stay on 5 stud

Edited by siztenboots, 26 February 2007 - 05:47 PM.

[Thorney]

I've been researching this for gawd knows how long and there is an alternaitve out there, I just need to test it. One thing though, what suspension are you running?

[Stu-7]

I've been researching this for gawd knows how long and there is an alternaitve out there, I just need to test it. One thing though, what suspension are you running?

Hi John,
Your post is very timely. It's the 3-way Nitrons i'm running.
In fact, I'm thinking of going 100lbs stiffer on the spring rates, See this thread HERE to reduce some of the roll. Thoughts?

Re: Hispec rear calipers, not worked out yet whether I would need larger discs to go with the 2-pots....I guess so (?)
re: brake bias valve, I guess on my current setup, all this would do is reduce overall braking force, when what I want to do is retain fron braking power and increase on the rear.

Edited by Stu-7, 26 February 2007 - 08:06 PM.

[vxr36]

I looked at bigger rears. There is an option that is quite pricy. Speak to Plans about the ultimate upgrade. They use a Ferrari rear caliper. It was too much for me... I don't track the car. There is little load at the rear when braking heavily inertia pushes the car forwards and onto the fronts so the rears can be smaller. A bias valve might solve your problems. R1CHY knows better, I am sure, but in the dry under heavy braking the rear brakes of a bike are next to useless as there is no weight at all on the rear tyre.... ON R1CHY's it is probably 30cm in the air!

Edited by vxr36, 26 February 2007 - 08:37 PM.

[meldert]

Had a great day at Bedford yesterday. My mate who was circulating with me in his S1 Honda, said my car looked very unsettled and squirmy under heavy braking, such as into the first corner of the lap.

If your car seems squirmy under braking it sounds more like you are locking up the rears... What tyres are you using front and rear?

[barrybethel]

I agree with Meldert. The standard kit should be ok for balance, if it is all working properly. If you want to upgrade to get more overall power and/or reduce fade etc then cool, but if you have a balance problem I would check tyre pressures, pad condition, disc condition and all that sort of sh*t first. Bearing in mind that the OE set up is a compromise so far as balance is concerned, obviously. In the wet if you could adjust it you would put more to the back and so on.

[Stu-7]

Had a great day at Bedford yesterday. My mate who was circulating with me in his S1 Honda, said my car looked very unsettled and squirmy under heavy braking, such as into the first corner of the lap.

If your car seems squirmy under braking it sounds more like you are locking up the rears... What tyres are you using front and rear?

I'm wearing Toyo R888's FR and rear. The rears aren't locking I don't think but, I can see why you take that PoV!
Depending on what data you want to look at there's a school of thought which says after upgrading the front brakes, the rears are not worked as hard and the OTHER school of thought says in upgrading the fronts, you are adjusting the OEM balance and favouring more front bias so, in order to restore the balance, you need to upgrade the rears.

Delving a little deeper, here's what I've gleaned:

In order to ensure vehicle stability under heavy braking, the rear brakes must not lock before the front brakes - correct? OK so...the lock-up point of the rear brakes is dependant on the friction of the tyre combined with the weight acting on the tyre ergo: the less weight you have acting on the tyre, the less braking torque is required to induce the locking point.

When you upgrade the front brakes, you are effectively generating more torque for the same pressure, everything else being equal ie: the overall brake bias moves more towards the front.
There is a school of thought that says the locking point of the rear tyres is effectively lowered as the front is retarding more energy from the car, taking weight off the rear ergo, no need to upgrade the rears.

HOWEVER...there's a school of thought which says in order to re-address the balance, you need to upgrade the rears. This exerpt is taken from Stoptech's website:

....they end up actually increasing the FRONT bias. How? By increasing the effective caliper piston area and the rotor effective radius, these two factors work together to increase the 'mechanical gain' of the front brakes, building more torque for the same pressure, everything else being equal. So, from a bias perspective we are not pushing the vehicle toward instability, but rather just the opposite - we are underbraking the rear axle! The obvious impact would be an increase in stopping distance - probably the one thing the new owner was actually hoping to reduce. Ironic. So, say you chose to install these big brakes on the front axle but want to maintain the OEM bias. What's the answer? Well, one way would be to invest in big rear brakes too which increase the rear mechanical gain to the point that the system is balanced once again.


Stoptech then go full-circle and say there's actually no need to upgrade the rears at all!

Finally, under an OEM bias condition, the rear brakes only contribute about 15-20% of all the braking force the vehicle generates, and when you install sticky tires you actually DECREASE the amount of work they need to do. Why? Because at the higher deceleration levels afforded by race tires, there is more weight transfer taking place, reducing the normal force on the rear tires and increasing it on the front (remember F=µN from above?). If anything, we now want to decrease the rear effectiveness. Ironic once again.

Well, either way, I can firmly say I am completely confused

What I can't ignore though is the FACT that my VX (with AP 4 pots on the front) under heavy heavy braking, tends to weave a bit at the rear. How is this so?

Thinking this through; this *weaving* may be completely unrelated to the braking force at the rear.

I am wondering, given that I am thinking about increasing the spring rates on the car < SEE HERE > that the relative (by that I mean on-track standards) softness of my spring rates is causing excessive dive under heavy braking on track. This diving takes even more weight off the rear and really feels unsettling. I think it's time to look into this further

[fluffy]

ok the rear of the car is squirming, that shows that the rear is light. Don't forget there is a pic on here somewhere of a vx with just the fronts touching the ground under heavy braking How about going for a thorney wing to add some downforce to the rear. That should stop it squirming? Alternatively carry something heavy like a couple of paving slabs in the boot. That should also aid in the downforce. I'll get my coat...

[meldert]

What tyre widths do you have? Original we have 225 at the rear and 175 at the front the brake balance with the original tyres and brake calipers is probably biased to the front so they will lock up first. This don't need to be the truth though since the car has an ABS to take care of the situation that ANY wheel locks up. On some new AUDIs for example the brakes are set up with a huge bias to the rear which makes the car extremely unstable when braking if you disable the ABS (by pulling the ABS fuse out for example) I have seen an AUDI doing a 180 spin on track at the end of the main straigt because of this... In our case we can be pretty shure that the brake have a good balance from the start without the ABS since they share this brake system with elises without an ABS. It is probably set up with a bit of extra bias to the front for extra safty, it is much better if the car locks up the front and by that brakes in a straight line, than if it locks up the rears and starts squirming and maybe even spin if you as a driver don't react accordingly. Well, lets modify the car a bit then: We start by adding 195 fronts keeping the width at 225 at the rear. The type of the tyre don't matter as long as they are the same all around. This will give us more grip at the front (good for removing some of that understeer) but it also increase the grip at front when braking, this will cause the fronts to lock up later than with the original set up. But i still think that they will lock up before the rears (I have this set up and that is my experience). Adding to the equation is also your GEO set up, if you have change to R888 you have probably had you GEO done increasing the camber at the front which is good for grip in the corner but not good for the grip in a straight line braking situation. This will ofcourse affect the brake bias too but I'm not sure by how much. If you have 205 up front and 225 at the rear we are getting to a situation where te car might get oversteered when cornering (if you dont get a stiffer ARB up front) and during braking you MIGHT lock up the rear first since you now have even more grip up front (a stiffer ARB will NOT help preventing this ofcourse). Everything depends on what margins we have on the brake balance from the start. Now, one way of changing this balance is to upgrade the brakes up front so that they get more of the brake force and therefor will lock up the fronts earlier again (even tough we have more grip at the front now). Doing this should settle the car down a bit again. Upgrading the rear brakes in a way that it shifts the brake balance to the rears will make that car stop at a shorter distance if you get the balance just right, but the car will feel a bit more unstable and if you shift the balance too much to the rear you might spin when braking hard (unless the ABS kicks in and thake care of the situation, but the car will feel squirmish before the ABS kicks in). This is in line what Stoptech tells you. But the numbers they are talking about (the rears have 15-20% of the braking force) is probably for a front engine car which at standstil have a wieght biased to the front and terfore more force on the front wheels... A car with a weight balance as ours ca 40% up front and 60% at the rears at stand still will benefit from this under braking. The weight will shift forward but it wont be as bad as on a front engined car. I have seen figures on a porsche 997 that says that it has almost a 50-50 weight distribution during braking, this means that it can have a lot of the brake force at the back without upsetting the balance, this ist great for shortening the stopping distance since it use more tyre area to stop the car. We have almost the same weight distribution at stand still as the porsche 997 so we should also be able to have quite a bit of braking force at the rear. Well none of this really helps you to make up your mind about upgrading the rears. But look at it this way, if the car feels unstable during braking now, adding more stopping power to the rear will only make it worse. The question is, what do you think about the car when you are braking, does it feel unstable, it seems like you are basing this question on something you friend saw when he was following you on track... If the car don't feel unstable you can pretty much ignore his opinion... Al cars will have weight transfer during braking, maby it's just this i he saw. And as a last comment eliseparts who sells the elise version (radial mountin instead but also made by hispec) of the same rear calipers as you post in the beginning of this thread. Actually sells two versions of the caliper one with increased piston area to be used together with AP four pots up front. So I guess that tha standard Hispec is made to keep in balance with the original front calipers, so you probably wont gain much brakinf force att the rears by changing to them. the main advantage with them is the 3 kg(!) less unsprung weight per side at the rear...

Edited by meldert, 28 February 2007 - 06:05 PM.

[walkes]

Could this just be a driving style / changing down at the time of braking / Lowish tyre pressure?

[chris]

the main advantage with them is the 3 kg(!) less unsprung weight per side at the rear...

Could you clarify ?

I can't see, how you can save 3 kg per side, only by replacement of rear calipper, even with ultralight caliper, what is the weight of OEM rear calipper ?

Edited by chris, 02 March 2007 - 12:44 PM.

[2-20]

the main advantage with them is the 3 kg(!) less unsprung weight per side at the rear...

Could you clarify ?

I can't see, how you can save 3 kg per side, only by replacement of rear calipper, even with ultralight caliper, what is the weight of OEM rear calipper ?

4 kg according to this link
Here