degoetz Posted September 2, 2014 Share Posted September 2, 2014 I have a 2014 Stalker Mspec with a tire and wheel combination they have never used before. The car was built to compete in SCCA Solo in Emod class. I am using the same tire and wheel size as the current national champion and a lot of the top competitors use. Frt - 9.5"w x13x 20"tall, rear 14"w x13x 23" tall Goodyear slicks. Under med hard to hard braking I can lock up the frt brakes I think too easily. I am used to racing a modified C5 Vette with ABS. I have been in contact with Scoot at Brunton Auto and he runs equal sized race tires on his although he is building a car for himself will have my combination for future competition. I believe that the with added traction of the wider and possibly the taller rear tire could use more rear brake bias with the mismatched tire combination. I have been trying to find out if anyone has ever added an additional brake proportionig valve to the front wheels and if that would cause any problems with having two proportioning vales in a system. should I remove the rear valve before installing the frt one? At this point I am just going to drive the car more and see if I can get used to it. Link to comment Share on other sites More sharing options...
xcarguy Posted September 2, 2014 Share Posted September 2, 2014 Can you dial more bias to the rear with the current setup? Also, is one front tire locking up before the other? Link to comment Share on other sites More sharing options...
degoetz Posted September 2, 2014 Author Share Posted September 2, 2014 (edited) The current proportioning valve is in the rear brake line and I can only reduce the force to the rear brakes. In straight line braking both will lock up, in a turn normally the inside will lock first, which is normal. I would think that being 1/2 the weight of my C5 Corvette, the stalker would stop better but right now the vette will out brake it. Edited September 3, 2014 by degoetz Link to comment Share on other sites More sharing options...
rnr Posted September 2, 2014 Share Posted September 2, 2014 Wont increasing the rear braking force do the same thing? Or are you saying that your rear valve is full open? Link to comment Share on other sites More sharing options...
JBH Posted September 2, 2014 Share Posted September 2, 2014 Has the car been on the scales? Link to comment Share on other sites More sharing options...
xcarguy Posted September 2, 2014 Share Posted September 2, 2014 I"m assuming the proportioning valve is full open (max rear brake). On the brake pedal assembly itself, do you have separate front and rear master cylinders. If so, do you have an adjustment (similar to this: http://www471.pair.com/stalkerv/gallery2/main.php?g2_itemId=6813)? If so, can you adjust the 'bias only' to your liking and then use the proportioning valve to fine tune? Link to comment Share on other sites More sharing options...
degoetz Posted September 2, 2014 Author Share Posted September 2, 2014 (edited) There is only one master cylinder with two lines running off of it. The proportioning valve was and still is turned full clockwise which I was told is max to the rear. I have no instructions with the valve and will try to adjust it this weekend and see if it is set the right way. There is considerable more rotating mas in the much larger and heavier rear wheels and why wouldn't they require more braking force to them over the fronts from the stock equal tire set up? Edited September 2, 2014 by degoetz Link to comment Share on other sites More sharing options...
jimrankin Posted September 3, 2014 Share Posted September 3, 2014 Compared to stopping the cars weight the wheel/tire mass is minor, more important to going/cornering than stopping. Even with a 50/50 corner weight you still apply somewhere around 70% of the cars weight and brake force to the front wheels due to the weight transferring forward under braking. Do you have access to a good brake shop that can read the PSI going to the front and rear under full foot pressure? Might be a problem with the rear brakes getting too much of the pressure due to a master cylinder problem. A lot of standard automotive MC's were set up "dual diagonal" not "front/ rear" so the pressure is equal instead of front biased. Link to comment Share on other sites More sharing options...
FE07 Posted September 3, 2014 Share Posted September 3, 2014 Can't answer your question about the 2 proportioning valves. The best way to fix the problem would be to add another master cylinder (one for fronts and one for rears) and run a brake bias knob. Might give you the adjustability your looking for. In regards to the rotating mass- braking hard enough to lock up your fronts is shifting significant weight to the front of the car effectively unweighting the rears and negating the increased mass of the rear wheel/tire. Shifting braking force to the rear should help to prevent front lock up. Just don't go too overboard as too much rear brake bias will not feel very stable. Jim Link to comment Share on other sites More sharing options...
jimrankin Posted September 3, 2014 Share Posted September 3, 2014 Just read my own post and realized I miss stated the line about "the rear brakes getting too much" when I meant "the front brakes" getting too much pressure. Also ditto on FE07's post about too much rear bias, great way to practice getting into corners sideways unintentionally until you get that dialed out. Still haven't got mine exactly right but not the heart stopping experience it used to be ;-) Link to comment Share on other sites More sharing options...
xcarguy Posted September 3, 2014 Share Posted September 3, 2014 degoetz, Can you post a close up photo of your MC? Link to comment Share on other sites More sharing options...
degoetz Posted September 3, 2014 Author Share Posted September 3, 2014 http://i644.photobucket.com/albums/uu163/degoetz/007_zpsab1b326a.jpghttp://i644.photobucket.com/albums/uu163/degoetz/002_zpsc05bd688.jpg The front left line goes to the rear wheels, the rear right line goes to the front wheels Link to comment Share on other sites More sharing options...
James A Posted September 3, 2014 Share Posted September 3, 2014 Wow, a vacuum booster, what does this car weigh? Seems to me if you size the master cylinder correctly you can get rid of the vacuum booster, could this be the issue? Link to comment Share on other sites More sharing options...
xcarguy Posted September 3, 2014 Share Posted September 3, 2014 degoetz, I think the issue may very well lay with the different size front and rear tires (that's a stab at best). What tire pressures are you running fron and rear? Link to comment Share on other sites More sharing options...
Davemk1 Posted September 3, 2014 Share Posted September 3, 2014 Hey Don - At the risk of insulting the driver (whom I know BTW) I think the issue could come as much from technique as anything else. If you are getting even lock up in a straight line then the bias is certainly in the ballpark. What I've found happening with drivers that go from a car with ABS (your Vette has very good ABS) to a car without ABS is that the driver needs to adapt. If you go raging into a corner and simply panic brake with all your might you will lock up something and it will most likely be the fronts. IMO the reason for this is simple but can be hard to explain........ If you are rolling along and do a brake test (not in a solo run) I'll bet you lift from the gas and wait a half a second (causing forward weight shift) and then you apply the brake in less than a full on panic mode. If you hit the brake pedal a bit more slowly you will again cause a bit more weight shift to the front which in turn will give the front tires better bite which in turn will allow more brake to be fed in..........it's a positive feedback loop of sorts. It's the weight shift that is the key thing here. If on the other hand you go from full throttle (weight shifted to the rear) and then stomp on the brake pedal you can get the fronts to lock before the weight shift can happen (it can only happen so fast due to inertia and shock damping) and without the needed weight shift the fronts can lock. There are a few ways to improve this of course - one can give more bias to the rear, which is really just taking some out of the front unless the car is set up with a bias bar, so that even under full autocross panic brake the fronts won't be as likely to lock. This has some real downsides of course in that when you do less of a panic brake and more trail braking you are much more likely to lock the rears which will make the fronts locking seem wonderful. Picture braking mid sweeper in Helena and you get the idea. In the end I think it will just take you some time to get used to not having ABS and having to feed in the brake just a tad slower to get that weight shift to happen before really standing on the brakes and making the car feel like it has dropped anchor. You can also accomplish this with left foot braking. Does the car have a stick or auto trans? If it's an auto left footing will be easier to master and you can apply a bit of brake while still on the gas and this will start the weight shift.........then you drop the throttle and press harder on the brake and it will kick ass. I hope that helps. If you try it and have little success the best thing you can do is fit dual masters and a bias bar and you can really fine tune the system. I'll bet this is easier said than done though. dave Link to comment Share on other sites More sharing options...
degoetz Posted September 3, 2014 Author Share Posted September 3, 2014 Dave The even lockup is both fronts left and right not front to back. I agree completely that for me there is a bigger learning curve coming from one of the best ABS systems out there. But I still think the 50% lighter car should stop at least as good as the Vette. I am gradually applying more pressure at the pedal because the car is not stopping as well I think it should be. I am going to just leave it alone the rest of this year and try and adjust to it. I have also changed the rake from 1&1/4' to 3/4' to help with what I think might be under steer along with they are sending me 2 more sets of front springs 25 & 50 lbs lighter. Why don't you co-drive one day this weekend with me and show me how to drive one of these 7 series cars. Link to comment Share on other sites More sharing options...
Davemk1 Posted September 3, 2014 Share Posted September 3, 2014 Dave The even lockup is both fronts left and right not front to back. I agree completely that for me there is a bigger learning curve coming from one of the best ABS systems out there. But I still think the 50% lighter car should stop at least as good as the Vette. I am gradually applying more pressure at the pedal because the car is not stopping as well I think it should be. I am going to just leave it alone the rest of this year and try and adjust to it. I have also changed the rake from 1&1/4' to 3/4' to help with what I think might be under steer along with they are sending me 2 more sets of front springs 25 & 50 lbs lighter. Why don't you co-drive one day this weekend with me and show me how to drive one of these 7 series cars. Hey Don - I don't understand what you mean by this - "The even lockup is both fronts left and right not front to back." The physics of this gets really interesting. A light car won't necessarily stop any quicker than a heavier car. I can hear the 'boo - hiss!" out there but there is a good physics thing going on here. The amount of braking force a tire can generate is based largely on how much weigh is pushing down on it. So a heavier car will generate more grip than a lighter car oddly enough. At the same time a heavier car requires more grip to slow it down and the two can come pretty close to washing out in the end. So just because your new car is lighter than the old does not mean it will stop shorter. I know that seems counter intuitive but it's the way it works. The way to make a car stop shorter it to give it enough braking power (pads to rotors) and optimize the set up so that under hard braking all four tires are doing the same amount of work and use the stickest tires you can find.......easier said than done I must say. But that is the goal. One issue you could have is cool mornings and very wide tires on a light car - you'll never get them warm. If the fronts are cold (the rears will warm with driving force) then all bets are off. Take tire temps and I'll bet the fronts will be shocking cool compared to the Vette which would warm them all up. From my experience I seriously doubt you would ever be able to feel a 25 or even 50 lbs lighter spring. The difference is the amount the front end will sink is VERY small (is they are 500 lbs now and they move 2" under hard braking then moving to 450lbs springs would allow the car to about 1/4" more). You'll get much more change by playing with the damping than you will the spring rate for gross adjustments like this. It would be great if you have 2 way adjustable shocks so that you could decrease the rebound damping in the rear (allowing it to rise more) while at the same time decreasing the compression damping in the front to allow it to sink more. This would allow more weight shift. Thanks for the offer on a co-drive. I'm sure it would be fun. But I never drive anyone else's car unless I can afford to buy it if i wad it up........and I can't afford your car. Give it time and learn the car before you go making changes......even if it goes very well it will take a ver years to get used to the car and make changes to the set up so that the two of you mesh. it just takes time and the more runs you can get the better. Good luck with it. dave Link to comment Share on other sites More sharing options...
degoetz Posted September 3, 2014 Author Share Posted September 3, 2014 If you are getting even lock up in a straight line then the bias is certainly in the ballpark. dave I am getting even lock up in the two front, not the front and back. The rear wheels never lock up. I don't know why this would indicate to you that the frt to rear bias is ok. Link to comment Share on other sites More sharing options...
Davemk1 Posted September 4, 2014 Share Posted September 4, 2014 I am getting even lock up in the two front, not the front and back. The rear wheels never lock up. I don't know why this would indicate to you that the frt to rear bias is ok. I'm sorry - I thought you posted that in a straight line brake test that you got pretty even lock up. Ideally one would start with too much front bias and front lock up and then adjust to the rear until you get too much in the rear and then move back toward the front some so that you get all 4 wheels doing the most work they can. I think it's a bit easier to have a small amount of lock up in the front than having any in the rear as it's easier to deal with. Dave Link to comment Share on other sites More sharing options...
xcarguy Posted September 4, 2014 Share Posted September 4, 2014 (edited) Hey Don - I don't understand what you mean by this - "The even lockup is both fronts left and right not front to back." dave Dave, Under hard braking, the front tires are locking up while the rears are not. While I agree that the bias is favoring the front wheels, I wouldn't necessarily call that being in the ballpark without some hands-on experience behind the wheel. I also agree that a driver has to learn a new car and that the new car has to be dialed in. However, I don’t think that’s the issue here as Don’s experience, and posts, would suggest he’s fully aware of a necessary learning curve as well as a need to get the car dialed in . . . . Hence one reason for this thread. What I'm getting from Don in his posts is that, based on his level (years) of driving experience, the car simply feels unbalanced in the area of aggressive braking. And it may very well be unbalanced; the car is running a radically staggered wheel/tire setup with the contact patch between the tire and the track definitely favoring the rear tire, and, power assisted brakes are being used to stop a car that weighs less than half of what the power assist was originally designed to stop. I think Don is looking for ideas, suggestions, etc. for getting the rear brakes a bit more engaged in braking. One of the problems he’s up against is that the proportioning valve for the rear brakes is ‘full open’ with no more room for adjustment, thus the reason for tossing the idea on the table of using a proportioning valve on the front brakes in his opening post. Don, Having said all this, what type of brake pads are you running on the front and rear? And tire pressures? Edited September 4, 2014 by xcarguy Link to comment Share on other sites More sharing options...
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