Minimum front ride height

[smsazzy]

My ride height is about 1.75 inches and I have not hit the ground under braking. I use the same spring pack you do, and my shocks can be moved by hand, so it is not that. The only explanation then is the angle of my turkey legs compared to yours.

[fvracer27]

My ride height is about 1.75 inches and I have not hit the ground under braking. I use the same spring pack you do, and my shocks can be moved by hand, so it is not that. The only explanation then is the angle of my turkey legs compared to yours.

Or you don't use the brakes as much

Do Vees need brakes?

[smsazzy]

My ride height is about 1.75 inches and I have not hit the ground under braking. I use the same spring pack you do, and my shocks can be moved by hand, so it is not that. The only explanation then is the angle of my turkey legs compared to yours.

Or you don't use the brakes as much

Do Vees need brakes?

Hence the topic was centered around a 1G braking assumption. Most anyone achieves that. Although I would submit that if my model is true, no matter how hard I brake, as long as the front is doing a significant amount of that work, it still won't hit.

Based on your model, a 1.75 ride height should hit with about 70 pounds of weight transfer. That's braking at what? .25G?

[Mystique Racing]

I think some of the math is off a bit on a few of the previous posts. Wheel rates are about 30lbs per side using a full spring pack which would be 60 lbs total for the front end.

Anyway, I also run my frame 1.75" above the ground in the front. Occasionally the frame will hit the rub strips in the cork screw at Laguna Seca and a couple of other places on very bumpy tracks. Overall it is not a problem. I have included a graph showing my actual front wheel movement under braking which is being measured with my front shock pots. Of course I have to convert the shock movement into wheel movement by creating a math channel which multiples the shock movement x the motion ratio. As you can see the front end gets very close to the ground under braking. The red strip is longitudinal G's

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[BLS]

Scott,

Thanks for the graph. With your frame at 1.75 inches above the ground, are your trailing arms level, or somewhere else?

Regards,
Barry

[tiagosantos]

Scott, thanks for pointing out the spring rate error! Oops..

Thanks for posting the data as well, that's awesome! Like BLS asked, I'd love to know the turkey leg angle on your car.. Should help me figure out if any of my understanding of the forces is correct, based on your data

[Mystique Racing]

I have raised the beam in my car in relation to the frame so I can run the car as low as possible with the trailing arms being level or slightly higher (front of the trailing arm) then level.

[tiagosantos]

Well.. Then if my original assumption of a 20in COG height was right, the weight transfer would be around 296lbs.. On your data, looks like you had 1.75in on one side and 1.6 on the other with 1.2G of longitudinal force. I'll average it to 1.675.. With a wheel rate of 60lbs, 1.675" of travel = 100.5lbs. At my theoretical 6" long turkey leg (lol..) and starting with a level turkey leg.. At 1.675" of travel, the anti-dive force would be.. 1.2G * 1025 * 0.6 (brake bias to the front.. could be higher?) * 1.675 / 5.75 = 215lbs.. + 100.5 from the spring, 315.5lbs.. That's not terribly far off! It could be an awesome coincidence though


When I have some more time, I'll do my funny math on some other parts of the graph, pick different braking forces, see if it still makes sense..

[Mystique Racing]

I am suspicious of your anti dive number. If the front brakes are doing lets say 60% of the work and the rear is doing 40% wouldn't the anti dive number be 1025 x .6 or 615lbs?

[tiagosantos]

That would the the entire force on the front wheels.. Well, for a 1.2G braking force, it would be 1.2 * 1025 * 0.6 = 738lbs. That force is pulling back parallel to the ground. Which is why the angle of the turkey leg is important - if the turkey leg is parallel to the ground, there is no anti-dive.. At 1" of travel, it means the beam is now 1" lower than the link pin end of the turkey leg. If the leg was 6" long, it is now cos(10deg) * original length, so 0.984 * 6 = 5.9in long (I worked out beforehand that 1" of travel is very close to 10degrees..)

Anyway, so the anti dive force would be the 738lbs multiplied by the leverage of the turkey leg.. Which is 1 / 5.9. So 738 * 0.169 = 125lbs. That's at 1" of travel.. And I believe this gets added to the spring rate at the same amount of travel. Wherever they add up to the added weight from weight transfer, the dive stops!

Again, these are all very broad generalizations and assumptions.. The 6" length for the turkey leg is completely from memory, it could be 10 or it could be 3. It would make a huge difference in the math! But I am very surprised that it somehow added up to being that close to your data.

[Mystique Racing]

I'm not an engineer and I may be out of my mind here but I still don't think the front end sees 100% of the decelerating force? Isn't the rear also absorbing some of that force?

[tiagosantos]

Yeah.. Which is why I'm multiplying by .6 (60%). If the car is 1025lbs and there's 1G of braking forces, that means there's 1025lbs being applied by the ground to the 4 tires. If the brake bias is 60% to the front, that hopefully means that the front tires are getting 60% of the braking force - so 1025 * .6 = 615, like you said before. If the braking force is 1.2G, that's 1230lbs of force (1.2G * 1025), 60% of that = 738lbs.

[Mystique Racing]

OK, missed that in your calculations.

Sorry,

[FV80]

An interesting thread for sure - my 'seat of the pants + finger out the window (CHANGING the brakes shoes)' meter says that at LEAST 70% of the braking comes from the front wheels...maybe even 80%. I do recall seeing some rather dramatic front end DIVES from cars in front of me at the Runoffs (I don't know HOW they got in front of me, but they apparently did ). It was mostly the Vortech's that stick out in my mind... possibly because there were about 4 of them around me most of the race.... and it was mostly at T6 - top of the hill coming out of T5. Again - just because it's such a good VIEW when you are about a car length back at that point. Stephen drives a Vortech and it certainly seemed to me that more than 1.75" of DIVE was in effect. Possibly they were RAISED just before braking from all that powerful TORQUE coming out of our MAMMOTH ENGINES!!

Steve, FV80

[tiagosantos]

Steve, the guys that got ahead of you were certainly mistaken, that should never have happened

Well my "theory" does not preclude cars from diving more than 1.75", it really depends on where they start from.. If the turkey legs are parallel to begin with, there will be dive, and quite a bit of it! If they're angled at normal ride height, then the anti-dive will work sooner and the car won't move as much. I believe the vortechs allow you a certain range of adjustment.

Oh and yes, having them angled would not only give you anti-dive under braking, but also pro-lift under power! This isn't a big deal for me, I can't seem to get more than 0.3-0.35G of acceleration forces out of my engine, but you guys with big power might have a problem

edit: as far as the braking bias goes.. I sure would love to see some data from brake pressure sensors, but man are those things expensive or what?

[problemchild]

Unless you factor in the front droop limiter amount, visually comparing dive under braking is not representative of anything.

A car whose front shocks length limit the car to static right height will have far less movement than a car whose front shock length allows the front to rise 1/2" above static ride height ..... regardless of arm angle, spring rate, shock valving, or whatever.

Much like jumping on a front end in the paddock to evaluate stiffness, the front shock length can mislead many people. Many people will think a car with zero-droop limiters is sprung stiffly, whether it is or not. They interpret "hard to move" and "minimal movement" as being sprung stiff rather than droop limited.

[jpetillo]

John, are you saying that under braking, the contact patch is basically trying to stop rotation of the wheel, which "tries" to spin the whole spindle/link pin carrier assembly..? I think that's what you mean by torque at the turkey legs. Sorry, I'm good at math, but never studied physics or mechanics in English, so I get lost in terminology very easily!

If that is what you mean.. Even if my COG height is way off.. A COG at 15 inches would still mean about 220 extra lbs at the nose under braking.. Considering our very low spring rates, even with the rising rate from the turkey leg angle going past horizontal - if the torque cancels out the anti-dive forces, what else is stopping the nose from smashing into the ground? That is a genuine question, by the way Is it the shocks? I am completely honest when I say that I don't fully understand what shocks do. I know what the idea is, but.. I just don't understand them. Gonna do some reading today!

edit: re-read your post and noticed that you had sort of answered my question. I need to go look at my car and take some measurements, how long are the turkey legs anyway? Damn garage is way too dark and cold, I've been staying away from that place for a while.

Tiagos, Sorry - I posted a few days ago and then went out of town.

Yes, I am saying that under braking, the contact patch is basically trying to stop rotation of the wheel, which "tries" to spin the whole spindle/link pin carrier assembly. This is in addition to all the other forces mentioned. The shocks do slow or delay this effect, but it will not prevent it for a long enough braking time, where he shocks will eventually provide no force when the motion stops. John

John

[jpetillo]

The act of the tire under braking is pulling the tire backwards in relation to the direction of trasvel of the car. That is what is pushing the nose back up in the air. At 1025 pounds and 1G braking, you have 1025 pounds of force pulling backwards on that lever. Figure out the ratio and you have the exact angle at which the two equal out. The weight pushing down is only 200 some-odd pounds based on the previous post, so it does not have to be at 45 degrees to win out.

I'm sure someone has already mentioned that the whole front end doesn't get the 1025 lbs of braking force under 1G braking. But you are also still leaving out some forces. I think I need to read on before commenting more. Sorry... John

[jpetillo]

Okay, I read through these. Nice post, but I think the numbers flying around need to be tightened up.

Tiagos, the CG is more like 10" or so. Also, the front spring rate (both fronts combined) is about 83 lb/in for a stock spring pack when the torsion arms are parallel. The first number is an estimated, the second was measured. With the 10" CG height, you're looking at more like 125 lbs downforce from braking, and about 1.5" of front end dive will take care of that with stock springs. So, ignoring any other forces (anti-dive or braking torque), that's an estimate for ride height.

Let me get all the numbers together, put all the forces in, and then post that. I'll put in Steven's anti dive and the braking torque as well. John

[jpetillo]

jstoezel & Tiagos

I did the calculation of the ride height change vs. braking G's. This included the complete double-torsion bar front suspension, the spindles & the frame, but assumed that the rear suspension did not move - was rigid. (I was being a bit lazy - sorry.) The result will include the anti-dive aspects that Steven was mentioning and the additional downward force from the front tire contact patch's braking torque. Friction in the joints and members bending was not included.

This also assumed that the front end had a stock spring pack and some figures that different folks gathered together for geometry and chassis F/R weight bias and estimated COM height. So I used of 10" for a COM height and a location back from the front contact patch of 49". The location of the COM between cars differs greatly. Numbers like 49-54" back from the front are typical. Also, the beam starts out at zero degrees (parallel to the ground) for static weight with driver (at 0 G's braking). This does not include inertial effects, so the numbers below is what you'd get after all has settled, but you might hit the brakes hard and overshoot the numbers below before it settles. I also did not include tire squish.

With all this you get the following:

1.0G braking: 0.72" of dive (1.50" if you just added weight onto the front axle)

This shows the effect of what Steven was suggesting for anti-dive. Note that the location of your COM significantly changes these values.

Let me know if it makes sense. I checked the calculation and it seems to pass all the tests I threw at it. So, it should be correct. John

EDIT: I just remembered that I still need to correct it for sprung vs, unsprung weight components. That will only reduce these dive numbers.

[tiagosantos]

John, thanks for the post. I'll try to do the calculations I was doing before with your numbers for CoG height and distance, and using your measured spring rate as well.. I don't think my reasoning was too far off, but yes, I was making some pretty wild assumptions I'm all setup to measure my car's CoG height and weight distribution - well, other than the car being in pieces and the engine in a crate!

Interesting that you measured the spring rate at 83lbs/in combined. I believe (I've never actually checked!) my front spring is missing one 'leaf', so I should really measure my spring to be sure.

Thanks again for posting!

[jpetillo]

Tiago,

It would be great to compare with your calculations. It sounds like the CoG calculation is going to be a small challenge.

About the measured spring rate of 83 lb/in combined, is this number different from what you measured or expected? John

[Mystique Racing]

Tiago,

About the measured spring rate of 83 lb/in combined, is this number different from what you measured or expected? John

Based on my experience I think this value of 83lbs of total front wheel rate is pretty close.

My M2 had a Ron Chuck front beam with the shortened springs, which are obviously stiffer, and it measured right at 100lbs of total front wheel rate.

[jpetillo]

I have a shortened one as well, and it measures 100 lb/in. John

[tiagosantos]

I haven't measured my front spring at all, was going by the numbers that I had seen thrown around here on the forum. I believe my front spring has been modified by removing one of the "blades", making it softer. I'll have to measure mine, another winter project