Has anyone ever measured their COG height with a view to doing a brake force distribution calculation?
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Has anyone ever measured their COG height with a view to doing a brake force distribution calculation?
Hi Keith, an intriguing question! Is this something that you calculate theoretically or physically?
Cheers Steve
Hello Steve,
I was thinking physically. It involves lifting one end of the car up and measuring how much weight transfers to the other axle. So not simple to do. Need weighpads and some equipment and strategy for lifting the car safely. And a finished car of course. I've never attempted it myself but the information would enable anyone to calculate the weight transfer due to braking and therefore accurately predict the likelihood of rear lock with a particular brake configuration.
Cheers,
Keith
Keith, I suspect that nothing like this has been done before, and with your particular background and skill set, I suspect that you are likely to be the one person who can enlighten us to what your question asks?
I for one will be intrigued to know.
Robin.
Hi
going by the fact that the car is about 44” off the ground, half that is 22”
the main mass is the engine, and then driver that is lower than the chassis Centre,
the CoG should be at about 16-18” off the road.
Ian
For those that really want to know How To Calculate Centre of Gravity Position – How To Adjust And Tune Suspension Secrets
I have looked online for some COG height values and found a quoted value for a Cayman (20", 508mm) and an Elise (470mm). I think the Elise would be lower than my Hawk (smaller, lower engine) so I have started with 20" and see how it goes with the calculation. I also found some information on kerb and kerb + driver weights on the Forum (courtesy StruanR, thank you) and I've used these with some adjustment to make a first attempt at a brake force distribution calculation. And if I've uploaded it correctly the results should appear next.
Attachment 22261
Many of you will have seen this kind of thing before. I have used a simple spreadsheet program from an old friend and brake engineer but I'm sure something similar must be available on the internet.
For those who are not so familiar with the brake force distribution diagram, a short explanation.
The blue line is the ideal brake force distribution characteristic where the brake force on front and rear axles is exactly matched to the vertical load on the axles and the available tyre/road friction. It is derived by calculating the vertical load due to initial static loading plus the load transfer due to deceleration and factoring that with the road mu value. It is important because if the actual installed brake force distribution of the vehicle always remains below this line the front wheels will always lock first on any homogeneous road surface from polished ice to dry tarmac.
The orange line represents the "optimum" vehicle installed brake force distribution to give 1g deceleration on a road mu of 1.0. In practice I wouldn't want to match this line because it allows no room for variation in brake friction (between the pad and the disc) which will occur and could result in rear lock but it gives a limit to the braking ratio between front and rear brakes I might want to consider.
The purple line is the actual installed brake force distribution I would get if I were to carry over the concept of the donor car brakes (Brembo 4-pot 42/38 fronts and 38mm rear pin sliders). It is a long way from the optimum, based as it is on a completely different vehicle layout. Its legal but the maximum deceleration is not going to be great.
So I want to have a look at rear calipers with bigger piston areas.
I found another piece of interesting information on the Forum, this time from Ken Tomblin where he stated the AP Works Gp4 Brake spec (Thanks Ken). That data gives the following result.
Attachment 22262
I do not know the actual weight distribution for a Gp4 car and i don't know the actuation spec (which would have incorporated adjustable brake balance) so the result has to be viewed with caution but nevertheless its interesting. The Gp4 spec represents a Braking ratio of 57.6% Front. My "Optimum" line is 51.8% Front and the "carry-over" spec is 68.9% Front.
So I am still asking myself how close I can safely go to the Optimum spec. This analysis is for straight line braking only. Its not for corner braking. If I have too much rear brake will that upset the cornering stability under braking? I don't know. But I can't be the first to have faced this challenge and plenty of you must have already found a solution you're happy with. Are any of you able to share your brake specification with me in order to help me decide mine? I could even collect your responses together in a data file for reference by future builders, if that hasn't already been done.
Cheers,
Keith
Hi
When I upgraded my brakes on my Hawk I went on the fact that the weight distribution front to rear is 40/60 just like a F1 car.
This give 55/45 brake forces so I fitted equal size and power brakes. For the balancing I only needed to put in a adjustable limiting valve to the rear.
This setup works very well and tested on the dirt and track, and easy to do on any Stratos.
For and ideal setup the rears need to grab first to reduce weight transfer from the rear, but with equal force which is hard to do.
Ian
Thanks Ian,Quote:
Originally Posted by Iowstratos
That's just the kind of info I was looking for. Can I ask what caliper piston size you ended up with and what master cylinder set-up you use?
Cheers, Keith
Hi Keith
I went for HiSpec 285 Billet 4 brakes with the standard Fiat 131 Hawk setup. The rear calliper has a built in handbrake.
Ian
Mine is standard Alfa 156 2.5, with a Willwood brake bias adjuster, wound all the way to the back & locked off for the IVA , braking is very good... even when I deliberately ask too much!
Not too scary so far...
Thanks Tim,Quote:
Originally Posted by westonTB
I think I'm probably going to start with the carry-over set-up and look at a rear brake upgrade post IVA. I think in that case a bias adjuster (to reduce pressure to the rear brakes) shouldn't be necessary. If you had yours adjusted for full rear pressure that makes sense.
Cheers,
Keith
Thank Ian, I'll have a look at HiSpec.Quote:
Originally Posted by Iowstratos
Cheers, Keith
Hi Keith
HiSpec should still have all the information for the mounting and disc bells on file.
Ian
John Cross Super Retro Rally X Stratos (Litton Corse I) uses same Wilwood forged Superlite 2 calipers and Disc O/D all around with 0.700" masters Ft / Rr,
adjustment on balance bar, + quick adjust lever type bias on rear line.
John states the Stratos type car with its weight distribution requires more rear brake bias, being raced on mix track surface conditions he would certainly know.
This setup also simplifies spare parts with commonality.
As previously mentioned it is probably very close to the 55/45 ratio split
From another source, Hawk estimated C of G 350mm...
Thanks for the information and advice. A 55/45 split looks like a reasonable target and achieving that with similar brake spec front and rear plus some form of adjustment (balance bar, valve) for fine tuning makes sense.
Coming back to the original question about Centre of Gravity Height the calculations show that whilst it has an effect (the lower the CoG the less the weight transfer and the more rear brake bias required), its not as critical as i had thought it would be.
The results show that the "Optimum" braking ratio changes from 52.8%/47.2% (Front/Rear) at 550mm to 48.2%/51.8% at 350mm. The "Optimum" allows no tolerance for brake performance variation so adding in some margin for that, 55%/45% seems sensible.
But, I shall build my car with the brakes I have and look at an upgrade for the rears later, after I've had a chance to drive it.
Happy Christmas,
Keith
I read the theory and books some time ago, and as far as my understanding goes, I thought that the potential for manipulating the centre of gravity height in the restricted envelope of the Stratos chassis and body that most of us have to work with has got to be pretty limited.
Sean probably has the most potential with his one off chassis.
What would you actually have to shift around in the chassis to achieve a 200mm difference in c.o.g height?
Hi John
I think to get C.O.G. Of 200mm i think you need to have very heavy balls as that will be the lowest part of the car!
Happy Christmas and good driving in the new year.
Ian
I believe you may be right Ian!
All the best to you and everyone here too!