Spring rate TVR Vixen
Discussion
Is there a simple way to calculate a rough base spring weight? Like taking the mass of the car (excluding unsprung weight for more accuracy) and dividing it by four (if using four shocks)?
Or is it not quite that simple,
, i.e. not simple at all,
Or is it not quite that simple,
- EDIT*
, i.e. not simple at all, Edited by Cerberus90 on Friday 21st December 12:30
I have a vixen 2500 and my car currently has 185lb springs on the front and two 90lb springs on each corner of the rear. These are paired with spax adjustable dampers. As a complete package I am happy with them although I do not think I would go any stiffer for road springs. My car had the original units on them when I received it and the original springs measured in at 140lbs each (only one for each corner). I picked the 185 front and two 90lb rear weights based on a bit of research I did on this site (as well as what was locally available in 12 inch lengths). I am far from an expert please consult others before you buy anything.
thegamekeeper said:
Look back at this topic on here and you will find if 100 people reply they will all give different figures. If you choose one of those "opinions" and they are not right then you will blame that person.
But isn't that what asking opinions is all about, from the answers received you may be able to make a more informed choice? Or, of course, you have the choice of totally ignoring it if it doesn't sound good or come from people who's opinions you value or respect.As for blaming someone for bad advice, I find that ridiculous. Forums are open and public places where anyone can have their say and the reader is totally responsible for what they do with that information.
If, for example, from those 100 replies the vast majority are suggesting around 180 - 220 lbs (and are satisfied with the handling of their car) that would be a good starting point, especially if that figure matches with suggestions from manufacturers and suppliers?
Cerberus90 said:
Is there a simple way to calculate a rough base spring weight? Like taking the mass of the car (excluding unsprung weight for more accuracy) and dividing it by four (if using four shocks)?
<..snip..>
YES !!!<..snip..>
Spring rate = (1406250 x d x d x d x d) / (N x D x D x D)
d = wire diameter in inches.
N = Number of active coils. The flattened coils at the ends are not considered active.
D = Mean diameter of spring. Half way between the inside & outside coil diameter.
Assuming it's standard spring steel and some other assumptions, but it's near enough.
Wire diameter is very important to get right as a small error here multiplies !
Yours look nice and clean though, normally I would say to wire brush first ...
Edited by RCK974X on Friday 21st December 21:23
RCK974X said:
YES !!!
Spring rate = (1406250 x d x d x d x d) / (N x D x D x D)
d = wire diameter in inches.
N = Number of active coils. The flattened coils at the ends are not considered active.
D = Mean diameter of spring. Half way between the inside & outside coil diameter.
Assuming it's standard spring steel and some other assumptions, but it's near enough.
Wire diameter is very important to get right as a small error here multiplies !
Yours look nice and clean though, normally I would say to wire brush first ...
Yes but that's just calculating the spring poundage not what's right for the car. Spring rate = (1406250 x d x d x d x d) / (N x D x D x D)
d = wire diameter in inches.
N = Number of active coils. The flattened coils at the ends are not considered active.
D = Mean diameter of spring. Half way between the inside & outside coil diameter.
Assuming it's standard spring steel and some other assumptions, but it's near enough.
Wire diameter is very important to get right as a small error here multiplies !
Yours look nice and clean though, normally I would say to wire brush first ...
Edited by RCK974X on Friday 21st December 21:23
I'm with Steve - everyone wants something different. I could tell you mine but its a race car and use is entirely different from road use.
Shocks have come a long way in the last 20 years but a spring is a spring.........If you really want to get an idea go to a shock absorber designer with overall weight, corner weights, unsprung weights, ride height, wheel rate, track, wheelbase and roll centres and he will be able to calculate bump amd rebound charateristics and spring rates.
Or buy some from Steve, who has more experience of the early cars than most of us put together
Oops, you're right - I misread the post....
I do remember reading an article somewhere which calculated an "oscillation rate" (natural frequency) in Hertz, and went on to say luxury cars have 1-2hz and sports cars should have 5-10 ? something like that anyway. It might have been a John Upham one from his racing Vixen stuff ?
As you say, preferences vary widely....
I do remember reading an article somewhere which calculated an "oscillation rate" (natural frequency) in Hertz, and went on to say luxury cars have 1-2hz and sports cars should have 5-10 ? something like that anyway. It might have been a John Upham one from his racing Vixen stuff ?
As you say, preferences vary widely....
I HAD saved that article from John Upham's Vixen web site (I think that's where it's from)
so I include it in case it's any use to you ...
(John Upham <j.e.upham@reading.ac.uk>)
Bob,
I'd be curious to know the spec of the springs. Are you running
two or four, what is their free length, poundage and i.d.?
A few years ago I actually spent a lot of time with computer simulations deciding on the spring requirements for my 1968 Vixen S2. By measurement, the standard springs are a pair of 125 lb/in at the front and a pair of 140lb/in springs at the rear (but 4 rear dampers). This results in a front wheel frequency of 1.06 Hz (64 CPM) and a rear wheel frequency of 1.42 Hz (85 CPM).
Both these values are more akin to a large softly sprung saloon than a sports car and the resulting excessive suspension travel probably accounts for the number of damaged dampers I have replaced over the years.
I have four 90 ft lb, 2.25 inch on my S3 but this set-up is too hard for road use with a GT6 diff.
My solution was to significantly increase the spring rates from standard, both to reduce the suspension travel to an acceptable ammount and to limit the body roll to 2.5 degrees to prevent excessive Roll Center movement. (The front is much worse than the rear).
At the rear I used 4 springs to avoid excessive stress on the bottom suspension tie bar.
Here is the spec:
Spring Rate Free Length Max Coil Bound Length
Front 300 lb/in 12.50 ins 7.25 ins (2 required)
Rear 140 lb/in 12.50 ins 7.25 ins (4 required ie 280 lb/in combined)
The resulting wheel frequencies are front 1.66 Hz (100 CPM) and rear 2 Hz (120 CPM) and actually gives quite a comfortable ride. I understand rear figures are similar to a Fiesta. When pressed to the limit the car does tend to slightly oversteer but this could be corrected by a stiffer front anti roll bar which currently is the standard bent piece of wire.
Are you running a Salisbury 4HU diff or the GT6 one as the weight difference will alter the ride height.
John Upham, 1970 Vixen S3
see http://www.scarlet.rdg.ac.uk/msport/bullnose_92_tv...
The standard GT6 diff was never up to a Vixens' performance, I think I broke 6 before I converted back to the Grantura / Vixen S1 setup using the BMC B series diff. It had the advantage of being very light (all alloy) and there were numerous ratios available but limited slips were excessivly expensive, although the excellent TVR rear suspension design seems to manage without the need for this.
Bob Brewer Milton Keynes, UK.
rwb@grantura.demon.co.uk (home)
bob.brewer@gecm.com (work)
so I include it in case it's any use to you ...
(John Upham <j.e.upham@reading.ac.uk>)
Bob,
I'd be curious to know the spec of the springs. Are you running
two or four, what is their free length, poundage and i.d.?
A few years ago I actually spent a lot of time with computer simulations deciding on the spring requirements for my 1968 Vixen S2. By measurement, the standard springs are a pair of 125 lb/in at the front and a pair of 140lb/in springs at the rear (but 4 rear dampers). This results in a front wheel frequency of 1.06 Hz (64 CPM) and a rear wheel frequency of 1.42 Hz (85 CPM).
Both these values are more akin to a large softly sprung saloon than a sports car and the resulting excessive suspension travel probably accounts for the number of damaged dampers I have replaced over the years.
I have four 90 ft lb, 2.25 inch on my S3 but this set-up is too hard for road use with a GT6 diff.
My solution was to significantly increase the spring rates from standard, both to reduce the suspension travel to an acceptable ammount and to limit the body roll to 2.5 degrees to prevent excessive Roll Center movement. (The front is much worse than the rear).
At the rear I used 4 springs to avoid excessive stress on the bottom suspension tie bar.
Here is the spec:
Spring Rate Free Length Max Coil Bound Length
Front 300 lb/in 12.50 ins 7.25 ins (2 required)
Rear 140 lb/in 12.50 ins 7.25 ins (4 required ie 280 lb/in combined)
The resulting wheel frequencies are front 1.66 Hz (100 CPM) and rear 2 Hz (120 CPM) and actually gives quite a comfortable ride. I understand rear figures are similar to a Fiesta. When pressed to the limit the car does tend to slightly oversteer but this could be corrected by a stiffer front anti roll bar which currently is the standard bent piece of wire.
Are you running a Salisbury 4HU diff or the GT6 one as the weight difference will alter the ride height.
John Upham, 1970 Vixen S3
see http://www.scarlet.rdg.ac.uk/msport/bullnose_92_tv...
The standard GT6 diff was never up to a Vixens' performance, I think I broke 6 before I converted back to the Grantura / Vixen S1 setup using the BMC B series diff. It had the advantage of being very light (all alloy) and there were numerous ratios available but limited slips were excessivly expensive, although the excellent TVR rear suspension design seems to manage without the need for this.
Bob Brewer Milton Keynes, UK.
rwb@grantura.demon.co.uk (home)
bob.brewer@gecm.com (work)
Adrian@ said:
What knowledge has the OP gained here... lots of people have heard me talk about getting rid of the '70's feel' when selling a set of shocks and springs to them (as Andy pointed out above this is the frequency relating the the '70 feel), I have corner weighting equipment and side stepped the the question as I have never set up a 2500 engined M Series chassis with a Vixen shell, it was always going to be an odd one.
Adrian@
Having the corner weights correct wont help a car that has incorrect springs. As stated above wheel frequency is what matters and that is a function primarily of unsprung weight wheel rate, overall weight and track/wheelbase. In most cases for a road car you cant effect corner weights anyway as you cant move components and the shocks wont have adjustable spring platforms.Adrian@
Perhaps the debate could be settled by setting the car empty, then again with someone in the driver's seat of average weight and measuring the change? I would set the final weights with half the driver's effective weight added to the passenger seat as standard. The corners would then always be out by half an average effective body weight whether carrying a passenger or not.
The mass change of fuel would not affect the corners as it would reasonably act across the whole axle, similarly with bags evenly spread across the back.
This area is easier to Google or find in text books by using 'diagonal weight jacking'. None of my books use the term corner weighting.
Colin.
The mass change of fuel would not affect the corners as it would reasonably act across the whole axle, similarly with bags evenly spread across the back.
This area is easier to Google or find in text books by using 'diagonal weight jacking'. None of my books use the term corner weighting.
Colin.
longone said:
Perhaps the debate could be settled by setting the car empty, then again with someone in the driver's seat of average weight and measuring the change? I would set the final weights with half the driver's effective weight added to the passenger seat as standard. The corners would then always be out by half an average effective body weight whether carrying a passenger or not.
The mass change of fuel would not affect the corners as it would reasonably act across the whole axle, similarly with bags evenly spread across the back.
This area is easier to Google or find in text books by using 'diagonal weight jacking'. None of my books use the term corner weighting.
Colin.
ColinThe mass change of fuel would not affect the corners as it would reasonably act across the whole axle, similarly with bags evenly spread across the back.
This area is easier to Google or find in text books by using 'diagonal weight jacking'. None of my books use the term corner weighting.
Colin.
If you think the change from weight of fuel only acts across an exle I would suggest you have never corner weighted a car. Its like balancing a 3 legged chair. The rear tank on my race Grantura is in the middle and runs equally either side of the centere line. It is balanced with me in it and about 25L of fuel - full of fuel the greatest impact is on the FRONT corners
As Steve says with your misses, granny or event he shopping in on a road car it all goes out the window. Adrian, a Vixen and an M series wont be that far apart as the basic chassis design is similar ie the rear roll centre is still higher than the front. OK on the road a bloody awful on the limit.
I included that piece for interest and background - I agree that in reality, everyone's preference is probably different, and comparing a Vixen tubalur spaceframe to a monocoque Fiesta is also a BIG stretch - the Fiesta is going to be FAR more rigid IMHO.
I wasn't trying to say "do this" or anything like that.
I put twin shocks on my S3 with the longer S2 bars, and had 80lbs springs made, and at the moment have original fronts.... may change fronts after finishing rebuild and driving it.
I wasn't trying to say "do this" or anything like that.
I put twin shocks on my S3 with the longer S2 bars, and had 80lbs springs made, and at the moment have original fronts.... may change fronts after finishing rebuild and driving it.
Edited by RCK974X on Thursday 27th December 22:43
Fiscracer said:
Colin
If you think the change from weight of fuel only acts across an exle I would suggest you have never corner weighted a car. Its like balancing a 3 legged chair. The rear tank on my race Grantura is in the middle and runs equally either side of the centere line. It is balanced with me in it and about 25L of fuel - full of fuel the greatest impact is on the FRONT corners
As Steve says with your misses, granny or event he shopping in on a road car it all goes out the window. Adrian, a Vixen and an M series wont be that far apart as the basic chassis design is similar ie the rear roll centre is still higher than the front. OK on the road a bloody awful on the limit.
If your tank is symetrical about the centreline of the car and you balanced the car without your weight accounted for, so an empty car, then filled the tank to any level you choose, what would happen to the balance? Would it be still be balanced, albeit with greater loads at each corner?If you think the change from weight of fuel only acts across an exle I would suggest you have never corner weighted a car. Its like balancing a 3 legged chair. The rear tank on my race Grantura is in the middle and runs equally either side of the centere line. It is balanced with me in it and about 25L of fuel - full of fuel the greatest impact is on the FRONT corners
As Steve says with your misses, granny or event he shopping in on a road car it all goes out the window. Adrian, a Vixen and an M series wont be that far apart as the basic chassis design is similar ie the rear roll centre is still higher than the front. OK on the road a bloody awful on the limit.
Colin.
longone said:
If your tank is symetrical about the centreline of the car and you balanced the car without your weight accounted for, so an empty car, then filled the tank to any level you choose, what would happen to the balance? Would it be still be balanced, albeit with greater loads at each corner?
Colin.
Is the tank not behind the centreline of the rear axle??, one would assume that the rears would increase and the fronts decrease??Colin.
Lets see who can display the formulae first
N
heightswitch said:
longone said:
If your tank is symetrical about the centreline of the car and you balanced the car without your weight accounted for, so an empty car, then filled the tank to any level you choose, what would happen to the balance? Would it be still be balanced, albeit with greater loads at each corner?
Colin.
Is the tank not behind the centreline of the rear axle??, one would assume that the rears would increase and the fronts decrease??Colin.
Lets see who can display the formulae first
N
longone said:
If your tank is symetrical about the centreline of the car and you balanced the car without your weight accounted for, so an empty car, then filled the tank to any level you choose, what would happen to the balance? Would it be still be balanced, albeit with greater loads at each corner?
Colin.
Why would you balance the car without the driver? I've not yet driven mine driver-less.Colin.
As I said its like balancing a chair with one short leg. Let me set a little question then. If the fronts are balanced across the axle by changing the ride height L/R and (and the rears are not, driver's side heavier) and you add say 50kg of fuel behind the rear axle what is the impact on front corner weights? Even increase? Even reduction? Uneven change - up or down?
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