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MG MGA Let's have a spring and check strap discussion BBS discussion at MG-Cars.net
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MG MGA - Let's have a spring and check strap discussion
| So we all know it's impossible to get new rear springs that sit at the correct height, and most likely will need to depend on a spring shop to rebuild our old ones. This is the route I took, and was pleased with the results. However, after over a year of driving on them I'm estimating that they still sit about 1/2" to 3/4" too high. I'm fine with that, but what I'd like to know is what is the "correct" distance from the check strap bolt on the frame to the bolt on the axle. I read a lot of posts in the archives about guys waiting for their springs to settle so they can get those check straps on. Well, that isn't good because you are now immediately out of travel. I have brand new straps on my car, and they sit a little slack at complete rest. I'm thinking they should be much more slack than that, but remember my springs are a "tad" high and I'm using the 8-5/8" straps which should be correct. Since I've rebuilt both front and rear suspension with new springs, poly bushings, etc. I've noticed an oversteer condition which I speculate is the check strap lifting my axle off the ground and making the rear end unstable. You racers and autocross guys- how much travel do you let your rear axle have? I realize a sway bar will help with the body roll but that's next winters project. I plan to fit the 9-1/2" MGB check straps which I think will put the travel back close to where is should be. Sound reasonable? |
| Mark J Michalak |
| Mark- The point of the check straps is A) Prevent the damper from being the rebound stop and killing the damper B) Prevent the spring from going into tensile load at full rebound which breaks the spring. Loaded camber specs should be in WSM, so you can figure out what the spring should be, But isn't! Free camber is 3.60", so that is the spring position at no load, and the strap should limit it to that, but of course different springs would take or allow different straps. That may or may not work with the dampers. FRM |
| FR Millmore |
| Ok, good stuff. So I should be able to remove the shock link, find the damper stop, and determine whether the longer strap will prevent the shock from taking the full weight of the axle. |
| Mark J Michalak |
| The measurements that FRM mentions are not in the Workshop Manual but they are in the factory produced Technical Data Book for the mga twin cam. The pushrod car has the same measurements. See Barney's site: http://www.mgaguru.com/mgtech/books/tdb/tdtc_j_rear_springs.pdf Mick |
| M F Anderson |
| Mark- Yes. Remember that the straps have some stretch - some bad ones have nothing else! So the strap should come solid a bit before the damper travel ends. You do know that somebody on MGExp is making nylon strap ones? And Mick/Barney/Abingdon tell us that the spring >should< be flat at laden as specified in WSM, FRM |
| FR Millmore |
| Yes. Notice the link above. Free camber should be 3.6" (unloaded and unattached), and 0" with full load of 450 pounds. If you pay a spring shop to re-arch the springs, when they are finished the main leaf of the spring should go exactly flat (straight) with 450 pound load, and the professional spring shop should be able to reform them to that spec. For many years now most if not all of the replacement leaf springs have been too tall, considerably more than 3.6" free camber, and never going flat with full load. The parts suppliers never seem to get the message, so as long as some people keep buying the springs and not sending them back, the suppliers will continue to sell what people will buy. For a quick check of ride height, start with 165-80-15 tires (very close to original tire diameter), a fully assembled car and no passengers. You can lay your hand flat with fingers on top of the rear tire, and your knuckles should contact the fender arch. For the front tire you should get three fingers vertically in same space. If the rear end sits too high, then the check straps will not have enough travel before going tight. If a check strap goes tight with normal body roll in hard cornering, it will lift the inside tire off the pavement leaving half the weight of the car on the outside rear tire. The overloaded tire then loses grip efficiency, and the rear end swings wide. You then get a sudden transition from neutral or under steer to rather dramatic over steer. If you take your foot off the throttle at that exact instant you will find yourself going backward or doing a complete loop out of control. Aside from being hazardous to your health (and others around you), this is not the quickest way to get around a corner. Check strap going tight in a hard turn on smooth pavement should be avoided at any cost. The check straps should never go taught under any driving conditions, except when you hit a substantial bump and get the car airborne. Do NOT lengthen the check straps, as this will cause the shock absorbers to have harsh and destructive impact when they hit bottom end of travel before the check straps go taught (and you might still lift the wheel off the ground). If you would lengthen the check straps, you might as well cut them off and throw them away, as you have defeated the intended function of the check strap. The only reasonable solution to the problem of too-tall springs is to correct the shape of the springs so they are not too tall. The parts suppliers should be supplying springs with correct height so customers do not have to individually repair/reform each spring individually at great cost and inconvenience. If the vendors insist on sourcing too-tall springs, then they should have them all modified/corrected in large batch processing before selling the things, as a simple matter of good economics. If every customer who receives a too-tall spring would sent it back, the ill-fitting springs would fairly quickly disappear from the market. |
| Barney Gaylord |
| There are two properties of a rear spring that determine the ride height of the car when fully laden, (1) free camber and (2) spring rate . The free camber given in the Workshop Manual is 3.6 but there is no mention of spring rate. However it does give rear suspension working load as 450 lbs. This can be taken either as the weight (with the car road ready) on each rear wheel or the load on each rear spring. To work out spring rate it is important to know which of these two it is. I have seen reference to a book on the MGA which gives the weight distribution of the car as 1100 lbs on the front wheels and 900 lbs on the rear. If this is correct then the 450 lbs is the rear wheel loading. The rear spring loading is obtained by deducting from this figure the weight not loaded onto the spring which is made up of the rear axle, brake drums + linings, wheel + tyre and the spring itself. Ive weighed the axle + oil (127 lbs), wheel (15lbs), tyre (15lbs), spring (22lbs) which gives a total per spring of 115lbs which should be deducted from the 450 which is 335lbs. It is generally assumed that the working spring camber is zero (Clausager p.71) so if the 335lb figure is correct then the spring rate required would be around 95lbs per inch. If 335lbs is not correct and it should be 450lbs then the spring rate will be 125lbs. In this case the weight distribution would have to be changed from 900/1100 rear/front to 1130/870 which would be surprising with the weight of the engine/gearbox (430lbs) at the front. Anyway I bought new springs years ago (MGOC) and I measured the free camber at 4.2 which is around a too high. I convinced myself that the extra half inch would not be too noticeable when the car was finished. At the time I wasnt worried about the spring rate because the dimensions of the spring which theoretically determine rate viz. leaf width, thickness, length, number all seemed spot on and as per W.Manual. One other thing that affects spring rate is the Youngs modulus of steel but measurements I had seen of YM for a variety of soft and hard steels (even stainless) only varied by about 5% so I wasnt too concerned about this. I went ahead and installed the springs but before replacing the body I measured the distance of the strap fixing points centre to centre whilst loading the back of the frame over the wheels with sand bags. I went up to a load of 605lbs (302 lbs per spring) and the spring rate worked out at 140lbs per inch i.e. considerably more than the above 95lbs. To cross check this figure I removed a spring and loaded this up and measured a rate of 150 lbs per inch ! The two are probably within experimetnal error. With this spring rate Im fully expecting the rear ride height to be far too much when the car is finished. Over this time I did measurements on the rear suspension and did a scale drawing of the area around the spring. With the spring in a zero camber state (fully loaded) the distance apart of the strap fixing point centres is 7 inches. The new and unused straps I had bought (reproduction) had fixing centres at 8.75 so there should be 1.75 of slack at full load. The distance of the tip of the rubber bump stop to the top of the axle tube is 1.5 so the axle movement allowed is roughly symmetrical. The shock absorbers have a maximum swing of +/- 45 degrees and when the car is fully laden the shock absorber arms are horizontal. If the top bump rubbers are left off the car a deflection of 3.5 would cause either the axle to hit the frame or the upper shock absorber limit would be reached (both are close). The axle banjo casing might even hit the hemispherical indent in the boot floor. If there are no straps on then a similar deflection of 3.5 in the opposite direction would reach the lower shock absorber limit but the axle would probably hit the exhaust pipe first. As I have gradually loaded my car with parts I have been keeping an eye on the strap fixing point distance. This still has got to go before I can even fit the straps and theres not much left to put back on the car (seats, carpets, hood frame, petrol, etc.) . The 0.75" is roughly equivalent to another 400 lbs of stuff I need to replace and there is no way I have this much to go. Ive been wondering what to do about this. It would be simple if there was some outfit that supplied correct springs but I dont know of any in the UK. I cant even get my old springs refurbished as they went to the tip years ago. Even getting some specially made with the correct spring rate wouldnt be easy because Ive found that measuring apparatus for spring rate is not commonly owned by spring companies. I think the reason my springs are so high in spring rate is because the steels YM is too high. Ive seen a couple or papers that seem to indicate that unusual heat treatment of the steel can give higher than normal values of YM. Maybe getting the springs retempered in the UK (assuming they are made in India) would bring down YM? This would cost over 100 a pair. There is another thing I could do as a last resort. If you look at the MGA springs they have seven leaves but the smallest one at the bottom is different from the rest in that it is flat along its length whereas the other six are curved. This means that the bottom leaf plays no part in determining the spring rate until the spring is fully loaded and flat. In effect it acts as a sort of overload leaf which many lorry/truck springs have. Whether this is intentional or it is just there to locate the spring nicely in the spring rubber and plate I dont know. However, if curved spring no. 6 is removed from the stack and replaced with a flat one of the same dimensions the spring rate would be lowered until the spring was fully loaded thereby lowering the rear height. This should be easy to do because leaves 5 and 6 are not clipped in the spring stack and could be removed without much effort. To get a feel for how much the spring rate is reduced by this I looked at the formula for calculating spring rate and the rate is directly proportional to the number of leaves (curved) used so making leaf number 6 flat would reduce the rate to 5/6 i.e 117 lbs. per inch which is still a bit high. Making both 5 and 6 flat would bring the rate down to 95 lbs. per inch which I reckon should be about right. The only negative consequence I can see is that when driving maybe more stress would be placed on the rebound straps. Anyway I may get some flat leaves ordered unless I see something better on Ebay first! If I decide to go this route Ill report back later. Hope this helps! .........................................................Mike |
| m.j. moore |
| Mike- Pretty good as far as you go, but... You would need to do the same adjustments for wheel vs spring load in front in your actual spring load/weight distribution calcs. And power unit weight has a fair component on the rear axle. Young's is not likely to change that much, and is generally assumed more or less as a constant for "steel". Exotic heat treat or alloy makes it "not steel". Leaf thickness is the most critical variable - how good are those Indian or any modern leaf rollers? Specs are given as fractional inches, but are certainly in thousandths in manufacture, to some statistical average, since the leaves are not surface ground to thickness. How do you measure thickness in thou when there is paint on the metal? Do the leaves have square cut or tapered thickness/width ends, over what length? Measuring spring rates on leaf springs is almost impossible, due to interleaf friction changes. For the same reason, any positional changes during essentially static assembly are pretty useless, borne out by experience. Substituting flat leaves for one or more lower leaves is a good move, as it effectively creates a step-progressive rate, a pretty good deal. But, Do Not just remove the lower leaves, as you create a horrible stress raiser/spring breaker at the center mounting. However, rate will not change in any direct way according to the fraction of leaves removed, as the length of leaves is also a factor, which you are altering - effectively changing the taper profile of a center loaded beam. The formula you used is only valid for a stack of equal length leaves, with no interleaf friction; it is effectively a stack of springs in series. For a spring of the usual configuration, the only direct variable is spring width, which you can control and adjust fairly easily - if you narrow the entire spring 10%, the rate will drop 10%. Another way to change static load height without making correct springs, and which also gives a bi directional progressive rate, is to use one or more leaves with reverse camber on top of the stack, something like #4,5,6 inverted on top, with flat replacements on the bottom. I have done this with great results - except now I cannot throw dead springs away, so they proliferate and reach out and bite me sometimes! FRM |
| FR Millmore |
| Mike and I are on two slightly different tacks here, but all towards the same purpose. If the shock links have 3.5" of travel then there should be enough room to lengthen the straps a bit before you run out of travel, providing your springs are only about 1/2" higher than stock. Higher than that, and you might need another option. (Please keep in mind that the obvious perfect world scenario is to have springs that are the correct height. However, when the spring shops available to you don't have a passion for getting sports car springs exactly right, and the only solution is to keep removing the springs and taking them back, I'm going to look for another option to keep the car drivable.) I think if lengthening the straps by 7/8" (rubber bumper mgb straps) causes the shocks to bottom out, then the next step would be to fit rubber bumper MGB shock links, which should be a corresponding 7/8" longer. The object obviously being to keep the shock arm horizontal at rest, even if your spring arc is too tall. You would then shim the bump stop down to match. |
| Mark J Michalak |
| Specifications for the MGA leaf spring are very specific, as noted in the Twin Cam Technical Data Book (link above). Free camber is 3.6 inches. Spring rate is 125 pounds per inch. Working load is 450 pounds, which brings the main leaf exactly flat when fully laden. Fully laden means two passengers, a full tank of fuel and a moderate amount of luggage. While curb weight of the car (with two gallons of fuel) is about 2000 pounds, fully laden is about 500 pounds more. Passengers sit closer to the rear axle. Luggage is mostly over the rear axle. Fuel load is aft of the rear axle. Collectively the added load (above curb weight) is mostly on the rear axle. The specified load of 450 pounds per spring is on top of the spring, not counting weight of the springs or the rear axle. Modulus Of Elasticity (sometimes called Young's Modulus) for steel is 29,000,000 psi (in tension or compression), plus or minus 1.5% depending on alloy and hardening. That means there is very little variation in spring rate regardless of variations of steel alloy or hardness or temper. If you temper a spring to reduce hardness it still has the same spring rate (but will carry less load before hitting the yield point). Springs do not ever change spring rate with age or even when they sag. Sag of springs after decades of holding a load is a form of cold creep where the part gradually changes shape. For the leaf springs under constant load it may be about 1/10 inch closer to flat every 15 years or so. With MGAs being 50 to 57 years old now, you might expect ORIGINAL leaf springs to be about 3/8-inch lower now than when new (if the car spent all of its life sitting on the suspension). What you perceive as sag or lowering of the springs with time is probably more affected by wearing thinner at the points where the parts are in friction contact near the ends of the leafs, and degradation of the mounting hardware. When you find new springs being considerably taller than used springs, it is not because the old springs are sagging. Springs will take a set eventually if they are subject to about 50% constant overload. So, if you load the MGA leaf spring up to about 675 pounds, forcing it about 1.80" past the flat condition, and hold it there for about six months before you let it loose, it might end up a half inch shorter. But it will not get there in your life time just by waiting for the car to settle. |
| Barney Gaylord |
| My old springs had a cracked main leaf. (across the centre hole) I ordered new from VB, and they came with 6" free camber. I dismantled both and combined the new top leaf with the old leaves and got about the 3.6" required. |
| Art Pearse |
| Barney, Following on from your comments above - what is the best way to lower an MGA at the rear for competition - is it with blocks so that in a static position the spring leaves are flat? Mike |
| Mike Ellsmore |
| Yup. In essence, to lower the rear of the car you raise the rear axle. Front end of leaf spring is fixed to the frame, and rear end is fixed with a rather short shackle, so it is not easy to raise the spring. Without distorting the spring in any way, the appropriate way is to leave the spring in original location and raise the axle by use of a spacer block and longer U-bolts. |
| Barney Gaylord |
| >With MGAs being 50 to 57 years old now, you might >expect ORIGINAL leaf springs to be about 3/8-inch lower >now than when new (if the car spent all of its life >sitting on the suspension). I measured my car's original leaf springs and compared to the dimensions on your site, and found them to be about 1/2" lower than spec. Not sure what, if anything, I am going to do about that. |
| Del Rawlins |
| Most people don't mind the rear of the car sitting a little lower (especially if the front does the same). But I have driven some in your area (all roads in Alaska), so I understand reduced ground clearance could be a problem. Without changing the springs you could install a longer rear shackle. It is what the hot-rodders sometimes do to raise the rear end for installing larger tires. |
| Barney Gaylord |
| Changing shackle alters roll centers, so that may cause handling changes you may or may not like, but I reckons AK roads maybe not a big deal. But, I think Del should trade his nice slightly lowered springs to guys with higher ones, so he can beat then down to spec, on a rotating basis! FRM |
| FR Millmore |
| FRM I didn't keep the leaf thickness measurements I took on my new springs years ago but I went out this morning to check them again. They worked out at 0.222" compared with the spec. of 0.219 but this was with over four coats of paint I put on as well as some they came with. But forgetting about the paint a 1.5% increase in leaf thickness would only give a 4.5% increase in spring rate which is nowhere near enough to explain my measurements of 140 lbs per inch. And whilst I agree with you that interleaf friction could affect drastically a spot measurement of spring rate it should have no effect when taking the rate from the slope of load/deflection. Also I can't escape the fact that the way my car's going when it's finished it will end up at least two inches too high when the initial spring camber was only 1/2" over specification. So I reckon anyone buying new springs these days can't just measure the camber in the shop before buying them. The only way to check would be to try them on the car soon after purchase and return them if the strap fixing centres didn't end up at 7" or so...................Mike |
| m.j. moore |
| Mike- I do not know what you are calculating with, to, or from, and I don't think it matters really, because there are far too many uncertainties. Real world experience says that cars being assembled, regardless of bouncing, weights, prayer or sacrifice have little height relationship to how they are after a hard blast down a bumpy road. You might attempt compressing the springs in situ, by wedging the car against the ceiling or chaining it to a beam with the jack on top, and jacking up the axle or similar, and then releasing it to see if the springs return to where you started; If they do, then maybe worry about it. And you actually can chain the spring to a stout beam and jack the center up to the point that you bend the spring and correct the static camber, but it will not change the rate. If you try, build a clamp so it does not break or bend at the center bolt. Takes a lot of overtravel - unless the spring is cheese. FRM |
| FR Millmore |
| Ground clearance is always a concern. You should see the condition of the underside of my frame. The low point cross tube under the firewall is almost completely flat on the bottom. And the underside of the frame rails has taken a beating too, although I was able to pull the worst of that out using a slide hammer coupled to a vise grip, along with the torch. I really don't know that an extra half inch would have made a huge difference though. There's just a lot of places where I won't be able to drive it when it is done. I expect some of the damage was done on the frost heaves when my dad drove it from Homer to Cordova back in '81 or so. It also didn't help that it was on 155/80R15 tires instead of 165s from about '85 on. The thing that really upsets me is that one of the latest fashions among whatever bunch of idiots are currently in charge of the streets around here, is to put in these massive speed humps when the side streets get repaved. I've seen a few that would come close to high centering an MG. |
| Del Rawlins |
| Del- When you live in such a place, you go into off road rally mode - skid plates and sump guards. Re speed bumps - sue them! It is required everywhere that bridges be over the legal vehicle height, and any deviations from this be marked, on site and before you get there at the last turnoff. Legal vehicle is 13'6", and bridges must be marked if less than one foot more than this. Should be the same for bottom clearance. So first they must have a bottom clearance reg, and then the bumps have to meet that with some specified tolerance. PA bottom clearance is such that a completely flat tire on one wheel will not result in any metal parts touching the road. So about 4" near the wheel, and so, 2" at centre. And they must provide a tolerance, so speed bumps over 2" are definitely out, and that is without the tolerance, which you will need a lawyer to earn his keep for. FRM |
| FR Millmore |
| > When you live in such a place, you go into off road > rally mode - skid plates and sump guards. Yes... This is what I usually drive:  |
| Del Rawlins |
| Del - The road looks great for an MGA! Mike |
| Mike Ellsmore |
| Hey, it's paved! When I was there with my MGA in '97, that would be one of the better roads. Somewhere around Anchor Point, maybe? |
| Barney Gaylord |
| You only say that because I don't have a picture taken in the opposite direction to show you. That's the beginning of the McCarthy Road. 60 miles of pure lane and a half, barely maintained, unpaved, automotive hell. The road was built in the 60's on top of the grade of the Copper River and Northwestern Railroad. Mostly they at least pulled the rails aside first before graveling it over, but there are places where the tops of rails can be seen. Ties and pilings often emerge, to the detriment of low hanging vehicles. I can usually make decent time over it in the Jeep, but I drove it a few weeks ago and it was the worst I have ever seen it. Pulling a small trailer became a 3 hour ordeal each way. Counting the 5 hours on pavement, a total of about 16 hours in order to do one 15 minute welding job on a piece of equipment, and I'll still have to haul it back out there eventually. |
| Del Rawlins |
| Oh, you mean this road: http://mgaguru.com/ak/pic7/7-6.htm http://mgaguru.com/ak/pic7/7-4.htm In 1997 the sign said: McCARTHY 60 Tramway access only at McCarthy BRIDGE OUT 59 miles ahead No Vehicle Access To McCarthy We drove a couple miles of it just to get the flavor of the place. Did they ever get the bridge fixed? |
| Barney Gaylord |
| Damn. Never thought I would see that. The bridge referred to had washed out decades previously. The state eventually built a new "foot bridge" at considerable expense, that is wide enough for some ATVs to get across. Therefore, they sunk concrete filled steel posts as barricades so people couldn't do this. Which the locals promptly cut out using a smoke wrench. So they welded them back in. And then they were cut out again. And so on, and so forth. I heard that they were welded back and removed something like 14 times before somebody was able to bring the situation to then governor Frank Murkowski's attention. They have not been reinstalled since. The aerial tramway, which had been built by the locals, was taken out when the foot bridge was built, but pieces of it are still laying there. There is also now a privately owned vehicle bridge for which a yearly pass can be purchased from a local contracting company. It costs a few hundred a year, so the only people who buy the pass are locals or people with property there. |
| Del Rawlins |
| Just to revisit this quickly, I fitted the rubber bumper MGB check straps which are 7/8" longer. I also installed the rubber bumper MGB shock links. This allows the check straps to stop the axle before the shocks bottom out. Even with the longer shock links, the bump stops prevent the shock arm from hitting its upper limit. You should note that the upper limit is actually the car body, so you should make sure to take some measurements of your own if you do this. My result was drastically reduced oversteer. The car handles so much better. I also don't get a spinal compression every time the rear axle hits a big bump at speed. As a point of reference, I can fit three fingers under my rear wheel arch, and I have an extra leaf added to my springs. I don't mind the ride height, as I have to deal with speed bumps in the city and the occasional two-track in the country. |
| Mark J Michalak |
| Hi Guys A simple no cost way of effecting some lowering of the car is to take the flat spring from the underside of the spring and place on the top (same effect as a lowering block) Many MGA racers mine included used this neat little trick. Mark |
| Mark Dollimore |
This thread was discussed between 29/07/2012 and 18/08/2012
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