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MG Midget and Sprite Technical fitted panhard rod, but now get more understeer BBS discussion at MG-Cars.info
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MG Midget and Sprite Technical - fitted panhard rod, but now get more understeer
| Fitted a panhard rod, its level and crosses behind the axle at around the filler plug height. At the same time i fitted adjustable shocks to the back. Previously there were some very stiff shocks that came off a mazda pickup... I went for a thrash up my favorite twisty road but find i get more understeer than before. On short sudden twists in the road i can still throw it in there and it behaves ok, but on a long sweeping bend i'm fighting the understeer. I expect this is caused by the lowering of the roll centre at the back ? Or maybe caused by the softer back end ? i have the shocks set at 'click No3' which is the softer end of the scale.(Gaz mini adjustable shocks ) I have a 5/8" ARB at the front and standard springs with 3mm thick lowering spacers. Rear is lowered by 1 1/4" using blocks. Car has all fibreglass body and is very light with the back slightly heavier than the front with no one sat in it. The lowering spacers/blocks were used to off set the increased body height due to the lack of weight rather than for performance reasons.. any advice ? |
| Andy Phillips (frankenfrog) |
| Andy, I also got more understeer after fitting a Panard rod. By softening the rebound of the rear shocks you get more understeer as well. I solved the understeer with a thinner ARB and 2 degree negative camber in front.. You can also do some experiments with tire pressure. (more in front, less in the rear tires). Longer softer front springs, stiffer rear springs, stiffer bump force in the front dampers, more weight in the back will help. Is the understeer symmetrical, in left and right hand bends? Flip |
| Flip Brühl |
| Hi Flip. I seem to notice it more in a long sweeping RH band (car is RH drive). My next project is for adjustable camber top control arms...I currently use the front shock just as arms as i have put my own tube shock conversion on, so i plan to make some new top arms that can be adjusted, similar to the photo, but adjustable... then i can play with the camber. I had my toe-in set before my test drive at 0.3 degs each wheel. 
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| Andy Phillips (frankenfrog) |
| Andy Try a 9/16 ARB and a click up on the rear shocks. |
| Bob Beaumont |
| There is also the effect of lowering blocks on rear axle steer. It will be understeer or oeversteer, can't recall offhand, and it's too late at night and I'm too tired to work out which right now, but one or the other will happen. Check by getting a couple of bulky friends to sit on one side at the rear and jack the body on the other side up a couple of inches. Then compare the centres of the wheels side to side and see which way they have moved (forwards and backwards) relative to the body. Take initial measurements in the static position before loading so you have a reference. If the low side (outside of corner) moves back relative to the inside, then you will have roll oversteer as the angle of the axle will try to steer the rear of the car to the outside of the corner. If it moves forward then it is roll understeer. Either way, something to be accounted for in final set-up. |
| Paul Walbran |
| Andy According to my chart Depending on wheel diameter, but I would imagine your tyres are roughly the same OD as standard 0.3 deg = 20' == 1.8mm----one side Therefore total toe ,both wheels would be --3.6mm In my opinion that is way too much for a sporty Midget I'd be going Zero toe and all sorts of things happen Your car will turn in a lot sharper and I reckon it will be enough to get rid of your mid corner understeer It was possibly hidden/disguised by having very stiff rear shocks, not good for a lightweight car I remember having stiff shocks on the rear of my racer and they were terrible, the rebound was that stiff that the springs/car,both being very light were not strong enough to force the shocks back out The result was, you would start off ok but after about a lap the body would be pulled down and couldn't rebound and handling got very taily/skippy After sorting shocks etc and setting toe to Zero my car was a neutral steerer with a 3/4" front bar and 550 lb front springs. I could introduce understeer or oversteer by adjusting one tie rod half a turn either way to get toe in or toe out as needed, Personally I prefer a car with a tiddle of understeer for circuit work but with a tiddle oversteer for hillclimbing where understeer can be a bit of a disaster Try Zero- It'll make a big difference Willy On your panhard bar ,have you set it up mounted to the body on the driver's side, If it's on the other side they can feel wierd sometimes--don't realy know why but probably the pendulum effect from the other side maybe ??? Also ,If your bar is at filler plug level your roll centre is now slightly higher than std. not lower |
| William Revit |
| Id say your going to be living in experimenter Hades for some time to come thats alot of modification Id say invest in a good paper and pencil note book and take notes of everything you do and what those spec. Result in You didnt mention what wheels, that will have an effect also Prop |
| Prop and the Blackhole Midget |
| Andy, The picture looks nice! Understeer in long sweeping bends: so the bump of the shocks is ok. Read "tune to win" by Caroll Smith and you will understand it all. Flip |
| Flip Brühl |
| Is the Panhard rod absolutely level with the driver in the car? Does the car sit level using the bottom of the sill as the reference (spirit level under the sill to check) with the driver in the car? It doesn't sound like you have lowered the roll centre by much or any amount at all. However, I have never found the Panhard rod (at different heights including not level) ever created an understeer problem and would be interested in any clear explanation as why it would. Dropping to a smaller size of ARB ought to reduce understeer. I suggest that to eliminate whether or not the Panhard rod is causing understeer you disconnect one end (or remove it but not the bracket) and go for another drive. You don't say how quick you are driving through a sweeping bend but I'd have thought the definition of a sweeping bend is that it isn't sharp and is challenging in a Sprite/Midget at between 100 and 110mph. |
| Daniel Stapleton |
| Looks like i committed the sin of changing two things at once... With the old very stiff shocks it was very tail happy, Now ive got soft ones i'm hoping that will increase the grip at the back, which it probably has.. I think i need to get used to it.Its the nut behind the wheel that probably needs the most adjustment... However, good advice as usual, so my plan is: 1. Take of panhard rod, drive the corner and see how it feels.(can do this on the way to the alignment garage) 2. Have the toe-in set at zero, then drive corner again. 3. Adjust the rear shocks by an extra click and drive corner again to see if there is any difference. 4. Based on test 3 either leave shocks alone or put back one click, or maybe an extra click... 5. Refit Panhard rod and drive corner again. Wheels are 13" x 5.5j Tyres are 175/70-13 pressures 26 at front 24 at the back Panhard rod fixes to the body on the LHS , and to the spring clamp plate on RHS. RHD car. Speed through the bend was only about 50-60 mph... |
| Andy Phillips (frankenfrog) |
| Andy, can you remind us what you base your choice of tyre pressures on, as usually the fronts should be softer than the back. The handbook says 22F - 24R at normal running weight, 22F - 26R at max weight, and increase all round by 4 psi for running at sustained high speeds. |
| David Smith |
| Nothing scientific, other than i know the front is supposed to be stiffer than the back so i assumed the greater pressure for the front tyres. Do the tyre pressures from the handbook still apply with newer and wider tyres ? (sorry Nigel) I can play with tyre pressures also ... |
| Andy Phillips (frankenfrog) |
| factory pressures applied to 145/13 radials and 155/13s which were an option back in the day. I think the principle of softer front than rear will apply to whatever is fitted, will give a bit more grip and should remove some of your understeer, and improve the handling balance. |
| David Smith |
| I'm in agreement with David. I had an email exchange with the head of Motorsport at Toyo and his recommendation for reducing understeer was to lower front pressures and raise rears. I generally run at 22psi front, 24psi rear. At one hill climb last year I forgot to check pressures before I ran. It was a hot day and the car understeered quite noticeably. I checked the pressures and the fronts had gone up to 24 and the rears to 26 due to the heat. Dropping 2 psi all round gave me much better grip on the next run. Apart from telescopic rear dampers, my suspension is as standard running on 155/80 Toyo 350s (though I wouldn't choose these again!) C |
| Colin Mee |
| OK, tyre pressures much more important than i thought.. I'll add a step to my plan: 1. Take off panhard rod, drive the corner and see how it feels.(can do this on the way to the alignment garage) 2. Set tyre pressures to 22 front and 24 rear, drive corner again. 3. Have the toe-in set at zero, then drive corner again. 4. Adjust the rear shocks by an extra click and drive corner again to see if there is any difference. 5. Based on test 4 either leave shocks alone or put back one click, or maybe an extra click... 6. Refit Panhard rod and drive corner again. Should keep me busy for a saturday morning... ;-) Luckily this sweeping bend is on a quiet side road... |
| Andy Phillips (frankenfrog) |
| Andy don't forget how much tyre pressure is affected by ambient temp. 22/24 is OK for cold setting UK climate (15 Celsius?). Straight after a quick or spirited drive they will easily be 26/28 hot, probably more. What's the ambient temp where you are? I see it's 35 Celsius in Bangkok, so if it were me I'd set the pressures to 25/29 to start with. http://www.tirerack.com/tires/tiretech/techpage.jsp?techid=73 http://www.tyrepal.co.uk/managing-tyre-pressures/477.html HTH |
| David Smith |
| 42 degrees last weekend ! Yes will follow your advice , wouldn't have thought about that otherwise :) |
| Andy Phillips (frankenfrog) |
| All tyres have an optimum pressure in terms of grip. Either increasing or decreasing pressure from this will will reduce the grip. To make things not too easy, the optimum pressure is different for straight line (aceleration/braking) and lateral (cornering) and depends on the aspect ratio of the tyres and rim widths. Motorsport tyres have a different construction that road tyres, (generally sidewalls are firmer) so the optimum pressure will be different and usually lower than a road tyre. Back in the days of tall tyres (now called 82 profile) the tyre walls deflected hugely under cornering. At normal pressures on the track the tyres would be running well up the sidewalls while at the same time lifting the inside half of the tread off the road. In these circumstances increasing the pressure increased grip. In the early 1980s here, "standard production" salloon car racing rules included a requirement that standard road tyres be fitted (no, not road legal race stuff - off the shelf shopping basket tyres) and the car that consistently won these events was running 60 psi in the front. Braking at these pressures was below optimum, but the cornering gain was enough to do the trick. As lower aspect ratio tyres became available, then the optimum pressure decreased substantially. All this is to say that when you experiment with the settings proposed above, also experiment with tyre pressures rather than make assumptions. The way I do it is to try a 4 psi difference front and rear (doesn't matter which initially) test the car, then reverse the pressure difference and test again. If there is little difference felt between the two tests, you are around optimum. If there is a change to under or oversteer you can work out which pressure was giving the best grip, then repeat the test using pressure changes in that direction until you reach the stage of no real difference. It's worth the effort! |
| Paul Walbran |
Also get people to take pictures of it, so you can see how much the tyres move about
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| Rob Armstrong |
| is that an illusion or are are your rear wheels much bigger than the front ? |
| Andy Phillips (frankenfrog) |
| Rob, good pic illustrating just how much better modern lower aspect ratio tyres sit on the road without lateral deflection. (Either that or you weren't trying, which I find hard to believe!) |
| Paul Walbran |
| Rear wheels same size as the front - lots of compression going on. Tyres are wide; 185/60 R14 I might have been trying a bit ;) Though not as hard as totally possible as it was my first attempt at a track. 
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| Rob Armstrong |
| here's another one - tread trying to stay flat. It seems to be a bit of a balance between less air so they heat up to the right amount, and not too little that they roll off the sidewalls... These were at 23 all round (hot) during this activity 
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| Rob Armstrong |
| Rob You think your tire deflection is bad... check out mine, I made a you tube video its almost at the start... the series of photos of the green car...I watch that and I still dont understand why the wheels didnt just break off Prop http://www.youtube.com/watch?v=QT_NgmMR30M&feature=youtube_gdata_player |
| Prop and the Blackhole Midget |
| I'm yet to understand why the books say higher pressures at rear. Clearly (to me!) the fronts should be the same or higher than the rear if the boot is empty and no passenger. With the A series I always had a 2 psi difference front /rear (typically 22 front/20 rear) With the K this didn't feel right so now I have the same all round. The first thing to check is to make sure the front tyres are not 'tucking under' whilst you are understeering - look at the shoulder of the tyre and you should see markings and evidence of road contact. Try dropping 2psi at the front an not the difference. If your car is very light, I'd be looking at around 20 front 18 rear. Assuming the tyres are all the same! Good luck |
| Dan Cusworth |
| Danny are you saying the manufacturer has been consistently wrong since 1960? |
| David Smith |
| "Danny are you saying the manufacturer has been consistently wrong since 1960?" Has been known to happen. There was afterall no useful change to the flow qualities of MGB inlet ports, an area in which a modest amount of reshaping gives big gains! Rob, That's more like it, one wheel alomst airborne :-) but looking at your sidewall deflection I'd be trying higher pressures (2 psi at a time) to see if it gained or not. It's certainly getting towards the stage where it could help. |
| Paul Walbran |
| Interesting A few years ago when i was running a Midget in hillclimbs etc I tested and tried all sorts of combinations of tyre pressure and ended up at 38-40 in the fronts and 32-34 rear I had always run lower and a bloke came up to me one day and suggested running 40 all round for a try so i did and it was like turning the handling switch on, but a tiddle taily so gradually dropped the rears till it neutralized and that's where it ended up just sayin willy |
| William Revit |
| mine behaves strangely if the rears are harder then the fronts. Going to give all of mine a bit more go next time out :) |
| Rob Armstrong |
| Willy Much the same experience on taller ratio tyres. 175-60's we run around 28-30. Difference with hillclimbs is that tyres don't get so hot as on the track, where pressure requirements can differ again. main thin in my experience is to regard what others have found as a good starting point, but experiment from there for your own set-up and own driving and conditions to find the optimum rather than take anything else as godpel. The gains can be equivalent to 00's or even 000's of cash spent on engine mods. Rob Interesting observation, that what I have found ... again driving my car in my style and conditions. Life would be so boring if we were all the same, the BBS would die overnight for a start!! :-) |
| Paul Walbran |
| PS one day I'll lose the phat phingers ... |
| Paul Walbran |
| Paul Couldn't agree more----check out the fastest cars first On engines ,I don't know how many times I've seen people build a high output engine and completely loose the plot by driving for power instead of driving fast/properly Sticky tyres Good sensible suspension mods Brakes ---then horsepower I'm not knocking engine mods, actually my personal choice is the more power the better BUT rule number one should always be to remember how to concentrate on going fast The pressures I mentioned were with 205/60-13 Advans on 8" rims Unfortunately CAMS in their wisdom won't allow 205 tyres on 8" rims here anymore for competition as they exceed the max recomended rim width for 205s ---- willy |
| William Revit |
| "The pressures I mentioned were with 205/60-13 Advans on 8" rims" ... I like it. Yes, that would need quite a bit of pressure to get the centre of the tread taking its share of the load :-) |
| Paul Walbran |
| Paul It's taken me most of the week to find, but I knew I had this pic. of the old midget somewhere. I used to set my son up with the camera when testing and he would get tyre/wheel pictures during cornering etc. He got quite good at it towards the end. This one is of the 205s on the big rims doing their stuff powering out of a left hand sweeper. cheers willy 
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| William Revit |
| Willy I was hoping your pics were going to be of your son stretching and hanging over the window ledge with wind in his face inches from the tarmac... snapping photos of your wheels and tires in action in the turns....haha Fathers day... great moments....hahaha Prop |
| Prop and the Blackhole Midget |
| I run the same pressures all round on my road car and only in the last year or so got interested in them. If a particular car runs better with higher tyre pressures at the front than the rear that surely doesn't mean all cars will. For example if I change the front ARB on my car to one of a different thickness do I need to change the tyre pressures as well? On my road car I have found the difference between hot and cold pressures is 3 psi. |
| Daniel Stapleton |
| This is from above, hard cornering either the old 175 tyres on a 5 inch rim. Rolly poly sidewalls! http://www.flickr.com/photos/34910348@N08/8053919225/in/album-72157631691229693/ |
| Rob Armstrong |
| Willy, that looked a bit serious sideways with lots more distortion on the rear thean the front ... |
| Paul Walbran |
| Ha, was just poodling round in old pics for something else and found this one of our UK-domicled Midget in the MG Live autosolo a couple of years back: 155-70's with 40-odd psi and still lots of distortion. Didn't know I had a pic like that, shows what happens quite well! 
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| Paul Walbran |
| Turn left young man THAT is a flexible piece of rubber doing it's stuff Go the Midget |
| William Revit |
| Here's another pic for fun, IIRC (it was a long time ago) these were std size 145/80x13 and on std rims. Ian 
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| Ian Webb '73 GAN5 |
| Good one Ian! Mine was so distorted 'cos I was trying to catch you :-) Were you there this year? |
| Paul Walbran |
| Paul No I didn't enter this year, I am having a year off doing a mechanical rebuild on my car, just doing the Toyo gearbox this week in fact. I just racked my brains; that photo was in 1980, IIRC the gearbox layshaft stripped halfway through the event, so no first or reverse. Ian |
| Ian Webb '73 GAN5 |
| Ian. If that's you driving, you look like you've just seen the JCB from The Italian Job coming in the opposite direction. LOL. |
| Bernie Higginson |
| Finally got some time to get home and work on the car. Went to a laser alignment place. Had toe in set to zero... With tyres pressured at 30 psi f&r which translates to 32 after a spirited drive the handling felt much better . I need to play some more with tyre pressures but was cut short by a clogged fuel filter (don't ask !) Anyway here is a picture of the laser alignment screen.. Is it normal to have such difference from side to side ? Ignore all the figures except the ones in the red semicircles, they have to put a random car into the computer to be able to do the measurement as there is no settings for a frogeye. Top is castor middle is camber bottom is toe in I confirmed with the operator that the camber figures both mean positive camber 
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| Andy Phillips (frankenfrog) |
| Side-to-side difference in camber is usually caused by the car not being level, typically the car will have settled a bit more on the driver's side. The side which is lower will have the camber more positive and the other side less positive or more negative. Your average castor is correct. The castor on that readout is well down from the 3 degrees it should be. 3/4 degree of castor on one side would reduce the slef-centring action of the steering by an unhelpful amount, and the car may well feel different on left hand corners from right. The castor in the spridget is engineered in by virtue of a 1" kick-up in the front half metre (approx) of the chassis rails. A common mistake amongst body repairers is to assume this kick-up is accident damage and so straighten it out. Reduced castor can also be caused by accident damage that bends the suspension mountings or arms themselves. |
| Paul Walbran |
| I have long suspected accident damage repair.. This is a RH drive car. The original drivers side A-arm was bent and i replaced it with a new one. I did both sides at the same time and installed polybushes as well , the old A-arms didnt have mountings for the ARB.. Biggest givaway was that there was a mounting for ARB on the drivers side but not on the passenger side... Yes it seems to corner better in a LH bend than a RH... due to the more positive camber i expect. I understood that lowering the car actually gave more negative camber ? The car always sits lower on the drivers side, i have had the rear springs apart and swapped every other leave over and measured them and got the same rate. This didnt seem to make any difference. Then i put 20mm lowering spacers on the passenger side , this made the car sit level. Would this cause the extra positive camber ? For the castor, i have noticed the front 'chassis rails' do incline upward slightly... but also, my shocks have some axial play (a lot actually, about 10mm) so that could be a contributory factor.. |
| Andy Phillips (frankenfrog) |
| Andy You really need to get your car sitting level and not by fitting a lowering block to one side That is fairly dodgey Start off by finding out which end of the car is causing the problem Sit a socket or something similar/small on a floor jack and jack the car dead centre under the diff housing till both back wheels are just off the deck If the front suspension/springs are good the body should be the same distance off the ground on both sides--but if there is a problem in the front the car will list to one side If the car is jacked up like this and the car is still level then the problem is in the rear and will show up by one wheel being further off the ground than the other THAT lowering block has got to go After you have sorted out the leaning problem, then you can start looking at camber differences castor isn't such a problem as long as the steering self centres and doesn't pull to one side. A level car and Equal cambers should be your priority Personaly if it were my car I'd be aiming to get that one degree twelve min. left hand camber down to match the half a degree on the right hand side willy |
| William Revit |
| Ok sounds like a plan, though a longer one than I would like... If the problem does turn out to be at the front what would the next step be ? |
| Andy Phillips (frankenfrog) |
| Depends what it is! You could start off by swapping the front springs from side to side and see if the problem follows the springs. If it does, there's a simple answer - new springs. If not, there are chassis issues which will have to be found with a lot of looking and measuring. Willy gives good advice about the camber to aim for. Generally speaking, as positive camber at the front is increased so does understeer so bringing the LH back to 1/2 degree will help. |
| Paul Walbran |
| Paul/Willy, OK, i am officially adopting you as my suspension guru's... :-) Yes Willys advice makes sense and is what i will go for.. Of course, before going through that procedure i need to remove the lowering blocks... Here are some pictures from when i did the front rebuild around christmas time... This one shows the passenger side, with car on its wheels with no spacers(left side of picture) and with spacers fitted (15mm thick, not 20mm as i said before) I think it sits too high,and put this down to having a fibreglass front and a lighter engine with aluminium cyl head (Datsun A14). So i think i need to get shorter springs anyway. 
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| Andy Phillips (frankenfrog) |
And here is the drivers side taken at the same time, with no spacers.
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| Andy Phillips (frankenfrog) |
| Is it worth getting the ride height better with equal spacers both sides as a first step ? then i would have a better baseline ? |
| Andy Phillips (frankenfrog) |
| Yes All should be equal But I d just jack it up first and assess what you have BUT also keep in mind that with a Midget/sprite there is a shoulder/locator/ledge up in the top of the spring tower where the top of the spring sits and you need to make sure the top of the spring is sitting up in there properly. --I've seen them with one side fitting properly and the other not and it makes the car sit crooked If you have aftermarket springs they might possibly be of a diameter that is fouling the locator and not allowing them to fully locate---worth a look at for starters Just looking at you pics, the car appears to be sitting fairly high |
| William Revit |
This thread was discussed between 02/06/2015 and 30/06/2015
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