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Understeer problem


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Can anyone advise me on how to cure the appalling understeer I experience when cornering my sit-on tractor lawnmower?

I can't make any adjustments to suspension settings, since there is no suspension. Recommended tyre pressure is already very low and efforts to cure the problem by running almost flat tyres have been unsuccessful.

I considered subtle alterations to the steering geometry and found that by bashing the track rods with a club hammer I could shorten or lengthen them with ease. Unfortunately it didn't cure the problem and I have reverted to factory settings (straight-ish track rods).

Weight distribution could be a factor, but I can't move the seat any further forward and anyway if I sit on the bonnet, I lose traction at the rear (wheels, that is).

The mower is a "Rally" purchased at Mr. Bricolage. With such a sporty name you'ld think they would have got the handling sorted. Frankly, I wouldn't dream of entering a rally on this machine - the steering is just too unpredictable, and a high speed crash on a forestry special stage or, worse, an alpine hairpin, are just too dangerous to risk.

Even if I was prepared to put myself at risk, I really couldn't ask my co-driver/navigator to join me in doing so.

Has anyone got any thoughts on this?

Patrick
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The problem could be the Ackerman Angle (king pin inclination). I solved my steering problem by moving the camber shims from  the front of the top suspension wishbone to the rear thus reducing the toe-out on turns by approx 3 degrees. 

Try it.

weedon

PS

Thinking further about it, that only applies to right hand drive garden tractors, if you have a left hand drive I am afraid I cannot help.

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Well, weedon, the steering wheel is in the centre, but the gear change on the right gives the game away, so I'm afraid your cunningly simple modification won't work.

What a shame, I was looking forward to a bit of tinkering with the Ackerman angle.

Dick, I've considered your brilliant suggestion of weight increase over the front of the vehicle, but rejected it on the grounds that the addition of any weight on such a lightly built chassis would have serious consequences to the power/weight ratio, and hence loss of acceleration.

Moreover, braking distances might be seriously increased, thus lessening the chances of overtaking at bends. Anyway, ultimately, increased mass means more inertia, which implies poorer adhesion to the track at higher cornering speeds. At best it's just a stopgap solution.

In the end I have to heed the words of Colin Chapman who said that weight is useless in a performance vehicle (he said "racing car", actually, but I'm sure he was including lawn mowers).

I forgot to mention there is no wing on the back or front. Perhaps the formula rules don't allow aerodynamic modifications? Does anyone know?

Patrick

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In the absence of tinkering with the KPI, try a pair of backless leather driving gloves, wooden gear knob, dangly dice and 2 big speakers behind the seat booming out drum and bass.  An added bonus is that it might even get rid of the moles.

weedon

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I'm going to try the driving gloves and wooden gear knob later today. They won't add any significant weight.

There's no mirror to hang the furry dice from, so I won't go that route.

I'm afraid the big speakers, too, must be ruled out on weight and moment of inertia grounds, even though they could well be the solution to the serious mole problem.

Dick, I'm going to switch from mulching blades to the grass collection variety, and engage the cutter on the approach to bends - it would soak up too much power on the straights - in the hope that the low pressure zone provided by the collection system will "suck" the machine down onto the track, thus improving front end grip. This is the nearest I can reasonably get to the Chapparal "hoover".

I may even try adding skirts to the cutter cowling to help maintain the negative pressure under it. This could benefit both handling and grass collection performance.

Patrick
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Having enlisted the help of the pit crew (Dick & myself) plus the innovative ideas of your own, you now have the unseen problem of extra fuel consumption.  But on this I have a solution.  This does involve assistance either from your wife (with nothing better to do) or the next door neighbour.  Using a seed hopper from the back of a friendly farmers tractor, position this over your fuel tank (wife/neighbour on a step ladder) and have them swiftly deposit 5 litres of fuel straight into your tank.  If you coordinate both hands and feet you should be able to master throttle, gear change, cutting deck, clutch and bonnet to be able to get this timed to just a few seconds.  Be careful on the restart, as in the excitement the wheelspin could leave very unsightly skid marks in the grass.

weedon

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sorry but you are all wrong the kpi cannot be adjusted only the caster and camber angles can .bending your track rod will only throw out the tracking .so I have come up with a plan first remove the grass cutter from the tractor then the grass box this should solve the oversteer problem and  lighten it enough to give more top speed and better fuel economy.second fit nitrousoxide injection this will give a 60% boost to bhp.

              you will wizzzzzzzzzzzzz round that garden in no time .

     dave

 

 

 

ps you will need to buy a lawn mower though??????

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Dave, it was understeer I was trying to correct, not oversteer. Put simply, the driver turns the wheel, but the front end of the machine tends to continue in a straight line. This can result in high speed "offs". Turning the wheel further can eventually result in turn-in, but the effect is unpredictable, and to spectators, it looks as if the driver is a complete wally. Driver confidence is shattered and lap times naturally suffer.

By the way, I don't understand your last comment. What on earth would I need a lawnmower for?

Today I tried the new systems. Driving gloves and wooden knob on the gear change, skirting added to the bottom edge of the cutter cowling. I practised engaging and disengaging the cutter clutch so that I could swiftly bring the suction into action on corners.

For a couple of laps response seemed to be improved, but soon the weight of the grass sucked up into the collecting bin on the back of the machine, began to affect the fore and aft weight distribution, bringing back the understeer problem.

The session was abandoned to enable further consultation on possible solutions to the problem.

I'm thinking of leaving off the collecting box and letting the grass be blown straight up into the air behind the machine. There might be some downforce to be gained from the reaction to this blasting of waste material upwards. (Newton's law I think) But would the downforce on the rear alone further exacerbate the front-end grip problem?

More thought and research seems indicated.

Patrick

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well Pat as an ex racing mechanic  I should know a little bit I worked for Cooper in the late 70`s for 2 years . your theory on the grass box is good but in reverse , yes remove the grass box but point the blower down this will lift the rear end give more weight to the front and might even give a bit of ground effect i.e. suction to the road/ sorry grass, when you exceed say 70-80 mph.

       on your question of why would i need a lawn mower                          if you do all these mods I suggest

 

             what do you cut the lawn with

                            dave

stand corrected did put oversteer when i should have said the other

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If you would like to go round anti-clockwise move your near-side (in the UK) off side (in France) rear wheel to off-side (in the UK) near-side (in France) front , with me so far.  Now with both big wheels on the off-side (UK) near-side (France) after a slow start you should be able to pick up a bit of momentum and hammer round and should not have to worry about sideways drift on the corners.  The downside is that after a while berms will begin to form on the outsides of the bends. Whilst some  people will think that unsightly its my belief that you are not concerned with, what is in effect big gouges in your lawn, more about how fast your Mr Bricolage Rally can go. The upside is that when the berms are big enough you should be able to go round like a Wall of Death.

Having had my Ackerman poo-poohed I checked with Mr Ackerman (I know him as Alf) and he confirms that on the Mr B Rally you cannot in fact alter the KPI as Dave pointed out, but you can on the John Deere 240. 

weedon 

PS

I also worked for a Cooper in the fifties, but in a brewery making barrels.

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Would these "berms" be something like the banking at Brooklands or Indianapolis?

I like the sound of it, but I fear understeer can be an even more serious problem on a banked circuit, as it can cause vehicle and driver to be launched off the top at high speed.

With the run-off area at the dreaded "Potager" corner now bristling with spiral steel tomato plant supports, it would take a brave man to take the high line on the banking at full "hare" throttle setting.

Anyway, I think the Clerk of the Course might send out the heavy roller to eliminate any berms as soon as they appear.

On a less pessimistic note, the team is considering a variation on the "Rudi Uhlenhaut" weight reduction technique. He it was who stripped the white paint of the pre-war racing Mercedes cars to lighten them, thus creating the legend of the "Silver Arrows". The thought is that stripping the paint of the rear of the bright red Mr. Bric Rally machine might help encourage some controllable oversteer. Of course, we would lose the aft end of a couple of go-fast stripes.

What to do for the best? Decisions, decisions.....

Patrick

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Testing is still impossible because of wet track conditions.

We don't have the budget for the steering wheel I'm afraid. Moreover, there is no tyre sponsor, so no access to compounds of varying hard/softness.

We've looked again at the assymetric configuration proposed by weedon, but we're doubtful as to whether we could actually turn right at the 3 right-handers on the course, at Woodpile, Tree-stump and Bonfire.

Track conditions are not expected to be acceptable for high speed testing this week, so there will be no further bulletins for a while.

However, if you have any better ideas than have already been suggested here, please post them.

Patrick
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It would appear that my people have been in conference with your people Pat, with the outcome that if I divulge anymore development secrets my seat for next seasons series is in Jeopardy.  This I find a little confusing as my seat appears to be intact and Jeopardy cannot be found when I did a search on my 1995 Auto Express Routefinder.  I can only deduce that this may be a red herring! but must now desist from passing on more closely guarded secrets after today.

Having driven my way up through the ranks, starting with a push cylinder model via an Electric Flymo, Petrol Flymo, Suffolk Colt, Qualcast Bushwacker and finally to a Formula 1 Westwood T1600 WITH sequential gearbox AND powered grass collector I find myself in a quandary (you may have one of these between the tree stump, bonfire and woodpile). I have permission to give you one more tuning hint after which I have to call a halt to anymore assistance.

Many times I have heard reference to "building a cushion".  You most probably have heard it yourself "Michael Wosname is in the lead and has his foot to the floor building himself a cushion".  This may be the key you are looking for but as I have no experience in that particular field cannot help you myself but suggest you ask the ladies as soft furnishings are their forte. Perhaps one of the ladies here might have an old copy of Womens Own magazine with a pattern for one, as I have heard that if you read a Teen magazine they describe how to have an orgasm but in the Womens Own you can find a pattern to make your own. 

Best of luck

weedon 

 

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Hi Weedon

           you might be able to help me here, being out of work and at a loose end I am also trying to find Jeopardy there seems to be a lot of work there at the moment , I to have tried route finder too but to no avail , if anyone knows where Jeopardy is please let me know, only last week I see that Rover has 30,000 jobs in Jeopardy  If can get one it would be great ?

              Dave

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For Sale: Mr. Bricolage "Rally" tractor-style lawn mower. One careful owner, some minor modifications including bare metal bodywork at rear, plastic nose cone and cutting cowl skirts. Tecumseh 12 hp overhead valve high performance engine, lightly used. Front tyres well "scrubbed-in" by expert user. Some minor dents and bodywork damage, could easily be removed with a hammer.

N.B. Not suitable for cutting grass. If you want to do this, buy a green one called "Lawnmaster", "Garden Chief", or some other such name.

To be sold with flameproof racing overalls in Mr. Bricolage colours, backless driving gloves, helmet and goggles.

Reason for sale: Neighbour has bought a quad bike and blown us off the track with vastly superior performance.

Once again the big budgets rule the race track!

Patrick
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  • 1 month later...

[quote]For Sale: Mr. Bricolage "Rally" tractor-style lawn mower. One careful owner, some minor modifications including bare metal bodywork at rear, plastic nose cone and cutting cowl skirts. Tecumseh 12 h...[/quote]

For Sale: Mr. Bricolage "Rally" tractor-style lawn mower. Two careful owners, some minor modifications including:

 

bare metal bodywork at rear (previous owner)

ungraded carbon fibre nose cone and cutting cowl skirts.

Tecumseh 35 hp overhead valve high performance engine with recently added t25 turbo and dump valve.

Custom turbo delivery pipe and high flow intercooler.

Large bore exhaust

Recently replaced Front tyres.

larger 23mm rear anti roll bar.

Poly bushes.

Some new bodywork.

 

To be sold with semi-flameproof racing overalls (some tears), one backless driving glove, scratched helmet and broken goggles.

 

Hospital visit forces sale.

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I've been working like a dog "amenage-ing" another gite (the 4th) and had no time to get the famous mower out. The now waist-high grass has made the track unusable, and anyway I had lost interest after seeing the neighbour on his quad roaring round at 4 or 5 times my speed.

I'm now considering the possibility of turning my petrol engined debrousailleuse into a strap-on pusher fan for a home-made hang glider, with lift provided by sails from my two windsurfers, stitched together.

This should provide some material for a thread in the Sports section, in due course.

Has anyone got a Teach Yourself Flying book they don't need?

Patrick
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what a sickener

                           just bought a brand new v-twin dogs whatist honda hydro ect ...  with twin dodas  ect and it under steers like a bar steward and 2 days old got a puncture in the front wheel. had 5 in the wheel barrow so far thats 1 for every week we have lived here went to use it today bloody hell another one

             thorns thorns thorns

                               dave

ps no wonder honda have no points in the G.P. must be the penus

just love that word............

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[quote]what a sickener just bought a brand new v-twin dogs whatist honda hydro ect ... with twin dodas ect and it under steers like a bar steward and 2 days old got a puncture i...[/quote]

Dave,  you want to watch those thorns and the penus, they can give you a nasty prick.

weedon

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