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ackermann "versus" castor angle


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#1 mariner

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Posted 24 October 2011 - 20:33

As I grew up I "learnt" that ackermann effect didn't matter for race cars because the inside wheel was never loaded at max. lateral G so its direction didn't matter much.

However recently I was re-reading the Allan Staniforth book "Competition car suspension" where there is a chapter explaining how important Ackermann effect is on racing cars!

Staniforth's argument seems to be that strong ackermann helps lateral grip by running the inner wheel at a higher slip angle than the heavily loaded outer wheel, rather than have the slip angles fight each other and this still important at high G

So I learnt wrong (maybe). Staniforth reckons that you actually want ultra ackermann with the steering arms virtually meeting point ahead of the traditional rear axle line meeting point if the rack is mounted well forward of the steering arms.

To be fair his experience is on Uk hillclimbs where sharp corners are frequent ( e.g Prescott).

The other way discussed on this site of getting better results on low speed corners is to use a high castor angle so that the wheels generate negative camber on sharp turns. The downside is that a jacking effect can ocour.

My questions from the above are:-

1) That ackermann is good even on race cars argument is more correct than "no ackermann" of old.

2) If high castor angle helps on the same slow corners as ackermann, which likely to be best

3) Is there any trade off betwen ackemann and castor ( i.e you need to optimise them) or can you use both as much as you like?



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#2 Greg Locock

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Posted 24 October 2011 - 20:52

As I grew up I "learnt" that ackermann effect didn't matter for race cars because the inside wheel was never loaded at max. lateral G so its direction didn't matter much.

My questions from the above are:-

1) That ackermann is good even on race cars argument is more correct than "no ackermann" of old.

2) If high castor angle helps on the same slow corners as ackermann, which likely to be best

3) Is there any trade off betwen ackemann and castor ( i.e you need to optimise them) or can you use both as much as you like?


I have heard of negative positive and zero recommendation for different circuit cars. Bear in mind that static toe is fake ackerman. Fatboy wrote a note about this a long while ago. One use for excessive ackerman is that it helps rotate the car into the corner. One use for negative ackerman is that it allows better matching of the max grip slip angle of the lightly loaded and heavily loaded tire.

Castor is something I'd be inclined to minimise, to around 3-4 deg.



#3 NeilR

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Posted 24 October 2011 - 22:51

I think it is worth recalling where Staniforth is coming from.
Remember that he is first and foremost a journalist and not an engineer, though he is also undoubtedly more experienced than I am and has rubbed shoulders with some good people.
The other aspect is that he was mostly interested in hillclimbs and a good setup for this may not work well on a circuit. Also generalising from one form of car to another is somewhat foolish e.g. a leading car I know of uses substantial toe out, very little ackerman and very large amounts of castor and a spool rear end which seems to work very well for him and how he drives.

#4 gruntguru

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Posted 24 October 2011 - 23:07

Simple things like a spool rear end will have a dramatic effect on what you do with ackermann, caster and toe. eg the jacking resulting from lots of caster and scrub radius is useful for unloading the inside rear on turn-in. This is essential to overcome the initial understeer inherent in a spool.

#5 NeilR

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Posted 25 October 2011 - 01:13

Simple things like a spool rear end will have a dramatic effect on what you do with ackermann, caster and toe. eg the jacking resulting from lots of caster and scrub radius is useful for unloading the inside rear on turn-in. This is essential to overcome the initial understeer inherent in a spool.



which is why I was using it as an example - one end will influence the other as part of the overall system. I find Beckman's Physics of racing more beneficial than Staniforth's books, though I do still really like staniforth's race and rally car source book.

#6 kikiturbo2

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Posted 25 October 2011 - 23:56

Last fast westfield I ran on track actually had anti ackerman.. was a bi%^h to push around paddock.. However, we took it on a really tight track, more like a big kart track, and we had to dial major toe out to get it to turn into corners.. Was fun experiment though..

#7 Lukin

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Posted 26 October 2011 - 07:47

My understanding is that you need the tyre data to know what is important and both of them work on different characteristics of the tyre.

Principally:
- How much Ackerman you need depends on the peak slip angle characteristics between high and low vertical load.
- How much caster you need depends on the optimal peak camber (vs. lateral force) for high and low vertical loads.

Secondary considerations have been outlined above, also can look at:
- Tyre construction; if blistering/damaging tyres is an issue on long straight, add more caster to get camber with steer
- Circuit Type; if longitudinal acceleration needs to be maximised running a lot of camber is a hinderence
- More caster can make it jack and disturb the contact patch loads on transient manouvres
- Braking Stability and tyre drag can be a function of the toe angle which is, as Greg said, a poor mans version of Ackerman

In essense, I think of Ackerman and Caster as ways of optimising the tyre grip between the inside and outside tyres when cornering. They are non-linear approaches to deal with the non-linearity of tyres.

They will be somewhat related, but everything is related when the aim of the game is to work through four small contact patches.

#8 Greg Locock

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Posted 26 October 2011 - 08:53

So the optimum ackerman and castor settings depend on the weight transfer, and the weight transfer depends on the max latacc, and the max latacc depends on the ackerman and castor settings.

grins.


#9 cheapracer

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Posted 26 October 2011 - 12:25

Principally:
- How much Ackerman you need depends on the peak slip angle characteristics between high and low vertical load.


Mind boggles; surface grip, surface temp, aspect ratio, sidewall construction, tyre pressures, anti-roll, wet or dry etc, all factors that determine ideal Ackerman.




#10 Pat Clarke

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Posted 26 October 2011 - 14:35

Neil,

You write about Stanny in the present tense. Unfortunately, he left us over two years ago and is sadly missed, especially in FSAE circles =(
http://www.yorkshire...forth_1_2347527

Pat

#11 MatsNorway

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Posted 26 October 2011 - 14:44

Ackermann is huge in rc car racing.

http://www.teamxray....fb53890c77cbfcf

Picture:
http://www.serpent.c...1...14&size=350

http://awesomatix.co...ringsystem.html


Edited by MatsNorway, 26 October 2011 - 14:46.


#12 cheapracer

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Posted 27 October 2011 - 06:48

Ackermann is huge in rc car racing.


Not surprising with little weight transfer, miniscule slip angles and wide tracks relative to wheelbase.


#13 Peter Leversedge

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Posted 27 October 2011 - 08:45

In dirt sprint cars I used to run no Ackerman and a lot of King pin inclination , 9 to 10 degrees [ with a locked rear axle ] as the car started to side and you turned the wheel to the right it loaded up the left rear wheel and helped straighten up the car.....................
You need to be real fit or have power steering as most do have now days

#14 NeilR

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Posted 27 October 2011 - 11:32

Neil,

You write about Stanny in the present tense. Unfortunately, he left us over two years ago and is sadly missed, especially in FSAE circles =(
http://www.yorkshire...forth_1_2347527

Pat



Quite right. Perhaps because his (like many authors) work lingers we discuss them as though they have never let us.

#15 MatsNorway

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Posted 27 October 2011 - 16:27

How do F1 teams go about ackermann and caster?

#16 MatsNorway

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Posted 27 October 2011 - 16:58

Not surprising with little weight transfer, miniscule slip angles and wide tracks relative to wheelbase.


not entirely. they slide around a bit. espesially on foam tires.. they don`t lose grip the same way as rubber tires does.
http://www.youtube.c...x-kazVYo#t=102s watch corner exit.

And if you look on this video you will se the touring cars got not that half bad amount of body roll going. they use the roll to get extra grip. Not fairly common to have traction roll on carpet tracks.


But yea. the more the tire prep in the carpet/tarnac the lower and stiffer they get due to traction roll issues.

Great race from the World championship http://www.youtube.c...;feature=fvwrel

#17 munks

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Posted 27 October 2011 - 17:56

Great race from the World championship http://www.youtube.c...;feature=fvwrel


That was a really good race. I'm also impressed by the driving skill ... last time I watched an RC race it seemed quite amateurish but these guys were incredibly precise.

#18 Tony Matthews

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Posted 27 October 2011 - 20:14

Quite exhausting - I bet the cameraman got a stiff neck.

#19 NeilR

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Posted 27 October 2011 - 22:53

How do F1 teams go about ackermann and caster?


To be perfectly honest I have no idea. I do know a couple of recently ex-F1 engineers and I suppose I could ask them, though they were primarily aero guys. I suspect that both are not critical to the planning of an F1 car given the need to perfect aero and tyre wear etc. Actually that would be interesting: the change in slip angles with increasing aeroloads and the overall effect on tyre wear.

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#20 MatsNorway

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Posted 28 October 2011 - 08:55

Quite exhausting - I bet the cameraman got a stiff neck.


You should try the onboard cam


NeilR

It would be interesting to hear from them.

#21 Tony Matthews

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Posted 28 October 2011 - 10:19

You should try the onboard cam

That's what it looks like from the passenger seat of my van. Anyone want a lift?

#22 gruntguru

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Posted 28 October 2011 - 10:43

I'm dizzy.

#23 cheapracer

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Posted 28 October 2011 - 18:15

How do F1 teams go about ackermann and caster?


One of the English F3 manufacturers let it out they run 100% Ack which surprised a few.

KPI and caster wouldn't be too hard to work out from a few good wheels off pictures.

You might be able to roughly determine Ack from some overhead Monaco shots too.

Edited by cheapracer, 28 October 2011 - 18:16.


#24 MatsNorway

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Posted 28 October 2011 - 20:43

One of the English F3 manufacturers let it out they run 100% Ack which surprised a few.

KPI and caster wouldn't be too hard to work out from a few good wheels off pictures.

You might be able to roughly determine Ack from some overhead Monaco shots too.



cool stuff. i would like to hear some filosofy around it too. by running more achermann they get more steering what do they do with the weight balance? more to the front then?

actually when i said more to the front i imagined red bull as they have their pods furhter forward. Not that its relevant to weight but im thinking the unavoidable lift from the pods has been moved as far forward as possible to give the rear more downforce. And clean air ofc. the front wing allready produces engough downforce to counter the mandatory lift from the sidepods.

i guess the best counter to understeer should be to first fix it with mechanical setup chances if possible.

Where is that fatboy??
and DaveW they should know a thing or two.

Edited by MatsNorway, 28 October 2011 - 20:44.


#25 NTSOS

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Posted 06 November 2011 - 17:00

Just out of curiosity and as it relates to suspension geometry for an efficient, straight towing trailer...........getting conflicting opinions on the necessity of toe-in for the proper tracking of a SINGLE axle trailer.....some say it's not required, others say a slight toe-in is always desirable.......waaah!

How about the importance of positive caster and is camber even a consideration for straight tracking?

Thanks!

John

#26 cheapracer

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Posted 06 November 2011 - 17:30

Just out of curiosity and as it relates to suspension geometry for an efficient, straight towing trailer...........getting conflicting opinions on the necessity of toe-in for the proper tracking of a SINGLE axle trailer.....some say it's not required, others say a slight toe-in is always desirable.......waaah!

How about the importance of positive caster and is camber even a consideration for straight tracking?

Thanks!

John


Toe in.

Caster not important as you have a couple of meters of trail already from your coupling to the tyre contact patch.

Neg Camber helps a little with stability over bumps.




#27 NTSOS

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Posted 06 November 2011 - 17:55

Toe in.

Caster not important as you have a couple of meters of trail already from your coupling to the tyre contact patch.

Neg Camber helps a little with stability over bumps.


Ok, interesting.......thanks for your response Cheap! :up:

John

#28 Tenmantaylor

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Posted 06 November 2011 - 20:08

I used to race 1/10th RC buggies (off road) and ackerman was quite critical. My memory is that it made most difference in the tighter corners when at or near full lock (the mechanism of the steering arms could be altered to change the amount of ackerman). It was usually desirable to have a high ackerman setting as the unloaded inside wheel could still scrabble and provide alot of grip. Negative ackerman was a big no no.

#29 gruntguru

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Posted 06 November 2011 - 22:39

Just out of curiosity and as it relates to suspension geometry for an efficient, straight towing trailer...........getting conflicting opinions on the necessity of toe-in for the proper tracking of a SINGLE axle trailer.....some say it's not required, others say a slight toe-in is always desirable.......waaah!

How about the importance of positive caster and is camber even a consideration for straight tracking?

If you use your friend's fancy tow hitch, you can use any wheel alignment settigs you like.

#30 NTSOS

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Posted 06 November 2011 - 23:05

If you use your friend's fancy tow hitch, you can use any wheel alignment settigs you like.


Ok good Grunt, now please tell me more.....what friend's fancy tow hitch?

You mean something like this perhaps?

Friends Fancy Tow Hitch?

or

Friends Fancy Tow Hitch2?

or

Friends Fancy Tow Hitch3?

I didn't even know he had a trailer?

Confusion ???

Thanks!

John



#31 gruntguru

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Posted 07 November 2011 - 02:02

No, it was friend's fancy tow hitch #4.

I checked back to YOUR POST and realised it was your friend's recommendation - not his hitch - sorry for the confusion. :)



#32 Ray Bell

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Posted 17 November 2011 - 03:02

Originally posted by cheapracer
.....Neg Camber helps a little with stability over bumps.


Hmmm...

I've seen trailers that have gained that negative camber through bumps!

#33 Lee Nicolle

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Posted 18 November 2011 - 10:02

Hmmm...

I've seen trailers that have gained that negative camber through bumps!

Me too, and they generally gain negative camber as the axle flexes with the load
Start square and try and keep it there. But a 2" squ axle sill flexes a lot.

#34 Lee Nicolle

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Posted 18 November 2011 - 10:08

In dirt sprint cars I used to run no Ackerman and a lot of King pin inclination , 9 to 10 degrees [ with a locked rear axle ] as the car started to side and you turned the wheel to the right it loaded up the left rear wheel and helped straighten up the car.....................
You need to be real fit or have power steering as most do have now days

Power steering is for girls!
But I agree with the rest. My old Supermod is about 5-7 caster with very little ackerman. When I tried it it was so bloody heavy to sterr,, and very twitchy too. so back to what is had
Most modern Sprintcars though use no ackerman and plenty of caster. Some use axles with little caster on the l/f. as does my old mod.

#35 NTSOS

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Posted 18 November 2011 - 23:10

Hmmm, I wondering if the frame attachment points for a long trailing arm suspension and/or frame doesn't flex with a solid 2" diameter axle with independant suspension.....does that not mean that a bump induced incidence will not cause a great degree of camber change? I am thinking of using a long trailing arm suspension design with air bags because I want the the rear end of the trailer to kneel because I'm getting old and don't need the drama of loading my '56 chevy.....have no clue about building a trailer, but I'll just try to do it intuitively/overbuild as opposed to using proper engineering tecniques and calculations....it's just easier to overbuild it. You know it's amazing to me as I'm getting to the point of old age that I would rather drink and talk about it, as to opposed to actually doing it.....dealing with mortality is sad in a way.....sorry, lost my head!

#36 Tony Matthews

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Posted 19 November 2011 - 00:50

Understood!

#37 NTSOS

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Posted 19 November 2011 - 01:46

Understood!


Thankyou so much for your comment/observation sir!

John

#38 cheapracer

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Posted 19 November 2011 - 07:18

Me too, and they generally gain negative camber as the axle flexes with the load
Start square and try and keep it there. But a 2" squ axle sill flexes a lot.


I think Ray was referring to older decrepit trailers that end up with neg camber over the years.

Neg camber is good because the center of the tyre's contact patch on the bump is inside the tyre's center of contact patch on the road and wants to turn the tyre inwards.


#39 Catalina Park

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Posted 19 November 2011 - 10:16

Last pair of trailer axles I got I ordered with 1° positive camber. So when loaded they were around zero degrees.

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#40 cheapracer

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Posted 19 November 2011 - 12:12

Last pair of trailer axles I got I ordered with 1° positive camber. So when loaded they were around zero degrees.


Sure but that's a tyre wear question answer not a stability question answer.

#41 Lee Nicolle

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Posted 19 November 2011 - 20:41

Hmmm, I wondering if the frame attachment points for a long trailing arm suspension and/or frame doesn't flex with a solid 2" diameter axle with independant suspension.....does that not mean that a bump induced incidence will not cause a great degree of camber change? I am thinking of using a long trailing arm suspension design with air bags because I want the the rear end of the trailer to kneel because I'm getting old and don't need the drama of loading my '56 chevy.....have no clue about building a trailer, but I'll just try to do it intuitively/overbuild as opposed to using proper engineering tecniques and calculations....it's just easier to overbuild it. You know it's amazing to me as I'm getting to the point of old age that I would rather drink and talk about it, as to opposed to actually doing it.....dealing with mortality is sad in a way.....sorry, lost my head!

If you want to carry a 56 Chev you need a tandem axle trailer. Use rocker or better rocker and roller springs with 2" solid axles. And aprox 7" long drawbar and you will have something that tows really well and backs well too.

#42 NTSOS

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Posted 19 November 2011 - 22:49

If you want to carry a 56 Chev you need a tandem axle trailer. Use rocker or better rocker and roller springs with 2" solid axles. And aprox 7" long drawbar and you will have something that tows really well and backs well too.


Thank you for your input Lee, have to lookup what a rocker roller setup is!

John

#43 Tony Matthews

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Posted 20 November 2011 - 10:00

Sounds like a bit of a Gene Vincent to me.

#44 24gerrard

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Posted 20 November 2011 - 14:30

Sure but that's a tyre wear question answer not a stability question answer.


Which only goes to show that if you can get it to move the right way it will meet all your needs.
Geometry Holmes, Geometry.
Front wheel drive racing minis with variable geometry rear ends need from 1.5 degrees to zero degrees toe out and positive ackerman.
2.5 degrees of caster will see the inside rear wheel off the ground if you are capable of driving it fast enough.
I f you are unfortunate to still have a subframe rear end, drill the shaft outside slots to give 1.5 negative and 1.5 toe out.
Same effect but with added rear tyre wear.

F1 everything is done for aero, any real development for mech handling stopped decades ago.
So long as the tyres dont fall off the rims thats all they care about.
Hmm Vettel might be on to something.

Oh yea and whether the driver can drive it also comes into it.

Edited by 24gerrard, 20 November 2011 - 14:31.


#45 Lee Nicolle

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Posted 20 November 2011 - 21:54

Thank you for your input Lee, have to lookup what a rocker roller setup is!

John

Rockers are a simple pivot on the rear of the front springs that locate the front of the rears. Rocker and rollers are a sexier version that ride and tow even better. You can have a slipper spring that tows well, but they will always tow 'heavy' and transmit a lot of bumps back through the towcar whereas any rocker style minimises that and they tow lighter and are easier on the tyre casings, less likelyhood of blowouts BUT they are no easier on tyre tread as any theoretical caster moves quite a lot.
I tow mine at 120kmh all the time even on secondary [paved] roads and you arrive relaxed, not stressed from hanging on to the towcar. And it is ultimatly a lot better for the towcar as you are not transmitting all those bumps through it. And easier on the car being carried as it has not been shaken and stirred on the trip!

#46 Lee Nicolle

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Posted 20 November 2011 - 21:55

Which only goes to show that if you can get it to move the right way it will meet all your needs.
Geometry Holmes, Geometry.
Front wheel drive racing minis with variable geometry rear ends need from 1.5 degrees to zero degrees toe out and positive ackerman.
2.5 degrees of caster will see the inside rear wheel off the ground if you are capable of driving it fast enough.
I f you are unfortunate to still have a subframe rear end, drill the shaft outside slots to give 1.5 negative and 1.5 toe out.
Same effect but with added rear tyre wear.

F1 everything is done for aero, any real development for mech handling stopped decades ago.
So long as the tyres dont fall off the rims thats all they care about.
Hmm Vettel might be on to something.

Oh yea and whether the driver can drive it also comes into it.

On Red Bull the tyres have fallen off almost literally this year. Spa in particular.

#47 Greg Locock

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Posted 21 November 2011 - 01:05

Rockers are a simple pivot on the rear of the front springs that locate the front of the rears. Rocker and rollers are a sexier version that ride and tow even better.

http://www.alko.com....ollerRocker.pdf

#48 Lee Nicolle

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Posted 21 November 2011 - 02:38

http://www.alko.com....ollerRocker.pdf

Thanks Greg, that is the roller and rocker type. As most will understand it spreads the load far more evenly over both axles which improves ride for towed and towcar.

#49 NTSOS

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Posted 21 November 2011 - 14:35

Rockers are a simple pivot on the rear of the front springs that locate the front of the rears. Rocker and rollers are a sexier version that ride and tow even better. You can have a slipper spring that tows well, but they will always tow 'heavy' and transmit a lot of bumps back through the towcar whereas any rocker style minimises that and they tow lighter and are easier on the tyre casings, less likelyhood of blowouts BUT they are no easier on tyre tread as any theoretical caster moves quite a lot.
I tow mine at 120kmh all the time even on secondary [paved] roads and you arrive relaxed, not stressed from hanging on to the towcar. And it is ultimatly a lot better for the towcar as you are not transmitting all those bumps through it. And easier on the car being carried as it has not been shaken and stirred on the trip!


With your detailed explanation and Gregg's link, I've got it......looks to be fairly easy to implement too........cool, thanks all so much! :up:

John

#50 Lee Nicolle

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Posted 21 November 2011 - 21:08

With your detailed explanation and Gregg's link, I've got it......looks to be fairly easy to implement too........cool, thanks all so much! :up:

John

I buy the complete assembly and just weld the brackets on the chassis and clamp the axles on. I have built 3 and rebuilt 1 car trailer and they have evolved over the years.
The other good thing about a rocker set up is that with about 8 ft ramps the back of the trailer droops enough when loading that is is a straight run up onto the trailer so you never hook exhausts etc. That with an 800k dirt car or a 2 ton Ford Galaxie or Landcruiser.
The springs generally come in 1.5 ton, 2 ton or 2.5 ton and I think larger too. I use 2.5 ton, though the last one had 2 ton and rode that bit better with the light car on it.And was a bit dohy with the big car!