Jump to content


Photo
- - - - -

Racer 'unstable' Under Hard Braking !


  • Please log in to reply
27 replies to this topic

#1 Sputnik

Sputnik

    Mini Mad

  • Noobies
  • PipPipPipPip
  • 129 posts
  • Location: Cape Town

Posted 06 May 2012 - 10:48 AM

Hi,

My 1330 racer seems to be unstable when braking hard. That is only at the two corners that I enter at full speed that is approximately 110 mph (not true) That is when braking between the 100 and 50meter mark from the corner.

Some specs:

I have the hard/soft combo tie rod bushes from minispares
Tie rods are modified standard ones to allow to be adjustable. The adjustable part screws in over the tie rods is double the thickness of the standard rods, and the original rods was tapped to fit. (see if I can post a picture tomorrow)
Cones are however old ones (pre 80's)
New set of lower arm niches for change camber
Shocks are gaz adjustable, running on 22 of the 32 clicks at the front. The back is much much softer.
Brake bias at back is nearly nothing (as I rolled my previous car due to back wheel locking up)
Running yoko A008's at 1.7/1.8 bar right around.

I know that I will be told to replace the cones and the tie rods and will appreciate those comments (as i am planning to do it in about two months time, but i would have then competed in two more races) although I will also like to know what else must be checked or looked at.

Also, thinking ahead on my side -
Can it be that the back of the car is to light, lifts and then get unstable?
Can the camber toe and caster setting influence it. Caster to max not to touch STD body work, toe out and 1/16th, camber about 2.5/3deg
Will the ride height play any role in it ?,






#2 leroy26

leroy26

    Up Into Fourth

  • Members
  • PipPipPipPipPipPipPipPipPip
  • 2,239 posts
  • Local Club: minisunleashed.com

Posted 06 May 2012 - 11:01 AM

stoopid question time
back to basic tyre pressures, and wheel balance?
which track are you unstable at?

#3 leroy26

leroy26

    Up Into Fourth

  • Members
  • PipPipPipPipPipPipPipPipPip
  • 2,239 posts
  • Local Club: minisunleashed.com

Posted 06 May 2012 - 11:05 AM

also wind some brake bias to the back!!
you know what happens on a bike when the front brake is applyed to much! :D

#4 Dan

Dan

    On Sabbatical

  • TMF+ Member
  • PipPipPipPipPipPipPipPipPipPipPip
  • 21,354 posts

Posted 06 May 2012 - 11:28 AM

Do you have decent locking nuts on those adjustable tie rods? What length of thread engagement do they have? I'm just wondering if they can shimmy and rattle a bit under heavy load. But yes all the suspension setup will affect the handling under braking as much as at any other time. I'd agree about winding some more effort to the rear brakes, although it depends on how you currently have the split set really.

#5 minidaves

minidaves

    Up Into Fourth

  • Traders
  • PipPipPipPipPipPipPipPipPip
  • 4,933 posts
  • Location: kent
  • Local Club: mine

Posted 06 May 2012 - 01:36 PM

define unstable, if items are pushed to the limit then they will squirm alittle

#6 bugs42

bugs42

    Stage One Kit Fitted

  • Noobies
  • PipPipPip
  • 69 posts
  • Location: Newcastle upon Tyne/Oxford

Posted 06 May 2012 - 01:54 PM

i dont even know if you can get a limited slip diff for a mini, and if you can weather they are ajustable, but IF you have one, diff settings affect braking stability quite alot, especialy on a FF car with little weight on the rear. <--- sorry if i just wasted 5 seconds of your life :P

rear toe settings can effect it, too much toe out/not enough toe in will make it squerm.

if the front is set too soft then it will pitch too far forward and lift the rear up more, but id suggest leaving that for optimum cornering over braking.

and again id say like everyone else brake bias, if the fronts dont want to lock up then move it forward slightly, you might be suprised at how far forward it goes before the fronts lock.

hope ive helped! :)

#7 amarcd

amarcd

    Speeding Along Now

  • Noobies
  • PipPipPipPipPip
  • 434 posts

Posted 06 May 2012 - 01:58 PM

this might sound silly ....but the discs could be warped that would give heavy vibration under hard breaking.

#8 jaydee

jaydee

    Crazy About Mini's

  • Members
  • PipPipPipPipPipPipPipPipPipPip
  • 8,565 posts

Posted 06 May 2012 - 02:08 PM

Warped discs will cause juddering at much slower speeds
Its not clear if hes referring to a judder or a balancing problem

#9 racingbob

racingbob

    Up Into Fourth

  • Members
  • PipPipPipPipPipPipPipPipPip
  • 3,061 posts
  • Location: Hampshire

Posted 06 May 2012 - 03:17 PM

have you taken it in for a full wheel alignment re check

my racer was running 5 deg caster

and yes maybe to much braking on the front

#10 Sputnik

Sputnik

    Mini Mad

  • Noobies
  • PipPipPipPip
  • 129 posts
  • Location: Cape Town

Posted 06 May 2012 - 03:22 PM

Sorry for such long posts, but I am trying to give as much as possible information to get the best possible advice ....

Thanks so far ..

Just to answer some questions, and explain my brake setup.

By unstable I am trying to explain the feeling of the back of the car 'stepping' out under hard braking, to the one side and it feels like to the right first. It feels like if the back is trying to come around ( the same feeling I had when my brakes locked up last time) Naturally I correct it and then it starts moving left to right, until I get of the brakes. you can say the back of the car is swaying, if it is the correct word to use.

At this stage my front brakes Is not locking up. I have a single master cylinder with Cooper s setup with a single line and pressure regulator on the back subframe. The brakes at the back are adjusted so that when the car is on the jacks, and the brake is applied, the back wheel can just, just be turned by hand, as per bill sollis' book.

Can it be that the grip on the back tyres are not as good as it should be ? It seems that the a008's rubber might be a bit on the hard side !

Edited by Sputnik, 06 May 2012 - 03:31 PM.


#11 mini13

mini13

    Up Into Fourth

  • Members
  • PipPipPipPipPipPipPipPipPip
  • 3,811 posts

Posted 06 May 2012 - 03:55 PM

could be rear toe, is it set to toe out? also try skewing the front damper settings... an extra click or two on the driver side.

#12 Wil_h

Wil_h

    Up Into Fourth

  • Members
  • PipPipPipPipPipPipPipPipPip
  • 3,244 posts

Posted 06 May 2012 - 04:42 PM

I's measure the caster and check it is equal both sides. The fact that it is 'set to maximum to avoid the bodywork' is far from ideal. If I set mine to maximum it'd be around 10 degrees. 3.5 to 5 is generally enough. I'd also say that the front camber is a touch on the high side. But the OP seems a bit vage on actual settings.

I'd advise a good geometry check to make sure all is equal side-to-side as a start.

#13 bmcecosse

bmcecosse

    Crazy About Mini's

  • Members
  • PipPipPipPipPipPipPipPipPipPip
  • 9,699 posts
  • Local Club: http://www.srps.org.uk/

Posted 06 May 2012 - 04:57 PM

You must use max rear braking - up to the point of rears 'not quite' locking. This si why it would be adjustable - becuase you will need a little less rear braking in the wet. have you excessive front track ? Wheels with large offset - and hopefully - all this on 10" wheels ? Rear track must NOT be toe-out, parallel or 1/16" toe IN. And a small amount of negative camber on the rear wheels too - maybe 0.5 degree. I think your tyre pressures are far too low anyway!

#14 mike.

mike.

    Crazy About Mini's

  • Members
  • PipPipPipPipPipPipPipPipPipPip
  • 6,176 posts

Posted 06 May 2012 - 05:16 PM

I'd fit some better tie rods anyway, regardless of any problems your having. Standard tie rods are pretty flimsy and flex a lot under heavy braking. For a racer i'd fit some big heavy duty rose jointed versions and reinforce the mounting tabs for them too.

#15 bmcecosse

bmcecosse

    Crazy About Mini's

  • Members
  • PipPipPipPipPipPipPipPipPipPip
  • 9,699 posts
  • Local Club: http://www.srps.org.uk/

Posted 06 May 2012 - 05:21 PM

Tie rods are under tension when braking - standard are fine.... I assume the subby is solid mount ?? And I would have the front dampers are max......... rears to suit.




1 user(s) are reading this topic

0 members, 1 guests, 0 anonymous users