Vibration........

[Rob's Lexx]

Hi guys,

Just got back from a round trip from Leicester to Norwich.

On my way there, all was o.k until 2 hours into the journey when the car started vibrating pretty noticeably when braking. It is more noticeable when slowing down from speeds over 65 mph and the vibration kicks in between 62-5 mph.

The vibration is felt through the stearing wheel and the gearstick jerks around like its possessed :o

there's no pulling to one side or vibrating when simply driving along,.....its only there when i brake.

can anyone diagnose the issue?

i was thinking warped front discs? but could there be another cause worth thinking about??

cheers in advance.

rob :)

[Tony-Bones]

A definitive reason.

THE NATURE OF BRAKING FRICTION

Friction is the mechanism that converts dynamic energy into heat. Just as there are two sorts of friction between the tire and the road surface (mechanical gripping of road surface irregularities by the elastic tire compound and transient molecular adhesion between the rubber and the road in which rubber is transferred to the road surface), so there are two very different sorts of braking friction - abrasive friction and adherent friction. Abrasive friction involves the breaking of the crystalline bonds of both the pad material and the cast iron of the disc. The breaking of these bonds generates the heat of friction. In abrasive friction, the bonds between crystals of the pad material (and, to a lesser extent, the disc material) are permanently broken. The harder material wears the softer away (hopefully the disc wears the pad). Pads that function primarily by abrasion have a high wear rate and tend to fade at high temperatures. When these pads reach their effective temperature limit, they will transfer pad material onto the disc face in a random and uneven pattern. It is this "pick up" on the disc face that both causes the thickness variation measured by the technicians and the roughness or vibration under the brakes reported by the drivers.

With adherent friction, some of the pad material diffuses across the interface between the pad and the disc and forms a very thin, uniform layer of pad material on the surface of the disc. As the friction surfaces of both disc and pad then comprise basically the same material, material can now cross the interface in both directions and the bonds break and reform. In fact, with adherent friction between pad and disc, the bonds between pad material and the deposits on the disc are transient in nature - they are continually being broken and some of them are continually reforming.

There is no such thing as pure abrasive or pure adherent friction in braking. With many contemporary pad formulas, the pad material must be abrasive enough to keep the disc surface smooth and clean. As the material can cross the interface, the layer on the disc is constantly renewed and kept uniform - again until the temperature limit of the pad has been exceeded or if the pad and the disc have not been bedded-in completely or properly. In the latter case, if a uniform layer of pad material transferred onto the disc face has not been established during bedding or break-in, spot or uncontrolled transfer of the material can occur when operating at high temperatures. The organic and semi-metallic pads of the past were more abrasive than adherent and were severely temperature limited. All of the current generation of "metallic carbon", racing pads utilize mainly adherent technology as do many of the high end street car pads and they are temperature stable over a much higher range. Unfortunately, there is no free lunch and the ultra high temperature racing pads are ineffective at the low temperatures typically experienced in street use.

Therefore - there is no such thing as an ideal "all around" brake pad. The friction material that is quiet and functions well at relatively low temperatures around town will not stop the car that is driven hard. If you attempt to drive many cars hard with the OEM pads, you will experience pad fade, friction material transfer and fluid boiling - end of discussion. The true racing pad, used under normal conditions will be noisy and will not work well at low temperatures around town.

Ideally, in order to avoid either putting up with squealing brakes that will not stop the car well around town or with pad fade on the track or coming down the mountain at speed, we should change pads before indulging in vigorous automotive exercise. No one does. The question remains, what pads should be used in high performance street cars - relatively low temperature street pads or high temperature race pads? Strangely enough, in my opinion, the answer is a high performance street pad with good low temperature characteristics. The reason is simple: If we are driving really hard and begin to run into trouble, either with pad fade or boiling fluid (or both), the condition(s) comes on gradually enough to allow us to simply modify our driving style to compensate.

Topic taken from the wim forum..... Hope it helps.

[Rob's Lexx]

so are you saying its likely to be warped discs caused by abrasion from the pads?

could it be anything else like dodgy caliper etc?

cheers

[Tony-Bones]

so are you saying its likely to be warped discs caused by abrasion from the pads?

could it be anything else like dodgy caliper etc?

cheers

Unlikely for such a young car.

[Rob's Lexx]

so more likely to be the discs that need replacing?

[Rikos]

You can get front discs and pads for £90 off eBay mate

[Rob's Lexx]

you got the link? cheapest i've found was mintex at about £115 + 15 postage

[Rikos]

The ones for £115 are the same prices have gone up.

There are some others on there as well

http://search.ebay.co.uk/brake-discs-lexus...PageNameZRC0022

Hope link works

[Rob's Lexx]

The ones for £115 are the same prices have gone up.

There are some others on there as well

http://search.ebay.co.uk/brake-discs-lexus...PageNameZRC0022

Hope link works

cheers mate but don't think i'll be putting 'Allied Nippon brakes' on the car.....might stick to mintex.

found this dealer but some bad feedback.....should i be weary??

LINKY

[el_gencer]

My car did something simillar, at around 60-70mph. I changed the front discs and that sorted the strange vibrations. I looked at the discs but they didnt look warped, in the end i thought sack it and changed them any way. I put DBA 4000's on, it was an expensive choice to make and would recommend that you have a garge have a quick look you discs before purchasing anything. Quick Fit does a free brake check i believe. However i wouldn't buy brakes from them, they'd probably be expensive.

[Rob's Lexx]

my next door neighbour is a mechanic and between us i'm sure we can jack the car up and spin the wheels and look for any sign of warping - problem is that its often not visible!!

[Rikos]

I agree wouldnt go for the allied nippon things i'm gonna go for mintex and £115 for front set is a bargain!!

But those discs on link look pretty good and the dealer has 98.1% positive feedback so thats not bad.

Have a look at what the bad feedback is?

Hope you get it sorted soon though mate pretty sure mine are gonna need changing as i've got same problem.