Tyre load index question

[QBee]

I am struggling to understand the meaning of the load index, not in general terms (the bigger the number, the greater the weight the tyre can carry), but in precise terms, ie for example, can I use an 83 or 87 load index tyre on my Chimaera, which are rated for around 500 kg, and what are the consequences of getting it wrong?

Also, does the load index requirement go up if the car is driven hard, for example on a sprint or track day?

I really need a tyre technocrat on this one please, or a top omniscient like Chimp-on-Gas

[spend]

QBee said:

I am struggling to understand the meaning of the load index, not in general terms (the bigger the number, the greater the weight the tyre can carry), but in precise terms, ie for example, can I use an 83 or 87 load index tyre on my Chimaera, which are rated for around 500 kg, and what are the consequences of getting it wrong?

Also, does the load index requirement go up if the car is driven hard, for example on a sprint or track day?

I really need a tyre technocrat on this one please, or a top omniscient like Chimp-on-Gas

Several tables, may vary a bit between manufacturers, I keep this as basic info:

Load index Load in kg per tyre
65 290
66 300
67 307
68 315
69 325
70 335
71 345
72 355
73 365
74 375
75 387
76 400
77 412
78 425
79 437
80 450
81 462
82 475
83 487
84 500
85 515
86 530
87 545
88 560
89 580
90 600
61 615
92 630
93 650
94 670
95 690
96 710
97 730
98 750
99 775
100 800
101 825
102 850
103 875
104 900
105 925
106 950
107 975
108 1000
109 1030
110 1060
111 1090
112 1120
113 1150
114 1180
115 1215
116 1250
117 1285
118 1320
119 1360
120 1400
121 1450
122 1500
123 1550
124 1600


Basically you cant get tyres with a low enough rating for the TVR's as they pretty much have a FOS built in, as low as you can get tends to be better as you get a nice compliant sidewall that probably does more damping than your expensive dampers as well as smoothing out road tracking as well as the undulations + some feel before it breaks away... It really has to be looked at in view of the sizes / profiles you need and the specific tyre (they aren't all the same & you can't afford to test each one!).

[QBee]

Thanks Dave. That reassures me. I have Federal 595 RS-Rs on at the moment, load indices 83 on the front and 87 on the rear, whereas my Toyo 888s are 91. The Federals do feel nice and compliant and are great on the road, but also handle well on track.

Your comments about ride quality do ring a bell with me. With the Toyos on the car, or my previous road tyres, which were also 91 load index, if i came away from a track day and forgot to soften the front dampers down from rock setting to sofa setting, the car felt awful all the way home, so much so that i had to get out and soften them off. With the Federals on the car I have had to leave them on rock setting, as my poor old fingers cannot shift the tiny damper adjusters, but the ride home has been ok. And as track tyres they felt fine, I was overtaking a Cerbera 4.5 at Cadwell Park that was being campaigned on Toyo 888s, so no complaints on the relative amounts of grip. I would have to look at a video of the car to see if the front dives more under braking than on the Toyos, as the car felt planted and predictable, not a killer TVR.

[Classichim]

The ideal track conditions would be as high as possible but with a high grip rating, that way the tyre wouldn't over heat when put to the extremes of track use.
If you have say 270kgs on the front tyres when stationary imagine the load under hot track use on the outside tyre, double that I'd imagine.
That said the Federals get totally abused in the Mazda super cup cars into the turns. Some guys don't brake just pummel it into the turns on the front end, under steer/ oversteer at every opportunity and the tyres still ok.
Look new when they come back in.
Out of interest the Mazda is the same basic weight of a Chimeara and the fronts are set to about 26 psi when cold, tyre pressure directly after a race is about 34, anymore and they just fall away.
Hope that helps.

[Classichim]

34 psi when hot is the perfect pressure for max grip. If you don't have a tyre temp gauge it's the easiest way to know if your getting the best out of the tyre,
A high degree of camber and you can attack the tyre into corners without rounding the edge.
Once the edge goes the tyre is always slower!
Two or three heat cycles and the tyre is ready to go and hangs on for tons of laps.

[QBee]

Interesting, thanks Alun