Wheel bolts for a Mondial 3.2 with 348 wheels | FerrariChat

Wheel bolts for a Mondial 3.2 with 348 wheels

Discussion in 'Mondial' started by caterus, Mar 28, 2017.

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  1. caterus

    caterus Karting

    Dec 24, 2010
    60
    Oxfordshire, UK
    Full Name:
    Tim
    Like many Mondial owners, I decided to replace my TRX wheels with 348 wheels assuming, like many users and suppliers, that standard 348 wheel bolts would suffice. But I was wrong and this note may help others avoid my mistake. My Mondial is a 3.2 (Euro Coupe) and I purchased pre-1993 standard 348 wheels. Other versions of the Mondial and 348 wheels may have different specifications and my conclusions may not apply.

    The key measurements are:
    1. Standard TRX wheel bolts (front and rear) have a 40mm shaft (i.e. excluding the bolt head) which protrude 23.7mm beyond a TRX wheel hub face.
    2. Standard 348 bolts (front and rear) have a 56 mm shaft which protrude 21.2mm beyond a 348 wheel hub face.
    3. Mondial 3.2 front wheel hub threads are inset by approximately 12mm while the rear wheel hub threads are inset by 9.5mm

    Consider first the front wheels. Because of the thread inset, a TRX wheel+bolt required about 7 turns to fully tighten (torque 73 lbs/ft) whereas a 348 wheel+bolt required only 5 turns. When fitted I found that a 348 wheel with 215/45 R17 tyre had sufficient clearance from the hubs and coil springs and for left and right full lock. The issue is therefore is 5 turns acceptable? Not according to Hill Engineering who recommend that a wheel bolt should have at least 6-7 turns. Based on my measurements, Hill concluded that a standard 348 bolt was too short to be totally safe and recommended a bolt with shaft length of 58.5mm to ensure full thread engagement.

    Consider now the rear wheels. Here the hub thread inset is approximately 2.5mm less than that of the front hub. A TRX wheel+bolt required about 9 turns to be fully tightened while a 348 wheel+bolt required 7 turns. The standard 348 bolt therefore passes the number of turns test. However, when fitted, the inside edge of a 348 wheel was just rubbing against the coil spring bottom cap (with spring fully extended) compared with a gap of approximately 20mm for a TRX wheel/tyre. Of course when the spring is under load the gap between wheel edge and coil spring will be greater (because the coil spring is angled away from the wheel) so the issue here is what gap is considered reasonable. Adjustment is available by fitting wheel spacers and longer wheel bolts.

    What did I do? Following Hill’s advice (who were extremely helpful and are highly recommended), I purchased 58.5mm bolts for the front wheels (Hill will supply odd length bolts for the same price as a standard bolt). This bolt requires 8 turns to fully tighten at which point the bolt just protrudes beyond the inner hub (about the same as a TRX setup). The decision regarding rear wheels was more difficult and probably more controversial. I decided to fit 20mm spacers and 76mm bolts with 245/45 R17 tyres and this required 8 full turns of the wheel bolt to fully tighten. At this point the gap between inner wheel/tyre edge and coil spring was close to that of a TRX wheel/tyre while the outer tyre edge still had reasonable clearance with the bodywork.

    Fitting 348 wheels to a Mondial 3.2, requires a judgement call relating to wheel bolt length. My judgement is that the use of standard 348 wheel bolts is not advisable and could be dangerous. If others disagree, I would advise very careful evaluation of their reasons.
     
    ellum likes this.

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