Hex Size Problem - CRC Wheels on Schumacher Shimizu Pre-Glued Tyres
26 August 2023
F1 rear wheel hexes have a diameter of 14mm across all car manufacturers (except the 3Racing FGX). Rear wheels therefore typically have a slightly larger diameter hex in order to fit smoothly over the axles hex.
However, I recently discovered a manufacturing issue with the CRC wheels where the internal hex diameter is less than 14mm, as shown in the photo above.
This meant that when I tried to remove the wheels, they were stuck fast. They wouldn't budge! In the end I had to remove the rear axle, ask a friend to hold the wheel on the edge of a desk, and hit the end of the axle with a metal punch and hammer! That certainly got them out, but it's not the simple process I'm used to when changing tyres :)
I contacted Robin Schumacher, Managing Director of Schumacher Racing, and this was the first they'd heard of this problem (and the wheels and tyres have been available for 3 years). I opened all three packets of XG-575D and XG-572C rear tyres that were on my shelf and measured them. They all have this issue, and one of the XG-575Ds had been sitting in my pit box for more than 12 months.
Robin was very helpful and commented, "I accept that the wheels are undersize, and we are looking into this. Also the X-Ray hex is oversize, so it is a combination of tolerance problems. It is unfair to blame only the wheel."
I then started measuring the rear hexes of my five F1 cars (a mix of Xray and Tamiya) and found that the hexes were not all exactly 14mm and were sometimes slightly larger (by hundredths of a mm). In addition some hexes were not uniform. For example, one Xray rear hex was 14.01mm in one location and 14.04mm in another. Now at this level of measurement it is easy to make errors when measuring by hand, but I agree with Robin that the alloy hexes are not all 14mm. However, they were all at least 14mm and therefore while this might exaggerate the problem with the CRC wheels, the fundamental problem is that the CRC wheel hexes are not large enough.
I then dug out a set of Ride VR, Volante VF1-RARSS and Contact JF1-28RA, all of which are pre-glued tyres. Their hexes were all greater than 14mm and fit my cars without issues.
Robin continues, "We will report the problem to CRC who make the wheels, and see if they can do anything to improve the fit."
In the meantime, if you don't want to encounter these problems, the excellent Schumacher Shimizu tyres are available as tyres only:
- Medium Rear - PS-0572 as tyres only
- Soft Rear - PS-0575 as tyres only
You will need to mount these tyres on another manufacturer's wheels. Exotek make wheels for the Shimizu. The old Pit Shimizu tyres could be mounted on foam wheels, so you may be able to do the same with the Schumacher Shimizu (instructions for Pit Shimizu here).
If Schumacher advise a solution in the future then we will update this article.
NB: this issue only affects rear tyres. The pre-glued front tyres XG-571C are not affected.
Photo below: Shows the wheel hex and how it was measured.
Photo below: Shows an Xray X1 2015 alloy wheel hex and how it was measured.