ZXR750 'H' Models Rear end weight loss

[ZXRRDave]

evening all. Some of you may have spotted my build thread on my ZXR750H1 Thunderbikes racer, to which I have already completed the major suspension work. Herein is a guide for rear end weight loss the Dave way. (the front end is now ZX7R in line with Bannon's Excellent thread in this guide section).

First up in the process was to weigh the existing H1 ZXR rear end. When the (extraordinarily high tech) method of plonking it on the bathroom scales was used it showed that the back end assembly of Swingarm, Wheel, Sprocket and carrier with chain (but no rear caliper) weighed in at a hefty 33ish Kilo's.

The bike I as collected rather helpfully came along with a swingarm for an H2 ZXR, which is A: Much prettier B: Slightly shorter (speeding up the handling) and C: a bloody sight lighter. Here's the 2 side by side.

Using the aforementioned bathroom scales showed there to be a 5KG difference just in the 2 swingarms. fortunately the H2 arm bolts straight into the H1 frame, so no mods required at that stage. However, I wanted to lose more weight - and the fact you can reliably moor a reasonably large boat with either of the standard wheels on a ZXR made that a good place to start.

Looking around, It was clear that the 5.5" rims for the G/J/A1p model ZX6R were both plentiful and cheap (about £40 plus postage gets you a good condition one with a disc, and about another tenner gets you the sprocket carrier) there seemed to be a reliable assertion from most people 'in the know' that the said 6R wheels were also a fair bit lighter. A 6R wheel was duly purchased and fitted with the part worn slick acquired with the bike for shakedown and build purposes and weighed against the ZXR wheel (fitted with a BT014 road tyre). This comparison (wheel, disc, Sprocket carrier) showed a weight of approximately 18kg for the ZXR assembly and 12.5kg for the 6R setup. Another 5 and a half kilos!

So, setting about the task of mating the 6R wheel to the ZXR swingarm the list of requirements were as follows:

Decide what Caliper will be used and sort a hanger

Sort out spacers

Sort out spindle difference (6R uses 24mm Spindle, ZXR 20mm)

work out chain alignment

Read on in Part 2!

[ZXRRDave]

So, the first challenge was the difference in Spindle Size. As mentioned, the 6R uses a 24mm Spindle, whilst the ZXR swingarm is set up for a 20mm Spindle. Since I already had an H2 Spindle and adjuster blocks it seemed silly to try and do a ton of machining to the arm and blocks to get a 6R spindle in (which I would have first had to acquire) so a quick head scratching session with the wheels, a set of vernier calipers and a notebook got the dimensions for the bearings in both the ZXR and 6R wheels, which then gave an inner, outer and width dimension for the bearings needed to sleeve the wheel down.

Those dimensions were entered into the handy ready reckoner on this site - which gave a bearing code of 63042RSC3 (20x52x15). I popped down the road to the helpful guys at 'Bearing Supplies (Norwich) Ltd' who had plenty in stock and bought enough of them to modify 2 rear wheels (dry and wet race wheels).

Measuring the bearings as they came showed that the inner race was large enough to meet the spacer that runs through the middle of the wheel between the bearings and provides lateral support to the centre of the bearing when the wheel is torqued in by the spindle and spacers, so no new spacer was required there.

Next on the list were spacers. A set of 6R spacers were sourced from Ebay, along with a 6R caliper mount for dimension checking purposes. Fortunately, when slotted into the wheel and slipped in to the swingarm it transpired that the 6R Spacers, Wheel, Sprocket carrier and caliper hanger were exactly the right width to fit the swingarm perfectly.

The assembled swingarm, wheel, Disc and sprocket carrier with spindle and spacers were weighed at this point and showed a weight of 20.5 kilos. Admittedly this didnt include the chain which was still attached to the original arm when weighed but it was a huge step forward.

Buoyed up by this piece of good fortune with the spacers widths, and aware of the fact that the ZXR caliper hanger didnt sit onto the 6R wheel/disc in the slightest, it was decided that further investigation into mating the 6R rear caliper to the ZXR swingarm would be the way forward. A discussion with Chris Sadd at CS Engineering in Norwich showed that there would be no trouble in effecting this conversion.

Now, the ZXR uses a Honking great Torque arm with a truck sized Opposed 2 piston caliper on the rear end which is utterly unnecessary for racing, whilst the 6R uses a reasonably svelte single piston sliding caliper, with a hanger that engages on a boss on the inside face of the swingarm - thus eliminating the torque arm.

Moving the spindle as far forward in its adjusters as it would go showed that the Caliper mount just collided with the inside face of the swingarm. The caliper mount was therefore chamfered slightly at the front end to negate this, as the socket the boss engages in is very deep - deep enough that losing about 3mm from the extreme of it's adjustment was not a concern. Chris carefully measured and aligned the boss on the inner face of the arm to allow the wheel assembly to move smoothly back and forth without snagging at an angle, then welded it in place.

He then machined 2 steel inserts to fit inside the 6R spacers and sleeve them down to the ZXR spindle - this also had the effect of making the spacers captive to the caliper mount and sprocket carrier when slid into place - helpful to prevent a spacer bouncing away across the paddock during a hasty wheel change if it starts pissing down 5 minutes before a race.....

With all this completed, the wheel was fitted into the swingarm and a 6R caliper, newly rebuilt with fresh seals and pads, was fitted. This done, a straightedge and laser showed that there appeared to be no runout in the newly fitted Renthal 520 racing sprockets - borne out by fitting the new Tsubaki lightweight 520 racing chain and it turning smoothly in line. You have GOT to love how kawasaki are so anal about using the same dimensions and alignments across bikes which are 10 years apart.

So, based on approximate figures, the rear end swap has saved a massive amount of weight - somewhere in the region of 13-14 kilos once the elimination of the Tonka Toy style Torque arm and honking great caliper are totalled up along with the savings on wheel, swingarm and lightweight chain and sprockets.

Its relatively safe to say I'm quite chuffed with it.

[Ascalon]

Fantastic work there mate. That's given me some pointers for weight saving on my J1.

However, I did have to fix a sentence for there - no charge.

You have GOT to love how kawasaki are so GOOD about using the same dimensions and alignments across bikes which are 10 years apart because of a tried and trusted design that needs no updating.

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