Lightweight Final Drive - 525 to 520 Conversions

[Shazaam]

This topic comes up regularly and it is always controversial.

One side argues that ANY reduction in weight is desirable while the other side argues that reducing the factor of safety in the final drive components is unwise. It’s important to note also that, after air filters and exhaust systems, 520 conversions are one of the biggest money-makers in the after-market.

So I’ll leave it to you to decide, but the argument goes something like this.

Pro-racers and countless amateur track and street riders have made the conversion from a 525 chain to a 520 set-up without incident. There’s a large amount of anecdotal information stating a positive experience with less-robust final drive components, so any concern-voiced is somewhat equivalent to being labled a “chicken little.” Reducing chain and sprocket weight is especially desirable because it is both unsprung and rotating weight, not just translational weight, so it has the same effect as increasing an engine’s torque output, that is, it makes it easier to accelerate the bike.

For example, the DID Exclusive Racing Series 520ERV2 chain has a tensile strength of 8,250 psi. with a weight savings over an 525HV chain of around 128 grams. Since the 520 and 525 chains need different sprockets, replacing the rear sprocket at the same time with an quick change rear having an aluminum sprocket will save an additional 475 grams.

The other perspective is that chains break, and that the reports of failure are not that infrequent, so installing a weaker set-up is just increasing the odds for failure. The size of a chain is based on specific engineering standards that reflect the engine displacement (and roughly, the maximum torque output) of a motorcycle using an appropriate safety factor to guard against failure from wear and the higher occasional loads. Among the reported consequences of a chain failure is severe damage to the engine casing and bending the transmission output shaft. From a safety point-of view, locking the rear wheel while cornering is, shall we say, undesirable.

There are some strong 520 chains and some weak 525 chains, but in general, a 525 chain has a higher tensile strength than a 520 because it has chain links with thicker side plates. Specifically, Ducati installs a DID 525HV chain standard on its liter displacement bikes that has a tensile strength rating of 9,120 psi. Popular conversions uses a DID 520EVR3 rated by the manufacturer at 8,650 psi, or a 520ERV2 rated at 8,250 psi - so as you can see, there IS a difference.

The size of the chain to be used is also affected by the size of front sprocket that’s chosen. Ducati models have a wide range of torque output and the size of the chain and Ducati’s selection of sprockets reflect this range of outputs. All Ducati current models, except the 748 and 749, come with 15-tooth front sprockets. The 748 and 749 series (with the exception of the 749R) all have torque outputs below the 78Nm of the original 916, so they are supplied with 14-tooth front sprockets. The 748’s get 520 chains. The more powerful 749R (82Nm) is 15-tooth.

The 749’s get 525 chains, probably more for parts standardization than for strength. I remember from the product introduction that that was one of the major design objectives of the new models.

Starting with the higher torque 916-series (and 749R), and continuing with the 996, 998 and 999, the factory moved to a 15-tooth front sprocket and a 525 chain.

Why?

Because more torque means MORE CHAIN TENSION, and a 15-tooth front sprocket lowers the tension in the chain by seven percent. A 525 chain has a tensile strength that’s ten percent higher, so you get an overall 17 percent stronger setup with a 15-tooth sprocket.

Still-higher torque SP, SPS, R and Corsa models output over 100Nm, so how do they survive with 14-tooth sprockets and light-weight 520 chains?

That’s easy.

Once you get over a certain torque level (for a given weight bike) the bike will wheelie before the chain tension exceeds it’s strength limits. At least for awhile ... chains on these bikes don’t usually see 15,000 miles of service.

So, what does this suggest about changing our final drive components? Three things.

First, as a general rule, it’s better to increase the rear sprocket size to avoid the higher chain tension resulting from a smaller front sprocket. Changing from a 15 to a 14-tooth front sprocket will result in a seven percent higher chain tension.

Second, the heavier the bike, the higher the chain tension needed to make it wheelie, and the higher the maximum chain tension it will experience. So, a 680 pound 916-plus-rider will generate a higher chain tension than (say) a lightweight Corsa-plus-jockey or even a high-power Suzuki GS-X. When a chain under tension elongates 10 percent, it needs replacement - frequently for a Corsa bike. This, I think, explains why the pros can run 520 chains, and why streetbikes shouldn’t.

Third, COMBINING a change to a smaller front sprocket with a change from a 525 to a 520 chain on a higher-torque model Ducati will significantly weaken the final drive load capacity. Reports of chain failures are common enough, so it may not be wise to ignore this point for the sake of saving 275 grams of chain weight.

That said, I’d like to report that a 14-tooth front sprocket on a stock 916 525 chain has worked fine FOR ME for over 25,000 miles.

Your mileage may vary.

[Chuckracer]

Another thing to possibly consider...on a couple of my 520 converted bikes from my past (my Hawk and my CBR600F4 race bikes), the 520 chain was slightly too narrow for the stock swingarm rubber chain sliders, and it ripped them up pretty consistantly. I stuck with 525 on my 749 for this reason. The swingarm pivot is high and I run a lot of rear ride height, so the chain runs on the slider a lot. Other's I know didn't have that problem, so, as usual, your mileage will certainly vary!

[Lars]

For the chain and sprockets, engine torque is only interesting if the compared models have the same gearing.
Of two models with similar torque, the one with the tallest (gearbox) gearing has least torque on the front sprocket. And you get a lot less torque in second than in first.

Smaller front sprocket increase torque load on both chain and wheel, bigger rear sprocket only increase torque at the wheel.

[tsal998]

great info, again, Shazaam. i've been wrestling whether or not to change my 998 (with a 999R engine) to a 520. maybe not a good idea...

[silversnl]

Also keep in mind the frequency of replacement. Most street riders will keep a chain for many thousands of miles where as racers replace chains a couple times per season (and pros likely every race weekend). The vast majority of the reason for that IMO is simply just in case. When there is money on the line, you don't want a simple item breaking causing a disastrous outcome.