GCN Tech Clinic: Rusty wax chains, mis-matched tyres and ERG mode

In the Tech Clinic this week, Ollie Bridgewood is answering your latest questions and sharing some insights from our comments section. If you have a solution or anything we cover in the clinic don’t be shy to share as there are often multiple ways to solve the same problem.

Is there any difference between using the big or small chainring on an indoor trainer in ERG mode?

A few factors are at play here that could result in a difference in feel. Firstly depending on what gear you are in there will be a difference in drivetrain efficiency. When you are in ERG mode you want to pick a gear with the straightest chain line as any deflection of the chain will result in a loss of watts, meaning you will have to ride at a greater output for the same measured wattage on the trainer. Typically using the big chainring and a gear in the bottom half of the cassette will result in the straightest, most efficient chain line.

When you're using ERG mode, turning a larger gear means the flywheel will be turning faster. A higher flywheel speed will mean that the resistance will have a more gentle curve when adjusting to a different power intensity. The concept is very similar to how riding at 300 watts on a climb can feel very different from 300 watts the flat. The difference in flywheel inertia will cause a difference in feel especially when changing between power intensities.

Does a waxed chain last longer on an indoor trainer?

One of the main considerations for how long a waxed chain will last on an indoor trainer has to do with how sweaty you get and where that sweat ends up. If you are a sweaty rider the salt will cause abrasion and corrosion on the chain. In this case, wiping the chain with a microfibre cloth after a ride to get the worst of the sweat off the chain will help prolong the life of the wax.

It really is difficult to define a hard and fast interval for when you need to rewax your chain. There are so many variables that affect the lifespan that using a fixed interval does not work. Instead, the easiest way to know when it is time to rewax the chain is to listen for when it starts to make a noise. A waxed chain will run silently until the wax coating breaks down at this point it is best to remove the chain and re-apply the wax.

Is there anything wrong with fitting a 28mm tyre on the front and a 32mm tyre on the rear of my bike?

There is nothing wrong with fitting mis-matched tyres, but we'd say that if you are fitting a 32mm tyre on the rear for added comfort, why not put a 32mm tyre on the front too?

The main thing that will prevent you from fitting wider and more compliant rubber to your bike is the limitations of your frame. Although tyres come in a listed width the actual size they inflate to will depend on the internal rim width of the wheels. A wider internal rim width will allow the tyres to sit in a more open profile meaning that they will measure wider than if they were mounted to narrower-rimmed wheels. If you are unsure how wide a tyre will inflate on your current rims there are plenty of helpful calculators out there that will give you a measurement.

If you are fitting the tyres for added comfort we would suggest going for the widest tyres that you can fit. If you can only fit a maximum 28mm tyre at the front then go for that but if you can fit 32mm rubber front and rear this would be our choice. Not only will it be more comfortable but it will also give you more grip and added puncture resistance.

Is a waxed chain more efficient than a belt drive?

Belt-drive bikes struggle to achieve the same levels of drivetrain efficiency as chain-driven bikes. Even an oil-lubricated chain will typically outperform a belt drive system in regards to efficiency. Belt-drive bikes are far less susceptible to dirt ingress and also typically last a lot longer than a traditional chain-driven bike however this increase in durability comes at the detriment of transmission performance.

Should I use my calculated optimum tyre pressure on a wheel-on indoor trainer?

Wheel-on indoor trainers have very specific needs that differ from the needs of a tyre out on the road. Firstly the likelihood of getting a pinch flat on an indoor trainer is almost zero and the need for an optimised cornering grip is also unnecessary.

The main consideration for indoor training is that the tyre has enough grip on the roller to prevent wheel slip under high torque loads or sprint efforts. This means that the correct tyre pressure on an indoor trainer could be considerably different from what you would use on the road and the best way to find it is simply through trial and error.

If you have any tech-related questions that you need answering, head over to this week’s Tech Clinic video on the GCN Tech YouTube channel and add your question to the comments along with #ASKGCNTECH. Or leave your question in the comments below.

Oliver Bridgewood

GCN Tech Presenter

PhD Chemist turned cycling enthusiast, bringing the lab coat to GCN Does Science