Technical FAQ: Taking care of unused tires

Properly storing tires

Dear Lennard,
I have a set of backup clincher wheels and tires I use when my daily set is damaged or in the shop getting trued. Can you tell me how I should store these with the tubes and tires on them? Does hot or cold affect them?
— Patrick

Dear Patrick,
I spray 303 Protectant on the tires and hang them on rims (or stack unmounted tires laying flat) in a dark attic. Storing in a cool place is best; neither extreme cold nor high heat is good for them. I have installed Ridge Vents to keep my attics cooler for storage of tires (among other things) in hopes of reducing the continued vulcanization that can harden the tread over time. But avoiding sunlight and protecting them from ozone damage are even more important. 303 Protectant blocks sunlight and is particularly important if they are not going to be stored in the dark.

Modern rubber compounds used in bicycle (and car) tires are formulated to resist cracking due to environmental hazards such as ozone, oxygen, UV radiation and heat, but that only goes so far. Tire manufacturers add waxes, antioxidants, and antiozonants to their rubber compounds to help protect tires against deterioration by ozone, oxygen, and heat, but the tire must be flexed frequently to squeeze the wax and some other protectants out onto the surface. If the tire just sits, the wax eventually disappears from the surface, allowing ozone, which breaks bonds inside elastomer molecules, to attack the rubber. That’s why the tires crack on cars left sitting for years, even if they’re parked in a dark garage. If driven occasionally, some protectants will come to the surface and prevent cracking.

Also, washing the tires a lot removes waxes, antioxidants and antiozonants designed to protect the surface of the tire, causing tires to age prematurely. That’s why at the end of the season I coat my cyclocross tubulars, which get washed a lot and are costly to replace, with 303 Protectant; I’m hoping that it will do some of the work that the waxes, antioxidants, and antiozonants were designed to do. And it makes them look beautiful and shiny.

I have recommendations from a number of tire manufacturers below. None of them recommend spraying a protectant on them, so maybe I’m being seduced by the good looks they provide. But I still have the same tires on four graduated-size little kids’ bikes that I made for my daughters 25, 23, 20, and 15 years ago. Little kids are too light to wear through all of that thick rubber, and they sit for long periods awaiting service for the next niece or nephew or friend’s child. I’m quite sure that the tires would exhibit heavy cracking now if not for the 303 Protectant, since they are stored in garages where light gets in and left out in the sun. The same goes for the tires on my wife’s bikes. She has a bunch of sweet bikes and goes years between using them, as her passion is riding horses; she spends her rare free time doing that and not riding bikes. The tires on all of her bikes look good and are not cracked, despite there being a window in the garage in which they hang. I believe that this is thanks to my coating them with 303.

I used to use ArmorAll, which offers lots of tire-care products, on my tires, but we had big problems with fish-eyes in the paint jobs on our frames when our paint shop was adjacent to an auto-detailing place or whenever an unpainted frame was transported in a car whose dashboard or upholstery had been treated with ArmorAll within the prior few months.

The excerpt below from a paper entitled The Pneumatic Tire put out by the US. Department of Transportation National Highway Traffic Safety Administration explains a lot of the reasons tires deteriorate during storage.

5. Aging of rubber
Rubber undergoes profound changes on storage that are accelerated at higher temperatures. Deleterious changes occur in tire properties after storage at ambient temperatures for five years or after use on cars for similar periods [29]. They are caused by a variety of chemical reactions:

(i) Ozone attack. Although the concentration of ozone in the atmosphere is quite small, typically only a few parts per 100 million, ozone reacts rapidly and efficiently with the unsaturated elastomers commonly used in tire compounds, leading to molecular scission [ed. note: breaking of molecular bonds, particularly in long-chain molecules]. However ozone cannot penetrate deeply into the material — reaction takes place at the exposed surface and produces a relatively-innocuous thin degraded surface layer, about 20 μm thick, which protects the interior. However, if a small tensile strain of the order of 10% is present in the rubber surface, then the scission reaction with ozone causes characteristic sharp cracks to form in the surface and grow inwards, continuously exposing new material to further attack. The cracks grow surprisingly rapidly. They become about 1mm deep after only two weeks exposure of an unprotected rubber compound to normal outdoor air with an ozone concentration of about 5 parts per hundred million. Thus, ozone cracking is potentially a serious problem in tire sidewalls where tensile stresses are commonly present both in storage and in use. Special additives, termed antiozonants, inhibit ozone cracking when added to the rubber compound in sufficiently large amounts, about 3%, probably by competing with rubber molecules for reaction with ozone. Butyl rubber is much less susceptible to attack by ozone than other common elastomers, at least at ambient temperatures, because it contains only a relatively small fraction of reactive C=C bonds in the molecular backbone.

(ii) Oxidation. Another cause of aging is reaction with atmospheric oxygen. Oxidation is slower than ozonolysis and oxygen therefore penetrates for some distance into the material before reacting. Thus, oxidation does not cause cracking directly although the oxidized material is often brittle and cracks on flexing. Depending on the relative rates of diffusion and reaction, the affected depth can range from several mm at ambient temperatures, when the process takes years to reach a significant stage, or a fraction of 1 mm at elevated temperatures when oxidation is rapid, taking only a few hours. Typical hydrocarbon elastomers undergo an autocatalytic reaction that results in addition of oxygen groups to the molecule and formation of new crosslinks by interaction with neighboring molecules. As a result the material generally becomes harder and eventually brittle. However another, generally minor, consequence of the complex oxidation reaction is occasional molecular scission and hence softening. This provides a convenient way of characterizing the sensitivity of a rubber compound to oxidation. Samples are stretched and aged in an oven at various temperatures, usually in the range 70oC to 130oC, and the tensile stress is monitored continuously over a period of several days. As oxidation proceeds and some elastomer molecules break, the stress falls and gives an indication of the extent of oxidation. The rupture reaction follows an Arrhenius dependence on temperature to a first approximation, with an activation energy of about 25 kcal/mole. Thus, an increase in temperature of 10°C causes an increase in rate of oxidation by a factor of about 2x.

Another way of assessing the sensitivity of a rubber compound to oxidation is to expose samples for various periods at elevated temperatures and then measure the remaining strength and extensibility at room temperature. A typical specification for aging resistance would require that the tensile strength does not change by more than a specified fraction, say 20%, and the extensibility does not decrease by more than a specified fraction of the original value, say 30%, after a period of aging of 7 days at 70oC or 22 h at 100oC.

(iii) Additional vulcanization. Vulcanization does not stop when the cured compound is removed from the mold. Continued curing takes place subsequently but at much lower rates, of course, depending on the temperature. As a result, if tires are stored or used at elevated temperatures the material hardens as more crosslinks are introduced, or softens (a phenomenon termed reversion) as those crosslinks already formed gradually decompose. These processes are a consequence of a series of complex reactions involving elastomer molecules, existing crosslinks, residual sulfur, activators and accelerators, and byproducts of the various intermediary steps in the crosslinking reaction. In conventional aging measurements, these processes are difficult to distinguish from the effects of oxidation, but they can be studied separately by aging samples in an oxygen- free environment. In thick rubber articles, material far from the surface, say over 10 mm deep, may undergo solely anaerobic aging because oxygen reacts before it diffuses so deeply. Passenger car tires, on the other hand, operate for long periods at moderate temperatures, so that oxygen may diffuse extensively before reaction. Thus, oxidation is regarded as the normal mode of aging of tire components.

(iv) Weathering. This mode of aging is rather ill-defined. Insofar as new aging processes occur, other than oxidation and ozone attack, they appear to be associated with irradiation by UV and sunlight. Radiation causes free-radical reactions that can initiate or catalyze oxidation and ozonolysis, as well as being itself a direct cause of crosslinking and/or molecular scission.

― Lennard

From Continental:

Don’t know about the 303 Protectant, but definitely cool and dark just like fine wine — on the rims and enough air to keep them round, although they will steadily bleed down depending on how long they are stored.
— Brett Hahn, Brand Manager
Continental Bicycle Tires NA

From Hutchinson:

The two biggest things to avoid that degrade tire life are UV and ozone. You can keep tires out of the sun to avoid the UV, but high ozone levels are probably outside your control, especially if you live in a city with high levels of air pollution.

I’d avoid extreme high-low temps as well during storage just to reduce the chance of the rubber drying out and cracking. Oh, and hang the wheels so the tires are not contacting the ground, as the pressure drops creating potential flat spots on the tires.
— Richard Goodwin
PR & Marketing Liaison
Hutchinson Tire North America

From Ritchey:

Avoid sunlight, moisture, and flatness. If possible, hang them up for storage. Otherwise it’s smart to keep them inflated (you’ll have to periodically check them) as once they go flat, the tire can develop cracks from being deformed too long.
— Sean Coffey
Ritchey Global Marketing Director

From Specialized:

Tires and tubes should be stored in a dark, cool, ventilated area. Heat, UV light, Ozone, and time degrade the rubber compound. The tires keep their performance level for about three years. Then the aging agents and softeners have dissipated enough to let you feel the difference in grip and suppleness.

However, it is safe to use tires up to six years after manufacturing.
— Wolf Vormwalde
Tire Product Manager
Specialized Bicycle

From a Former Bicycle tire engineer:

Yes, I agree: a cool dark place is the best to store spare wheels … plus wine and salami!

Back to the wheels; it’s also better to keep the tubes inflated (a couple of bars will be enough) if the wheels are on the ground, with the wheel weight completely on the tire.
— Alberto De Gioannini
Founder, Effetto Mariposa

From Michelin:

Regarding tire storage, here is Michelin’s response. Note that in the coming weeks, Michelin will actually be publishing a complete official document on the subject of storing Michelin bicycle tires.

Concerning storage, we recommend storing our tires away from lights (especially UV rays), in a place with low humidity between 5 °C and 35°C. We do not advise to use any chemical products to clean or store our tires. We do not have any age limit of use, since the performances of a tire strongly depends on how it has been used and/or stored.
— Nick Legan
Dispatch Colorado