Since the moment of its invention, the bicycle has come a long way from primitive “boneshaker” until today’s marvel of engineering. It’s construction remained very simple for a long time and it was used mainly for road traveling.
In the 70s there was a revolution in world of cycling – the creation of a first mountain bike. New bikes gave riders opportunity to explore places previously completely inaccessible to them. New disciplines started to appear in cycling.
Dirt jumping and Street Trial are becoming more and more popular each day. In this article we will focus mainly on them.
Dirt & Street
Dirt-jumping – the name fully reflects the essence of this sport. You ride a bike on a specially build track full of trampolines made out of dirt. Take offs and landings make great impressions on audience, one of the most gripping sports to watch live
Street-trial fans also have something to brag about. Their stunts use elements of urban terrain and infrastructure and require perfect control of the bike
Although these two disciplines have a lot in common, bikes for them have some distinctions.
Dirt & Street bikes characteristics
For dirt jumping riders usually use BMX bikes and modified mountain bikes. These bikes are quite heavy and have only a rear brake. Street bikes are similar but they have some unique features
In these two disciplines only single speed bikes are used. Almost all manufacturers produce multi speed bikes and then market them for dirt or street riding. When choosing a bike for these disciplines, multi speed bikes should be excluded from the list of options.
If a bike has multiple gears that means that the manufacturer has saved some money on frame and other important components. On top of that a multi speed bike in dirt and street disciplines is simply useless. It only increases weight and cost of a bike.
If you are just getting into this sport it is OK to purchase a fully built “complete” dirt/street bike from manufacturers. While you gain experience you can upgrade and improve your bike to fit your needs and personal preferences.
The frame is most expensive and most important part of the bike. Key characteristics of a frame is its geometry and material from which it is made. From these factors depends bikes usability in particular discipline.
Dirt and Street bikes are both subjected to extreme stress, so there are special requirements for their strength. These materials are usually used in frames of these bikes:
- HI-TEN. This is a steel with improved quality, which is used for frames on entry level “complete” bikes. Frames from this material are heavy, and yet they do not have the strength necessary for this discipline. This is the first part of the bike that needs to be replaced when upgrading.
- ALUMINIUM ALLOYS. Frames from these materials are less heavy and are not susceptible to corrosion. They are not good at dampening the vibrations and they have no maintainability. At high stress levels aluminum accumulates fatigue which will sooner or later lead to structural destruction. Nonetheless, there are quite few fans of these materials among dirt and street riders.
- STEEL ALLOY. These are the frames that use chromium-molybdenum alloy (Cro-Mo or “chromol”). They are able to absorb most of the energy during the landing and soften it. These materials are used on mid to high level bikes. The frames are made with variable wall thickness wich increases strength and reduces weight.
- MAGNESIUM AND TITANIUM ALLOYS. Lightweight, strong, expensive. As well as frames made of composite materials they are beyond the scope of this article, their main area of application is in professional cycling. Multimillion dollar contracts more than cover their costs.
From all that is said above we can make a conclusion: Chromium-molybdenum alloy (Cro-Mo or “chromol”) still remains a best option for when it comes to choosing a frame material. It provides optimal strength to weight ratio. Aluminum also has a lot of fans but in my opinion it is better to leave it for the road bikes, gravel bikes and cross-country bikes, as they experience far less stress.
For each discipline different frame geometries are used. It will be uncomfortable to ride street on a dirt bike and vice versa. Unlike other types of cycling, bike size does not play a major role here. The length of a top tube is what important in these disciplines.
Key feature of dirt jumping bikes is their length. The length of front triangle is around 570-590mm. Rear triangle is also long 370-410mm.
Head tube angle is 68-70 degrees. This ensures a smooth landing and minimizes mistakes. This geometry increases jump hight and air time but it is not suitable for street riding because of bad controllability.
On the other hand we have street bikes with the shortest top tube length in any discipline. The length of a front triangle is usually around 545mm. The rear triangle is around 345-350mm. Head tube angle is 71-74 degrees.
Dirt bike chainstay is parallel to the ground or has a negative angle. In this case the bottom bracket is lower than the rear hub wich makes the center of gravity of the bike lower, this has a positive effect on acceleration dynamics and adds stability in flight. It is easier to control on sharp turns. Street bike have a higher position of the bottom bracket. This is made for more responsive control ability, also lets you install pegs on your bike.
Don’t get too carried away with increasing hight of the bottom bracket. This will drastically decrease bike stability and control when jumping. A slight bottom bracket hight increase does the job, but if you don’t have enough hight to install pegs, than you need to install a longer front fork.
On dirt and street bike frames the dropouts are usually horizontal, which allows you to adjust the length of wheelbase.
Choice of a wheel size is another hot topic that causes a lot of debate. Dirt and Street bikes both use 24 and 26 inch wheels. 24 inch wheels are widely considered to be more versatile and suitable for both disciplines. There’s also a setup in dirt bikes where front wheel is 26 inches and rear wheel is 24.
For Dirt bikes 26 inch wheels are more comfortable. Bigger wheels soften the landings and handle better on uneven surfaces.
For riding Street with small jumps 24 inch wheels are the best option. Smaller wheels are lighter, stronger, easier to control when doing stunts. Bikes with smaller wheels are much shorter, this makes them more agile, maneuverable and responsive.
Choosing wheel size also depends on riders hight. If the rider is taller than 180 cm (5.9 ft) 26 inch wheels will work great. If the rider is shorter than 180 cm (5.9 ft) 24 inch wheels will be a better option.
For Dirt bike we use semi-slick tires because they have to have good rolling ability and at the same time stick to the ground. For street slick and semi-slick tires are an option.
Double walled rims is a must on any wheel type.
A balanced approach to the choice of a fork is very important. Dirt and Street bikes use two types of forks: rigid and (relatively) short travel suspension fork. Suspension fork with 130mm of travel is well suited for big jumps, aggressive riding and landings on a flat surface. It is also better choice for beginners. Keep in mind that a good suspension fork is not cheap. Bikes in bike shops are usually fitted with simple and cheap suspension forks which decrease the price of a bike an increase shops profit.
For a street bike it’s better to use a rigid fork. There are no big jumps so suspension isn’t needed. Rigid fork allows for a better control over bike. Big plus of this fork is that is not expensive. You can use saved money for a better frame.
Most important criteria of a rigid fork is the material from which it is made of. The Best material for a rigid fork is chromium-molybdenum alloy (Cro-Mo or “chromol”) 4130. Most of the forks in bike shops do not have butting and heat treatment, this leads to cracks and bends.
Standard axle for dirt and street bike is 14 – 20mm it provides necessary strength under load. I do not recommend using 10mm axle, cause it will break quickly.
Stem and Handlebars
Handlebars length depends on riders shoulders width and his physical condition. For bigger riders 700mm or more is suitable. For ordinary riders the width of handlebars should be no longer than 680mm.
On bikes with rigid forks the handlebars should be made out of Cro-Mo.
On bikes with suspension fork aluminum will do just fine.
Hight of the handlebars on a rigid fork should not be less than 3 inches. For suspension fork – not less than 2 inches.
But mostly this depends on personal preferences.
Shorter stem (35-50mm) allows better bike handling, since the arms of rider are closer to front wheel axle. Such stem is ideal for riding Street.
Bigger people with longer hands, who ride Street on short wheelbase, can ride with longer stems (50-55mm). Otherwise, they will feel discomfort.
Break types on these bikes don’t play significant role. What does is their presence or absence. Some Dirt/Street riders don’s use them at all. For beginners I will strongly advice to keep the brakes on the bike. This will help to avoid serious injuries.
You can install simple V-brakes on your Dirt/Street bike, but disk brakes are still remain priority. Hydraulic disk brakes are better when it comes to performance, but mechanical disk brakes are cheaper and easier to service.
The seat should be soft and wide, so it will be easier for a rider to hold a bike mid-air with his legs. Hard seat is going to hit your knees all the time and bring a lot of discomfort.
For people who have decided to get into extreme sports protection is vital. First thing you should do is get yourself a helmet. You can get a full face helmet but it’s a little heavy, hot and obscures your vision a little. For beginners I would recommend starting with an ordinary helmet and than moving on to a full face.
You should also think about other protection like knee pads, elbow pads and gloves.
In conclusion: should be noted that there are a lot more nuances when it comes to dirt/street bikes. Understanding will come with experience. Some technical questions are being disputed to this day, and there is no one correct answer. Personal preferences also play big role. Just start riding, chat with more experienced riders. Practice makes perfect.
What are your thoughts on this topic. Leave a comment below.