Why do a Fork Conversion?
Motorcycle forks have a 100+ year old history and it makes sense that the newer the front end, the better the performance will be. Like any technology, testing, trial and error over decades have aided in continuous improvement. The progression took us from springed / girder type forks to telescopic forks. Telescopic suspension isn’t specific to bikes, the principle was first used on air-craft landing gear in the 1930’s. The same type of suspension is being used today – although far more sophisticated in terms valving, spring and compression rates, material technology and more. This doesn’t mean you can’t go with an old school girder front-end, but don’t expect to see any performance improvements come with that – in fact you’ll see the opposite. If you’ve chosen to go down that road then the reason is probably aesthetic or you’re looking to keep the build as original as possible. This route is perfectly okay as long as you’ve determined how the bike will be used. Take a look at some of the work produced by Max Hazan and you’ll see what we’re referring to. Some builds may not be completely practical, but they’re relatively functional artworks that deserve the attention they get. There’s no question here when it comes to amount of quality and labour intensive craftsmanship that goes into each build.
Setting aside minor sub-reasons or nuances, a fork conversion is done to achieve two primary things; improve performance, and/or improve the look of a motorcycle.
A common upgrade is converting from standard telescopic forks to an inverted/ upside down (USD) front end. With this upgrade you can achieve quite a lot in terms of style and performance. Usually, what you’re doing is removing old 1970-80’s or earlier forks that have seen better days. Even if they’re in good condition, the forks will often feel quite spongy, lack torsional rigidity and dive each time you hit the front brake – and because of smaller brakes take quite a while to bring you to a complete stop.
By making the big move to a USD fork conversion you’re killing several birds with a single stone:
- Better spring performance and adjustability – (sag, preload, rebound and compression adjustment).
- Better stability due to an increase in torsional stiffness.
- Less weight.
- Better braking – (often replacing drum brakes, or single disc brakes to large diameter twin discs).
- Better reliability and cheaper servicing costs.
In terms style points, USD forks provide a sporty premium look and can be a real positive for bikes where a bit of visual mass is needed up front. They also come in various anodised finishes such as black, silver, gold, and more; although changing the colour scheme isn’t the most difficult thing in the world. The old forks are likely to look worn and be somewhat battered and bruised. They can be refurbished to look like they just came out of the factory, but factory finishes then and now have also changed significantly.
Styling is personal, and we’ve gone with factory options and refurbished front-ends in the past, but we do more fork conversions than not and constantly get asked about our fork conversion kits – so the performance and style upgrades are something that are difficult to ignore.
The rest of this article gets a bit a technical. It contains useful diagrams and dimensions, but feel free to skip through to sections that are relevant to you. It is designed to educate (as a guide) as well as entertain, but it’s up to you to apply common sense and judgement. The information is not exhaustive so when in doubt, chat with a local authority for guidance relevant to your country and/or state. If your local authority doesn’t have any guidance on extensive motorcycle modifications then get in touch with an engineer or someone competent to point you in the right direction.
At some point in time we’ll cover some of the more common regulations and modification guides in and around Australia, USA and the EU – similar to our Tech Article on Noise.
Motorcycle Steering Angle, Trail, Wheelbase and more
Converting a front end is not always as simple as swapping over old with new. There are a number of things that need to be considered, and with a little bit of homework and forethought you’ll be on the right path to doing a fork conversion the right way.
Bear one thing in mind as you do it, when you change the front end you will change the stability and performance of your motorcycle. This sounds obvious, but it also sounds a bit intimidating at first. However, it shouldn’t be, there’s a good chance that you will change those aspects of the motorcycle that better suit the kind of riding or comfort that you require in your custom build (assuming you go from old to new technology).
To understand what we’re changing and how it affects the ride let’s look at a few common terms and how their changes affect the stability of a motorcycle:
- Steering angle;
- Fork Offset; and
- Centre of Gravity (CoG).
Steering angle / rake is the angle of the steering head with respect to the vertical. It is essentially the caster angle, and directly responsible for tracking and stability in straight lines. The smaller the steering angle, the more responsive the steering is to small movements. The larger it is, the more effort it is to get the bike to move. This is why cruisers have a greater steering angle (about 32o) than sport bikes (about 25o). By increasing the steering angle you also increase the trail.
Trail is the distance between the centre of the contact patch of the tyre and where the centreline of the head-tube meets the ground. The longer the trail the more stable the bike is at higher speeds as it auto-aligns when encountering any severe road vibrations or bumps. It also means more effort for the rider to initiate a turn. If you increase the steering angle you will increase the trail. With some older bikes where the axle is not centre to the fork line, the trail will change more so when a new front-end is installed. We recommend searching up the specifications of your bike to try and stay close to the original dimensions. More importantly, don’t reduce the trail unless you want a significantly more engaging ride. If you must, stay above 90mm.
This is the dimension between the centre of the front and rear axles. Generally speaking, the longer the wheelbase, the more stable the bike is on the road as it distributes its weight over a longer distance. The shorter it is the more responsive the bike will feel in corners and the less effort a rider will need in moving in and out of bends. Wheelbase is also important to consider for braking and accelerating. If you reduce the wheelbase, more weight transfer will occur over a shorter distance during hard braking, conversely it’ll be easier for the front wheel to lift up off the ground when accelerating. The stability offered by the wheelbase is different to that of the trail dimension. A longer wheelbase will not re-align the front wheel at high speeds. Which is why trail is still crucial to get right.
This is the distance between the centreline of the forks and the centreline of the head-tube. By changing the fork offset you change several aspects the bikes geometry like the trail, wheelbase, Centre of Gravity (CoG), and the weight distribution. For example, if you reduce the fork offset, you increase the trail, but reduce the wheelbase, you also increase the height of CoG (unless the forks are shorter), and increase the weight on the front axle.
It is a common dimension to tinker around with especially as you start with your fork conversion. If you’re installing a set of new forks from the ever popular range of Honda CBR, Yamaha R1 & R6, or a Suzuki GSXR you’ll find that the original forks have considerably shorter fork offset (usually between 30-40mm) than your original 1970’s – 80’s motorcycles (usually ranging from 45-55mm). A straight swap could work well as you increase the trail, but you need to consider what this does to the wheelbase and the weight distribution of the bike. Another thing to keep in mind is that as you bring the forks closer, you have less space between them and the tank, thereby reducing the steering lock angle. This isn’t usually a problem unless you do a lot of riding in car parks at very low speeds.
Centre of Gravity
The CoG is the point at which the entire mass of the motorcycle can be thought of as being concentrated at, and if it could be pinned in the middle of space, would hang naturally without leaning to one side or the other. It is the point at which the entire mass of the bike is at equilibrium. By changing the geometry of the bike you change the CoG. For example, if you change the fork offset but retain the same length in the front end, you essentially raise the front of the bike upward, the CoG naturally goes up as well. If you reduce the ride height of the rear you reduce the height of the CoG. If you extent the rear wheel, thereby increasing the wheelbase, you shift the CoG to the rear.
The higher the CoG from the ground, the less effort required during turning. The lower the CoG the more stable the bike will feel when traveling in a straight line.
An important aspect that will dictate the CoG is the fork length. Depending on the type of build you may need to increase the length of the front suspension as well as the rear, such as when converting to a tracker or a supermoto. If you intend to keep the bike in as neutral of a stance as possible, such as a café racer or a brat style bike, it may be appropriate to have slightly shorter forks to counter the upward movement caused by the reduction in offset. You will also need to consider the front wheel. Modern front ends usually have 17 inch wheels, older BMWs or Hondas can range from 17, 18 to 19 inch wheels. Depending on the tyre profile size you could try match the overall diameter, but in some cases the front wheel will be a different size to the rear.
Changing the rear shock height then also needs to be considered because you want to ensure a manageable transfer of weight between the rear and the front during braking. Whilst the front will do most of the braking, the rear shouldn’t be unweighted excessively otherwise you risk making it less stable. Choosing a suitable fork length and spring rate is also important to consider to avoid excessive nose dive. By choosing forks suited to a particular weight of motorcycle you can strike a good balance. For example our Estoril Blue BMW K75 build shown below comes in at 197.8kg (wet weight) and the Honda CBR 929 front end was used on a 194kg, motorcycle. Depending on the rider, this can be adjusted even further, but it’s a good base to start with. The weight distribution between front and rear was 51/49.
We won’t dive into adjusting front and rear suspension in this article, but to get the most of your build this should be done.
One of the dilemmas faced when doing a fork conversion is creating a mismatch between the front and rear wheel. There are several common approaches in this case:
- Live with the new front wheel. Often the size of 300mm + brake discs will hide the fact that the front wheel is different to the rear to the casual observer, although in some cases it’s more obvious than others. The benefits of living with this approach is that you reduce the rotating mass of the front wheel (aluminium alloy and considerably lighter). This improves handling and reduces weight and is the most cost effective approach.
- Do a swing-arm conversion or fit a like for like rear wheel. This is a great approach and results in less weight and better overall performance (now reducing the mass of front and rear wheels). But it can be quite complicated and expensive as you start fiddling with even more geometry, drive sprockets, chains and more – the numbers, and time really start to add up.
- Fit custom spoked front and rear wheels. This provides a balance of performance and style. There is no doubt that spoked wheels provide that vintage look that most are after when designing a cafe racer etc. However, the cost of custom spoked wheels can often double the cost of fork conversion. Why? It’s likely that you’ll need custom front and rear hubs, need to get the wheels laced and trued, and so on.
- Retain the original wheel by fitting brake disc / rotor adapters to suit. These can be custom made to suit your particular project by a competent workshop with a lathe and mill. We also offer them for BMW K and R series builds with the original second gen snowflake wheel. The benefits include better braking and the visual aesthetic that comes with old-school front and rear wheels. If you’re looking to do a set of custom adapters for your build bear in mind bolt strength, hub-centric mounting, a potentially new axle and ensuring that the old wheel is suitable with the caliper spacing.
Each approach has its pros and cons. Ultimately, it is up to you to decide which direction to take. Whichever approach is taken, be sure to do research into that aspect of the build (particularly relevant if you’re planning to extend the swing arm, or lace your own wheels and so on).
Fork Conversion – Installation
How you approach your fork conversion depends highly on your build and whether parts are readily accessible or not. Depending the compatibility of your donor front end and the motorcycle you may be able to use a combination of new steering stem bearings and spacers to get it to fit. The most important aspect is to ensure the top and bottom bearings match with the outer diameter and height of the race for your intended bikes head tube, and have the inside diameter of the bearing fit the new head-stem. In some cases you may be lucky enough to get them to match, then all that needs to be done is to buy new bearings, go to a local workshop and have them press them on for you.
If the new head-stem is a little bit too tall, you can use a combination of spacers to fill in the space get the system to work. When installing the new front-end be sure that the bearing preload is suitable.
In most cases however, getting this to work perfectly is unlikely because the combination of bearings required for the inner and outer diameters simply aren’t available. If your donor head-stem is too short or far too long (where spacers can’t be used), you’ll need a new head-stem at the very least. Getting a custom head-stem can be expensive to source especially because machine workshops are likely to do a one-off. If you have the material, a lathe and the ability to make the correct sized threads, you can attempt to do this yourself. All that’s left to do is press out the old and press in the new.
The simplest way to do the conversion is to get an aftermarket fork conversion kit that will be suitable with your desired forks and the model bike you’re working on. There are numerous available on the market and the main differentiator between each is the range of bikes they cover, price, design, and material choice.
For example, we offer conversion kits that suit BMW K Series bikes, and Honda CB Series from 69 to 83. We’re always expanding our range so be sure to check out our product pages for more information. We offer the most competitive in price options and use the best aerospace grade aluminium available for the task. Design is subjective – We love it, our customers do too, but we know that there’s a possibility that not everyone will. Whichever kit you go with, be sure it suits your style and your ride.
With an aftermarket kit you should get the steering stem already pressed into the bottom triple, bearings should be included, and where applicable the bottom bearing should already be packed and pressed onto the stem. The top bearing and triple will need to be installed when you mount the entire triple tree on the bike. Usually all that is required is knocking out the old races from the bikes headtube and knocking in the new. A plug and play kit shouldn’t take longer than 30 minutes to install – not bad for an entirely new front-end!
If you purchase an aftermarket kit that is not a plug and play you may need to find a workshop to press in the headstem, and bearing for you (make sure you pack it the right way and with high temperature bearing grease). This becomes less of a plug and play approach and will end up costing more in time and money but still gives a good end result. The last point of discussion is material choice. OEM manufacturers and most aftermarket kits are made of 6061 T6 Aluminium. This is a high quality aerospace grade alloy that offers good strength and ductility. Aftermarket race option set ups use 7075 T6 Aluminium, which offers almost twice the strength but is a little less ductile. If the geometry of the triple tree is identical, the 7 Series alloy will offer up to 25% more rigidity in the system. This means less flex and more feel of the road – essentially your forks are doing all the work as intended. This may not be necessary if you don’t intend to ever get on the race-track, but then again this rings true for more than half of the performance upgrades for cars and motorcycles, we still like to have the best installed.
Common USD Fork Specifications and Dimensions
Here is a table of some of the most common forks used on café racers, bobbers, and brat style custom builds. The table provides some useful dimensions to help you determine what forks to look out for. It is a table that we developed with the available information online and measuring some of the forks that we’ve had in our workshop. It’s here because this information shouldn’t be kept a secret. We hope that it’s useful for those of you willing to make a leap into a fork conversion for your build.
Each of the fork variations in this table can be used with our plug and play fork conversion kits, or other kits in the market. If you have the know-how and tooling available to create a custom triple tree and / or steering stem, you can rely on this data for multiple conversions. There may be some significant carry-over from year to year but if it’s not included in the table below it’s because we haven’t been able to confirm it. We ask you to submit any information for other forks and their years so that we can add them to the table as a catalogue of dimensions for other builders, tinkerers, and enthusiasts to use. Let’s help strengthen the community by sharing important knowledge around. Dimensions can be submitted by emailing us at firstname.lastname@example.org . We’ll update the table on a weekly basis.
|Year||Make||Model||Top Fork Diameter* in mm||Bottom Fork Diameter** in mm||Fork Spacing in mm||Rotor Diameter in mm||Fork Length*** in mm|
|2000||Honda||CBR900 (929 RR)||50||54||214||330||710|
|2001||Honda||CBR900 (929 RR)||50||54||214||330||710|
|2002||Honda||CBR900 (954 RR)||50||54||214||330||710|
|2003||Honda||CBR900 (954 RR)||50||54||214||330||710|
|2008||Suzuki||GSXR 600/750 K8||50||53||207||310||730|
|2009||Suzuki||GSXR 600/750 K9||50||53||207||310||730|
|2010||Suzuki||GSXR 600/750 K10||50||53||207||310||730|
*measured at where the top triple is mounted
**measured at where the bottom triple is mounted
***measured from the top of the fork to the centreline of the axle.