Jimny Front Bump Stops

I had no intention to spend big bucks on a set of rubber bump stops for the Jimny. What's the point its just rubber something else should fit?

So with that ideology and with a limited search for one in the market found the Alto's rear bump stops could be a suitable alternative. It was and along with the spring change this was put in. Its about half an inch longer than the stock, give or take.

I was not really happy with it because the first segment was hollow. Well, that is kind of good makes it sort of dual stage bump stops but may be a bit too soft in the running and are not pointy like the rear. I like pointy.  Said a great man in 2012.

Against my wishes i decided to import a set of bump stops with a plan in my mind. Jimnybits 25mm bump stops were chosen and they come with cone type (progressive) rubber buffer. The plan was simple I wouldn't use the supplied bump rubbers but intended to use the Jimny's original rear bump stops in the front.

It wasn't a straight swap, never thought it would be as it isn't advertised like that. The threading on the aluminium bushing is M10x1.5. The threading on the OE Jimny rear bump stops are M10x1.25. The long stud supplied with the Jimnybits aluminium bump stop bush is an M10x1.5, this is required to mount the aluminium bushing on to the vehicle.

What now needs to be done is to make the lower part (about 1.5cm) of the thread on the bushing M10x1.25 and leave the rest as is. So the lower part of the bushing can take the OEM Jimny rear bump stop and the top the M10x1.5 stud supplied for installation.

Sensible thing to do is to drill out the section of the thread that needs to be re done and do a helicoil insert. This is how it should be done, can't tap a coarse thread to fine. But that isn't how I done. I took the M10x1.25 tap and ran it through the coarse thread till required length to cut new fine threads within that coarse thread.

This seemed to work, the bump stop screws in smooth and fits tight all right. Its a bump stop, all it needs to do is hold on and don't fall off. Guess this is good enough for that. If and when the rubber engages with the axle the shock is transmitted through the metal base to the aluminium bushing and not the threads. Think I will a add a drop of blue Loctite to prevent it from any accidental loosening.

The stock Jimny bump stop seems to be approximately 60mm length. Not pictured but the Alto rear bump stop is about 10-15mm longer give or take couple of mm's. The Jimnybits 25mm extended bump stop (rubber and aluminium bush combined) is 80mm which seems about right. My hybrid bump stop (Jimnybits aluminium bush with Jimny OE rear bump stop) is about 100mm. It doesn't mean that suspension travel sees a hard stop at the new length, being conical these are progressive bump stops there is more give than a block type bump stop.

So now I have nice pointy bump stop with means to go longer (wouldn't be necessary) and easily replaceable, both OEM and both locally, if I need to in future. Being Maruti it is a lot easier and cheaper to buy the OEM bump stops than to try and source any universal type bump stop rubbers which has these M10x1.5 threads.

There is no need to import the bush either if one can do the legwork and follow up. Any lathe workshop would be able to machine a set for you easily with the correct thread. I would have done so in the past.

Why?

The compressed length of Dobinson IMS at the front is ~395mm and the compressed length of stock shock absorber is ~380mm (as given on TGR, who is a friend). I haven't measured at what compression length of stock shocks the axle engages the stock bump stops. But I assume the stock bump stop length is kept as is by Suzuki to work with stock shocks to engage the axle before OE shock bottoms out.

The Dobinson IMS bottoms out in compression 15mm before the stock shocks does. This could mean the stock bump stop may or may not engage the axle before the IMS bottoms out internally in compression. If it bottoms out internally before hitting the external bump stops it could damage the shocks over time. I don't think the internal bump stops (if there is any) will be up take the pounding. In my opinion it is wise to extend the front bump stop by 15 mm to save the IMS from bottoming out internally.

The hybrid bump stops I have here for my Jimny is about 40mm longer but this is not required and will limit the suspension up travel so is not recommended. The first solution would be enough, i.e., the Alto's rear bump stop for Jimny's front.

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Lifting my Jimny

One could do two things

1) Buy a lift kit which would have shocks, springs, cross member drop, trackbar riser bracket and a replacement brake hose for the short rear brake line.

2) Assemble a lift kit which need all of the above but one could mix n match stuff based on how one wants it to ride.

Most lift kits come with same manufacturer shocks and springs, but it is not strictly necessary or required to match the springs to the shocks for these 4WD lift kits because a lifted Jimny is no track car.

There are manufacturers themselves give the same set of springs with different types of shocks with different valving and different construction which don't behave the same. Then there are manufacturers who pair the same set of shocks with differently rated springs. I could cite various examples for both.

Its all good if one doesn't pair a spring that is too stiff with a shock which has no control over it, don't pair a stiff spring with OEM shock. All aftermarket shocks are rated to be used with much stiffer springs so they can handle it.

The most popular and well known springs and shocks kits for the Jimny come from either Japan or Australia. Kits and components also come from other countries UK, Italy, China, South Africa etc but they are less popular and limited in choices than what is made available by Japan or Australia.

The problem

If one were to go with a kit especially one that is made for the or by the Australian brands which are meant to be used with a reasonably loaded Jimny it could turn out to be a little too stiffly sprung for a lightly loaded or no load Jimny. Except probably for the OME kit with their close to stock rated* standard springs and twin tube shocks.

*Rated - I usually mean spring rate but could also mean preload. Also used elsewhere in this blog in this context.

So what could work for a reasonably loaded Jimny with - metal bumpers, roobar, stuff for camping, towing, and a boot that is a kitchen - wouldn't work for me which doesn't have any of these and a spring rated to carry all these would throw me around inside the cabin. Except for the OME most Aussie kits even with their lowest rated springs maybe a tad too stiff for my use case.

The solution

Why not use an OME kit or a Japanese kit? Both are excellent options but one caveat.

Good shocks are expensive some not so good shocks are also expensive because market is blind. So when paying money to get a set of shocks either locally or imported to me it makes excellent sense if they are sort of lifetime shocks? Like if they can be rebuild and used again and again? Or at least the thought that the shocks are able to be rebuild and used for ever? It makes sense that one is not paying shipping and duty over the actual cost either directly or indirectly for a wear and tear times. It does to me.

Make that two caveats. When paying a lot it would be nice if the shocks were also mono tubes over twin tubes? Mono tube rebuildable?

So that ruled out the OME twin tube shocks available for the Jimny and the Kayaba/KYB made adjustable twin tubes shocks which almost all Japanese kits are clubbed with (pretty sure each would have their own custom settings). They are not rebuildable. The premium shocks sold by the Jimnybits of UK are adjustable and rebuildable, but they are hydraulic shocks and not gas charged to the best of my knowledge.

That brought me to the only reasonably priced rebuildable mono tube and that is the Dobinson IMS. Its not adjustable, which is a shame, because it could have been but then they wouldn't be able to sell their MRR shocks. Mrrrrrrr!

Dobinson IMS40 numbers

Front : Compressed 295 mm* (from Aus FB group), Extended ~450 mm (measured myself from base of pin to center of eye).

Rear: Compressed 335 mm* (from Aus FB group), Extended ~495 mm (measured myself from center of eye to eye).

Stock shocks numbers (from TGR)

Front: Compressed 280 mm, Extended 415 mm.

Rear: Compressed 305 mm, Extended 450 mm.

* A note on compressed length measurements of Dobinson IMS:- Being  high pressure gas charged monotubes it is almost impossible to fully compress the shocks physically with hands and body weight. It can be done crudely with a jack with top mount bolted in the vehicle as I attempted it but the shock could bottom out and I wont know it (again being monotubes) so did not proceed further because of risk of damaging it. The compressed length numbers were shared by a Aus FB group user who said he got it from Dobinson.

The springs

Had to think long and hard about this and decided definitely not Australian except if it were the OME no load springs. At this point I was not very particular about having 5 door springs.

The Japanese have a lot of options when it comes to the springs from many manufacturers and different types. Taniguchi stood out. They were a very well respected name and the springs are made by Chuhatsu a top spring manufacturer which is also a Toyota subsidiary, and their spring catalogue looked well defined, each made for a purpose. So it was decided, Taniguchi it is.

Taniguchi had a few things going for it, one of which as I have already mentioned its made by Chuhatsu the other bits that caught my attention was they were dual rate springs with soft initial rates and that they had the Jimny lean taken care of by having different spring lengths on the left and right sides, both front and rear.

The dual rate spring with soft initial rate and stiffer secondary rate gives a very good ride at city speeds and over bad roads; at higher speeds the secondary rate which is stiffer than stock aids in good handling. Combined with the progressive IMS shocks the feeling is good on both city and highway. So the elaborate guessing game, sharing notes and mental math worked out great.

Supporting mods

Front castor correction - I didn't buy the castor correcting bushes as I went the full monty for the castor corrected radial arms. BUT, its a big irritating "but", the manufacturer doesn't have stock (informed after payment) and I've been waiting, as I type this for, more than 20 days. Neither their emails nor their quote had this mentioned and the website showed as they are in stock.

Front cross member drop - Same story as above and again same manufacturer as above and so the BUT still holds because they are to ship together. They are a pain.

Brake flex hose extension - I like the OEM rubber type extended brake hoses. Why rubber type? Easy to inspect. Proven hassle free application for a daily driver. So bought a set of four from Showa Garage and they are sorted.

Rear Panhard bar/trackbar riser - The rear trackbar is corrected with the Taniguchi riser bracket. The JB74 or Gen4 3 door ones do fit, all Gen4s have the same fitting and the same captive bolt for the stock Panhard bolt.

Bump stops - Strictly necessary for the IMS lift because the compressed length of the IMS shocks are longer than the stock Maruti shocks. You need the bump stop to engage the axle before the IMS shocks bottoms out, or risk damaging it. An additional 15 mm extended bump stop will be enough but can use longer if they are cone types. In the photo below I used an Alto rear bump stop which is longer by about 20mm. This will be changed to a cone type progressive longer bump stop.

Good to have supporting mods

Front trackbar/Panhard bar correction - On the Jimny what applies for the rear with regard to correcting the rear Panhard also applies for the front as both suspension layouts are identical. But most don't do this except maybe fit an adjustable Panhard road to correct the axle offset. The reason being the draglink and the trackbar need to be parallel to or face bump steer. So ideally both should be corrected together. But lets see! To correct this either the chassis mount could be dropped or a riser bracket can be used on the front axle just like the rear. The former does not raise the roll center but raising the mount on the axle will. But either ways having it parallel or close to parallel to the axle provides better road manners.

Rear castor correction - Whilst an incorrect rear castor does not affect steering it has its benefits. For one it corrects the drive shaft angle at the rear diff and two twists the axle forward so the rear fittings such as shock mounts and trackbar mounts are back to being level as stock which would put less stress on their bushing. Usually done on 50mm plus lifts but its not a rule.

Did I miss any? Will update if I remember.
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Bilstein Monotube vs Dobinson IMS Monotube - Why does it ride different?

If springs are the same?

A gas charged monotube shock absorber is a single tubular structure holding the oil, a shaft with piston/valve, a free floating separator piston, and a gas chamber.  The piston rides up and down the tube in the oil and further down tube there is free floating piston suspended as a separator. The job of this piston is to separate the monotube body into two halves - one containing the oil and piston and the other containing high pressure nitrogen gas. The monotube suspension was pioneered by Bilstein.

The function of the high pressure nitrogen gas is to apply pressure on the gas by means of the floating piston to avoid cavitation. If the oil is aerated the damping properties of the shock absorbers change as the rate with which the piston pushes the now aerated oil through the valve body. Applying a constant pressure on the fluid mitigates cavitation and the shock provides a consistent damping.

The other type of shock absorber is a twin tube and each have their own pros and cons. Twin tubes usually have linear valving and the gas pressures, if gas charged, are very very low.

If monotubes were pioneered by Bilstein why does a Dobinson IMS Monotube ride much better than an equivalent Bilstein Monotube? Two reasons - type of valving in mass market Bilstein monotubes and gas pressure.

Valving

All Bilstein shocks listed above have digressive damping, stiff at lower (piston) speeds and they ease up at higher speeds. But a sudden large impact on less than perfect road will jolt the vehicle and have experienced this many times on the roads here. Good for track and circuit.

Dobinson IMS shocks have a progressive damping curve, not as stiff at slower speeds and gets progressively tighter as speed increases. Good at low vehicle speeds and sudden impacts as one would expect on un sealed roads.

Gas Pressure

Both Bilstein and Dobinson IMS are monotube shock absorbers with internal reservoirs for oil and high pressure gas separated by a free floating piston. The Dobinsons IMS has a recommended fill pressure of 150PSI of Nitrogen. Bilstein holds a higher pressure N2 charge in their internal reservoir monotubes with recommended fill pressure between 180PSI and 200PSI and with a factory fill pressure at 200PSI. The Bilstein is at a minimum 30PSI to a maximum 50PSI higher pressure compared to the Dobinson IMS. This higher internal gas pressure in a Bilstein acts on the piston shaft giving it a gas spring effect and a static preload. Lower the gas pressure less the static preload.

A question asked frequently: Why does my car sit higher after I put a set of Bilstein monotube shocks (on stock springs)? Ans: High gas pressure.

Especially true if the vehicle was previously on a set of any low gas pressure or hydraulic shocks which were replaced with a set of Bilstein monotubes. The high gas pressure adds higher static preload to the initial weight needed to get the shaft moving. This combined with the spring rate adds to the total suspension preload at rest and at lower speeds. The effect of gas spring preload is more at low piston speeds and weans off at higher piston speed.

I have used Bilstein monotubes in two of my own vehicles, an Octavia RS Mk1 with stock and VW OE sport springs and in an Innova Crysta 2.8 Diesel with stock and Eibach lowering springs. Also had a friend's VW Vento 1.6TDI with me for many months (when he left the country) which had a B12 coil over kit. Although the B12 coilover was supplied with springs as a kit, it rode very similar to the other two. Not pleasant on our less than perfect roads, except for the Crysta.

Run off the mill Bilstein monotubes including the B12 coilover rode harsh except for the one I put in the Crysta. It was well behaved because the set was made for the previous Innova and the Innova Crysta I put it in weighed a good 200Kg more than the old Innova. This additional weight had a calming effect on the Bilstein's gas spring at low piston speeds.

Rebuildable?

Dobinson IMS is rebuildable by design and they have a rebuild kit available for the monotubes, they have a gas port to let out gas pressure and charge it post rebuild.

Bilstein supply all the minor and major internal parts as spare parts for their shock absorbers. The rebuild process of the Bilstein is a bit more involved because they have not included a gas port in their shock body. So First a hole needs to be precision drilled at a specific location to let out gas pressure and then use a high pressure Schrader valve (supplied as spare by Bilstein) installed in the shock body to charge the shock.

Both can be revalved during the process of rebuild.

Is there a better way that I could have put this?

I wish, watch the video this is what I wanted to convey in this blog entry about gas pressure. Also read the comments.

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